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100 BGP Questions Flashcards

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1
Q

Can router on different subnet become bgp neighbor?

A

Yes, BGP neighborship can be formed between Routers across different subnets. Unlike IGP protocols, BGP requires TCP connection across Routers which may be on same subnet or different subnets.

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2
Q

Which layer does BGP work on?

A

BGP is an application layer protocol and works upto Layer 7 of OSI model.

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3
Q

What is the difference between eBGP multihop and ttl security?

A
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4
Q

Which protocol and port number does BGP use for neighborship?

A

BPG uses TCP protocol and port number 179 for forming neighborship.

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5
Q

How do I configure BGP?

A

The basic configuration of BGP requires below commands –
router bgp < as-number >
neighbor < ip-address >
remote-as < remote-as-number >

Below is sample configuration –
R1( config)# router bgp 100
R1( config-router)# neighbor 10.0.0.2 remote-as 200
R2( config)# router bgp 200
R2( config-router)# neighbor 10.0.0.1 remote-as 100

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6
Q

What is the use of “BGP best-path as-path ignore” command?

A

As a default behavior of BGP path selection, BGP prefers the path with the shortest AS_PATH. This step is skipped if we have configured the Cisco “bgp best-path as-path ignore” command.

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7
Q

How do I configure BGP with the use of a loopback address?

A

Below is the configuration configure BGP with the use of a loopback address -
R1 Configuration - Interface Loopback0 ip address 1.1.1.1 255.255.255.255
! Interface FastEthernet0 ip address 10.0.0.0.1 255.255.255.0
! router bgp 100 neighbor 2.2.2.2 remote-as 200 neighbor 2.2.2.2 update-source Loopback0 > > > > > > this command specifies that the TCP connection with BGP peer should be established using loopback interface.

! ip route 2.2.2.2 255.255.255.255 10.0.0.2 > > > > > > Static Route for reachability to remote BGP peer End
R2 configuration -
Interface Loopback0 ip address 2.2.2.2 255.255.255.255
!Interface FastEthernet0 ip address 10.0.0.0.2 255.255.255.0
! router bgp 200 neighbor 1.1.1.1 remote-as 100 neighbor 1.1.1.1 update-source Loopback0 > > > > > > This command specifies that the TCP connection with BGP peer should be established using loopback interface.
! ip route 1.1.1.1 255.255.255.255 10.0.0.2 > > > > > > Static Route for reachability to remote BGP peer End

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8
Q

What is the order of preference of attributes when applied to one neighbor in BGP?

A

BGP works through these attributes in this specific order when choosing a path -
Highest weight (This is a cisco proprietary value)
Highest LOCAL_PREF Prefer a route that is locally sourced
Shortest AS_PATH ORIGIN Lowest MED External BGP routes are preferred over internal BGP routes If no external route select path with the lowest IGP cost to the next hop router for IBGP. The most recent route The lowest BGP router ID.

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9
Q

What does a next hop of 0.0.0.0 mean in the show ip bgp command output?

A

A network output in the BGP table with a next hop address of 0.0.0.0 means that either network is locally originated via redistribution of Interior Gateway Protocol (IGP) into BGP, or via a network or aggregate command in the BGP configuration.

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10
Q

What are the well-known communities of the BGP community attribute?

A

Below are well known BGP community attributes -

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11
Q

How does BGP behave differently with auto-summary enabled or disabled?

A

In the latest Cisco IOS releases, auto-summary is disabled by default. When auto-summary is enabled, it summarizes the locally originated BGP networks to their classful boundaries. Auto-summary is disabled, the routes introduced locally into the BGP table are not summarized to their classful boundaries.

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12
Q

What formats can I use to configure the BGP community attribute?

A

In latest releases of Cisco IOS (12.0 onwards), we can configure communities in 3different formats – Decimal format Hexadecimal format AA:NN format By default, Cisco IOS uses the older decimal format. In order to configure in AA: NN, where the first part is the AS number and the second part is a 2-byte number, issue the “ip bgp-community new-format” in global configuration. As an example, 3 formats of community 10: 20 will be as under – 655380 0x0A0014 10: 20

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13
Q

How can I verify if a BGP router announces its BGP networks and propagates them to the global BGP mesh?

A

Below are the commands to verify the IP blocks are announced to BGP neighbors ISP: “show ip bgp neighbors [address] advertised-routes” for sent networks “show ip bgp neighbors [address] routes” for received networks

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14
Q

When and how should I reset a BGP session?

A

Traditional approach was to tear down the BGP session with single neighbor/ all Neighbors/ peer group - R1# clear ip bgp {* | neighbor ip | peer-group} Below Outbound Soft Reconfiguration Re-sends complete BGP Table -
R1# clear ip bgp {neighbor ip} soft out Below Inbound Soft Reconfiguration Re-sends complete BGP Table - R1# clear ip bgp {neighbor ip} soft in Another option is to request a neighbor to resend routing information, without bringing a session down - R1# clear ip bgp {* | neighbor ip | peer-group} in

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15
Q

Is there any special configuration needed on PIX/ ASA to allow BGP sessions through it?

A

Below diagram will be referred to while showing the step by step configuration required on ASA/ PIX to allow BGP sessions through it -
Step 1 – (Configure access list to allow TCP port 179 / BGP as below – access-list BGP-NEIGHBOR extended permit tcp host 192.168.10.1 host 192.168.20.1 eq 179 access-list BGP-NEIGHBOR extended permit tcp host 192.168.20.1 host 192.168.10.1 eq 179 Step 2 – (Allow TCP option 19 in TCP Map) – tcp-map BGP tcp-options range 19 19 allow Step 3 – (Create a class map to match the BGP Traffic using the ACL above) class-map BGP-CLASS match access-list BGP-NEIGHBOR Step 4 – (Use Global Policy to apply all the actions) - Policy-map global_policy class BGP-CLASS set connection random-sequence-number disable set connection advanced-options BGP

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16
Q

What is an autonomous system number (ASN)?

A

AS numbers are globally unique numbers that are used to identify ASes, and which enable an AS to exchange exterior routing information between
neighboring ASes. An AS is a connected group of IP networks that adhere to a single and clearly defined routing policy. There are a limited number of available AS numbers. Therefore, it is important to determine which sites require unique AS numbers and which do not. Sites that do not require a unique AS number should use one or more of the AS numbers reserved for private use, which are in the range from 64512 to 65535.

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17
Q

What is the BGP path selection criteria?

A

BGP works through these attributes in this specific order when choosing a path - Highest weight (This is a cisco proprietary value) Highest LOCAL_PREF Prefer a route that is locally sourced Shortest AS_PATH ORIGIN Lowest MED External BGP routes are preferred over internal BGP routes If no external route select path with the lowest IGP cost to the next hop router for IBGP. The most recent route The lowest BGP router ID

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18
Q

What is the difference between always-compare-med and deterministic-med?

A

BGP router subcommands affect and influence the MED-based BGP best path selection process. Both commands are not enabled by default; and both commands are separate and independent – enabling one does not automatically enable the other.
Entry #1 – AS_PATH 300 100, MED 150, external, NEXT_HOP 4.4.4.4, RID 4.4.4.4 Entry #2 – AS_PATH 200 100, MED 200, external, NEXT_HOP 2.2.2.2, RID 2.2.2.2 Entry #3 – AS_PATH 300 100, MED 100, internal, NEXT_HOP 3.3.3.3, RID 3.3.3.3 When “bgp deterministic-med” is enabled - Entry #1 is the best of its group (Entry 1 and Entry 3) Entry #2 is the best for its group (Entry 2 only) Entry #1 and Entry #2 are compared eventually. Since the entries are from different ASes and the bgp always-compare-med command is not enabled, the MED is not considered in the comparison. Entry #1 is selected as the best path as it is an EBGP route. When “bgp always-compare-med” is enabled - Entry #1 and Entry #2 are compared first. These entries are from different neighboring ASes, but the MED is used in the comparison as the bgp always-compare-med command is enabled. Entry #1 is selected as it has a lower MED. Entry #1 and Entry #3 are compared next. The MED is used in the comparison again. Entry #3 is selected as the best path as it has a lower MED.

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19
Q

Do internal i.e. iBGP sessions modify the next hop?

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 9). Kindle Edition.

A

Internal BGP i.e. iBGP preserves the next hop attribute learned from eBGP peers. This means we are required to enforce the iBGP router to advertise itself as next hop and not the external BGP peer. In order to make sure we can reach the eBGP next hop, following are the options – Include the network that the next hop belongs to in the IGP or Issue the next-hop-self neighbor command The BGP route is otherwise unreachable.

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 9). Kindle Edition.

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20
Q

Do external BGP (eBGP) sessions modify the next hop?

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 9). Kindle Edition.

A

Yes, Routes advertised to eBGP peers will have Next-Hop attribute changed to eBGP routers IP address.

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 9). Kindle Edition.

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21
Q

Do external BGP (eBGP) sessions between confederations modify the next hop?

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 9). Kindle Edition.

A

No, eBGP sessions between confederation AS do not modify the next hop attribute.

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 9). Kindle Edition.

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22
Q

In external BGP (eBGP) sessions, which IP address is sent as the next hop?

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 10). Kindle Edition.

A

In eBGP peering, the next hop is the IP address of the neighbor that announces the route.

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 10). Kindle Edition.

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23
Q

Does the route reflector change the next hop attribute of a reflected prefix?

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 10). Kindle Edition.

A

By default, the next hop attribute is not changed when a prefix is reflected by route reflector.

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 10). Kindle Edition.

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24
Q

How can I announce a prefix conditionally to one ISP only when I lose the connection to my primary ISP?

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 10). Kindle Edition.

A

BGP advertises routes from its BGP table to external peers by default. The BGP conditional advertisement feature provides additional control of route advertisement depending on the existence of other prefixes in the BGP table. Normally, routes are propagated regardless of the existence of a different path. The BGP conditional advertisement feature uses the non-exist-map and advertise-map configuration commands to track routes by the route prefix. If a route prefix is not present in the non-exist-map command, the route specified by the advertise-map command is announced.

ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 10). Kindle Edition.

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25
How can I configure BGP to provide load sharing and redundancy in my network? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 10). Kindle Edition.
Below are the choices in BGP while load sharing in the network – a) When equal cost multiple links between eBGP neighbors - Using static routing for Loopback Address as a BGP Neighbor. b) When Dual-Homed to One Internet Service Provider (ISP) Through a Single Local Router - Using maximum-paths command c) Load Sharing When Dual-Homed to One ISP Through Multiple Local Routers - Using as-path prepend d) Load Sharing When Multihomed to Two ISPs Through a Single Local Router - Using weight and selective prefix receive/ advertisement - Using Local preference and as-path prepend ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 10-11). Kindle Edition.
26
How much memory should I have in my router to receive the complete BGP routing table from my ISP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 11). Kindle Edition.
Cisco recommendation is to have a minimum of 512 MB of RAM in the router to store a complete global BGP routing table from one BGP peer. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 11). Kindle Edition.
27
What are the benefits of configuring BGP peer groups? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 11). Kindle Edition.
Below are the key benefits which can be reaped out from BGP peer groups – Reduction of the resource of BGP devices when it makes the updates to the BGP neighbors. Reduction in amount of configuration which is requires to be done on BGP enabled device. Configuration becomes simple and easy ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 11). Kindle Edition.
28
Why do I see the same route twice from the same peer in BGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 11). Kindle Edition.
Two entries are seen due to soft-reconfiguration configured. Both the unmodified path and the modified path, which depends on the inbound policy, if permitted, are stored in the path table for the prefix. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 11). Kindle Edition.
29
What is synchronization, and how does it influence BGP routes installed in the IP routing table? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 11). Kindle Edition.
If your AS passes traffic from another AS to a third AS, BGP should not advertise a route before all routers in your AS learn about the route via IGP. BGP waits until IGP propagates the route within the AS and then advertises it to external peers. A BGP router with synchronization enabled does not install iBGP learned routes into its routing table if it is not able to validate those routes in its IGP. BGP synchronization rule - Do not advertise a route learned from IBGP to an external neighbor unless a matching route is learned from an IGP. This was an old rule. However, with newer Cisco IOS 12.2( 8) T and later, this feature is off by default. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 11-12). Kindle Edition.
30
How do I know which Cisco IOS software release supports a particular BGP feature? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition.
Use Software Research (registered customers only) in order to quickly find which Cisco IOS software release supports your feature. Link to “Software Research” is - https:// software.cisco.com/ selection/ research.html ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition.
31
How can I set the Multi Exit Discriminator (MED) value on prefixes advertised to external BGP (eBGP) neighbors to match the Interior Gateway Protocol (IGP) next hop metric? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition.
The set metric-type internal route-map configuration command causes BGP to advertise a MED that corresponds to the IGP metric associated with the next hop of the route. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition.
32
What is the default BGP ConnectRetry timer? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition.
The default BGP ConnectRetry timer is 120 seconds. Only after this time passes does the BGP process check to see if the passive TCP session is established. If the passive TCP session is not established, then the BGP process starts a new active TCP attempt to connect to the remote BGP speaker. During this idle 120 seconds of the ConnectRetry timer, the remote BGP peer can establish a BGP session to it. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition.
33
What does r RIB-Failure mean in the show ip bgp command output? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 12). Kindle Edition.
RIB-Faliure in “show ip bgp” command may be due to one of the below reasons - Route with better administrative distance already present in IGP. For example, if a static route already exists in IP Routing table. Memory failure. The number of routes in VPN routing/ forwarding (VRF) exceeds the route-limit configured under the VRF instance. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 12-13). Kindle Edition.
34
How can I redistribute internal BGP (iBGP) learned default-route (0.0.0.0/ 0) route into EIGRP/ OSPF/ IS-IS? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
By default, iBGP redistribution into IGP is disabled. Issue the bgp redistribute-internal command in order to enable redistribution of iBGP routes into IGP. A sample configuration for redistributing a iBGP learned default route 0.0.0.0/ 0 into EIGRP is shown in this output. Configurations for OSPF/ IS-IS are similar. router bgp 65200 bgp redistribute-internal ! router eigrp 200 redistribute bgp 65200 route-map DEFAULT ! ip prefix-list default-route seq 5 permit 0.0.0.0/ 0 ! route-map DEFAULT permit 10 match ip address prefix-list default-route ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
35
How can I filter all IP routes advertised to a BGP neighbor except the default route 0.0.0.0/ 0? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
The specific routes can be filtered if you use inbound filter-list, distribute-list, prefix-list and route-map all at the same time for the same bgp neighbor. This is the order of operation: Filter-list Router-map Distribute-list (or) prefix-list ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
36
Is it possible to track an interface and change the route availability? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
Yes, it is possible to track the state change of an interface and route availability with the Enhanced Object tracking. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
37
How does IP RIB Update allocate memory? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
IP RIB Update allocates the prefixes, and attributes are held in chunks. It is not possible to free the entire chunk until every element in the chunk is freed. If more routes are learned, then those free elements in the chunks are used. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 13-14). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 13). Kindle Edition.
38
What is the command to see IPv6 BGP neighbors? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
The “show bgp ipv6 unicast summary” command is used to see the IPv6 BGP neighbors ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
39
Why are there no statistic results when I use the debug bfd events and debug bfd packets commands? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
It is the normal behaviour, as bfd hellos are sent in sub minimal seconds and in case you run debugs for that, the router cannot handle. So the bfd messages are seen in debug only when flaps happens. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
40
Can I run two BGP process on single router ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
Only single BGP process can be run on single Router. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
41
Difference between hard reset and soft reset in BGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
42
What are different BGP message types ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 14). Kindle Edition.
BGP has 4 message types - Open Keepalive Update Notification All BGP message are unicast to the one neighbor over the TCP connection. OPEN Message - Open messages are used to start a BGP session by requesting that a BGP session be opened over an existing TCP/ IP session. Once two BGP routers have completed a TCP 3-way handshake they will attempt to establish a BGP session, this is done using open messages. In the open message information about BGP router will be available. Routers use this message to identify itself and to specify its BGP operational parameters. Open message is always send when the TCP session is established between neighbors. KEEPALIVE Message - If a router accepts the parameters specified in Open message, it responds Keepalive. By default Cisco sends keepalive every 60 sec or a period equal to 1/ 3 the hold time. UPDATE Message - Advertises feasible routes, withdrawn routes or both. NOTOFICATION Message - This message is sent whenever something bad has happened, e.g. an error is detected and causes the BGP connection to close. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 15). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 15). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 15). Kindle Edition.
43
What are various BGP states? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 15). Kindle Edition.
IDLE State: verifying route to neighbor - BGP refuses all incoming connections. No BGP resources are allocated in idle state, and no incoming BGP connections are allowed. Connect State: BGP waits for a TCP connection to be completed. If successful, the BGP state machine moves into OpenSent state after sending the OPEN message to the peer. Failure in this state could result in either going into Active state or Connect state, or reverting back to idle state, depending on the failure reasons. Active State: (Attempting connectivity to neighbor) In this state, a TCP connection is initiated to establish a BGP peer relationship. If successful, BGP sends its OPEN message to the peer and moves to OpenSent state. Failure can result in going to the Active or Idle states. OpentSent State: (Open message sent to neighbor) After sending an OPEN message to the peer, BGP waits in this state for the OPEN reply. If a successful reply comes in, the BGP state moves to OpenConfirm and a keepalive is sent to the peer. Failure can result in sending the BGP state back to Idle or Active. OpenConfirm State: (Neighbor replied with open message) The BGP state machine is one step away from reaching its final state (Established). BGP waits in this state for keepalives from the peer. If successful, the state moves to Established; otherwise, the state moves back to Idle based on the errors. Established State: (Connection between neighbors established) This is the state in which BGP can exchange information between the peers. The information can be updates, keepalives, or notification. Ques 44. Which command is used to disable BGP neighborship? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 16-17). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 16). Kindle Edition.
44
Which command is used to disable BGP neighborship? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
Neighbor < neighbor-ip > shutdown. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
45
What are values of keepalive and Dead timers in BGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
Keepalive - 60 seconds Hold-down - 180 seconds (3 multiplied by Keepalive) ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
46
How many public and private AS numbers are there? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
Public AS Numbers- Range from 1 to 64511 Private AS Numbers - Range from 64512 to 65535 ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
47
BGP Route selection criteria? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
BGP works through these attributes in this specific order when choosing a path - Highest weight (This is a cisco proprietary value) Highest LOCAL_PREF Prefer a route that is locally sourced Shortest AS_PATH ORIGIN Lowest MED External BGP routes are preferred over internal BGP routes If no external route select path with the lowest IGP cost to the next hop router for IBGP. The most recent route The lowest BGP router ID. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 17). Kindle Edition.
48
Does the router have to be restarted after a new BGP Neighbor Maximum Prefix is configured? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 17-18). Kindle Edition.
If the new maximum number of Prefixes is larger that the current maximum, there is no need to soft/ hard clear the BGP session, and reload is not required. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
49
Is there a command to check the advertised routes along with the prepend of the AS-paths? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
Below are the options to verify - Check the BGP AS PATH Attribute on Peering device. This is one of the easiest ways to check whether the router performs AS PATH prepending or not. Run debug on BGP updates (in outbound direction) and then check for prepends. Use an access-list while you debug BGP updates. Another option would be to take a packet capture on exit interface and see what update is being sent on the wire. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
50
How does neighbor soft-reconfiguration inbound command function? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
The command “neighbor soft-reconfiguration inbound” causes the router to store all inbound received routing policy updates, for example, a duplicate table is stored in the memory for each peer. This method is memory-intensive and not recommended unless absolutely necessary. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
51
What does the %IPRT-3-ROUTEINSERTERROR: Error inserting routing entry error message mean? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
This error message indicates that there is not enough memory to accommodate BGP prefixes, learnt from neighbors. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
52
What is the difference between when a route is injected in BGP via redistribute command or a network command? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 18). Kindle Edition.
When you use the redistribution of IGP into BGP to advertise the route, then there is no need to specify the network statement for all the subnets individually. Also when the route is obtained from any other routing protocols into BGP table by redistribution, the Origin attribute is Incomplete (?) When you specify the network command then it is Internal/ IGP (i). ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 18-19). Kindle Edition.
53
How do I verify Layer 4 forwarding summary information? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 19). Kindle Edition.
In order to view the summary information on Layer 4 forwarding, use the “show mls cef summary” command. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 19). Kindle Edition.
54
What are differences between IGPs and EGPs? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 19). Kindle Edition.
55
What is between AS-OVERRIDE and ALLOWAS-IN in BGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 19-20). Kindle Edition.
As a standard behavior of BGP remote site (using same AS Number) prefix received by other site of same customer will drop the route since it sees its own AS Number in the received packet. The methodology to circumvent or mitigate default behavior of BGP is to use the BGP features of “AS-Override” or else “Allowas-in”. While former is used by PE to modify the AS Number in AS Path so that prefix is not dropped, latter is implemented in CE device to introduce an exception in BGP AS path loop prevention mechanism. The difference between both terms is shared in below table - ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 20). Kindle Edition.
56
Name several path attributes of BGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 20). Kindle Edition.
57
Why is there a problem with iBGP in large networks? How can this problem be solved? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 21). Kindle Edition.
There must be a full mesh of iBGP sessions, in other words: each BGP router within an AS must have iBGP sessions with all other BGP routers in the AS. By requiring that all information in iBGP is learned directly from the router that learned the information over eBGP, there can't be any loops in iBGP. The full mesh requirement can be solved using either route reflectors or confederations. Route reflectors distribute iBGP information from one router to another, which is normally not allowed in iBGP. Since the clients of the route reflector get all iBGP from the route reflector they don't need to have iBGP sessions with all other BGP routers. Reflectors add additional path attributes that allow them to detect and eliminate loops. In Confederation, the AS is split into a number of sub-ASes, so the iBGP full mesh is done within each sub-AS and a modified version of eBGP is used between sub-ASes. To the outside, the confederation behaves like a single AS ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 21). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 21). Kindle Edition.
58
Name BGP path attributes to control incoming and outgoing traffic ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 22). Kindle Edition.
Inbound Traffic Flow can be influenced by manipulating the following attributes - AS-Path Prepending MED Outbound Traffic Flow can be influenced by manipulating the following attributes - Weight Local Preference ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 22). Kindle Edition.
59
My BGP is showing 0.0.0.0 as router-id; what could be the possible reason? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 22). Kindle Edition.
BGP Router-ID of 0.0.0.0 is set when below 3 conditions of assigning Router-ID fail - Use the address configured by the BGP router-id command Use the Loopback interface address with the highest IP address Use the highest IP address of the interface ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 22). Kindle Edition.
60
If my BGP neighbor is stuck in idle or active state, what should I do? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 22). Kindle Edition.
The reason for BGP neighbor stuck in idle or active state are – Wrong AS Configured Misconfigured Local IP or Per IP address Authentication issues TCP port 179 allowed across the peers Multihop and peer TTL misconfigurations Usually, there are configuration issues that stop the BGP connection from getting established. It can be a wrong AS, misconfigured local IP / peer IP address, authentication issues, and others. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 22). Kindle Edition.
61
Explain BGP site of origin (SoO). ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 22). Kindle Edition.
SoO is a BGP extended community attribute and stands for Site of Origin. SoO uniquely identifies the site that has originated the route.SoO is also useful to prevent routing loops and sub-optimal routing, especially used to prevent routing loops on dual homed sites. SoO value needs to be defined in one of the below Naming Conventions – A 16-bit autonomous system number, a colon, and a 32-bit number, for example: 45000: 3 A 32-bit IP address, a colon, and a 16-bit number, for example: 192.168.10.2: 51 SoO Rule of Operation: SoO is applied on updates coming to PE1 from CE router and when these routes are advertised as VPNV4 route to PE router PE2, the PE2 router doesn’t advertise the routes back to the CE router hence avoiding the routing loop. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 22-23). Kindle Edition.
62
What do you understand by BGP split-horizon rule? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 23). Kindle Edition.
The BGP split-horizon rule governs the route advertisements between IBGP peers, which specifies that routes learn via IBGP are never propagated to other IBGP peers. The BGP split-horizon rule prevents R2 from propagating routes learned from R1 to RT3. Similar to the split-horizon rule in the distance-vector routing protocols, BGP split-horizon is necessary to ensure that routing loops are not started within an AS. As a result, full-mesh IBGP peering is required within an AS for all the routers within the AS to learn about the BGP routes. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 24). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 23). Kindle Edition.
63
Describe BGP communities. Name well-known communities. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 24). Kindle Edition.
The BGP community attribute is a numerical value that can be assigned to a specific prefix and advertised to other neighbors. When the neighbor receives the prefix it will examine the community value and take proper action whether it is filtering or modifying other attributes. By default the community attribute is removed from the update before being sent to the neighbor. To allow community values to be sent to a specific neighbor the command neighbor x.x.x.x send-community must be applied. BGP has default 4 well known communities that can be used to mark prefixes; listed as follows: Internet: advertise these routes to all neighbors. Local-as: prevent sending routes outside the local AS within the confederation. No-Advertise: do not advertise this route to any peer, internal or external. No-Export: do not advertise this route to external BGP peers. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 24). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 24). Kindle Edition.
64
Can I use BGP instead of any IGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 25). Kindle Edition.
BGP can be used instead of ay IGP, however this would not be best recommended approach. Below are some of drawbacks of using BGP (iBGP) instead of IGP inside a network – Statically defined neighbor only with BGP and no dynamic neighbor discovery BGP uses Complex Path Selection Process unlike IGPs which determine the best path based on simple metrics. Scalability issues like fully mesh network in iBGP is another challenge. Though route reflectors/ confederation resolves the problem to a scale, still route recursion causes problems in iBGP. BGP default Hello and hold timers are 60 and 180 seconds which are pretty higher than IGPs (10 sec and 40 seconds respectively). This further delay convergence time in BGP. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 25). Kindle Edition.
65
Types of BGP routing table? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 25). Kindle Edition.
In order for BGP to be able to perform its functions it stores this information is a special type of database called the BGP Routing Information Base (RIB). BGP Routing Information Base consists of three parts as explained below – Adj-RIBs-In – This BGP information refers to routing prefix received from neighbor without applying any filtering or attribute manipulation. Attribute modifications or route filtering is applied after Adj-RIB-In. Loc-RIB – BGP maintains its own master routing table called the Local Routing Information Base (Loc-RIB). Best routes is selected after applying routing policies on the routes available in Adj-RIBs-In. Whenever an Adj-RIB-In changes, the main BGP process decides if any of the neighbor’s new routes are preferred to routes already in the Loc-RIB and it replaces as required. Adj-RIBs-Out – This table refers to Routes/ NLRI selected from Loc-RIB after applying outbound routing/ filtering policies. This table stores the routing information that was selected by the local BGP router and the advertised to its peers through BGP update messages. Below diagram will help understand placement of 3 BGP table types for BGP routing flow starting with Route learning till its advertisement to neighbors – ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 25-26). Kindle Edition.
66
What is route reflector and why it is required? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 26). Kindle Edition.
While configuring iBGP neighborship between BGP speaking neighbors in same AS, administrators need to be careful of iBGP rule (BGP Split Horizon) of preventing routing Loops. The rule governs that “for Route advertisements between IBGP peers, routes learnt via iBGP are never propagated to other IBGP peers. In order to address the Split Horizon rule iBGP neighbors need to be configured in full mesh. The formula to compute the number of sessions required for a full mesh is n * (n – 1)/ 2, where n is the number of BGP-enabled devices. Because of the internal BGP (IBGP) full-mesh requirement, most networks use Route Reflectors to simplify configuration. A Route Reflector (RR) is an iBGP feature that eliminates the need for a BGP full-mesh topology and allows iBGP to scale in large networks. The route reflector mechanism allows a BGP speaker (an iBGP router) to act as a route reflector that advertises (reflects) the routes it learns from one iBGP router to other iBGP peers within the AS. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 26). Kindle Edition.
67
What is no-synchronization? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 26). Kindle Edition.
The BGP synchronization rule states that if your AS passes traffic from another AS to a third AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP. The no synchronization command tells the iBGP routers that you don’t want them to “synchronize” iBGP with your internal routing protocol. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 26-27). Kindle Edition.
68
Default BGP timers. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
Keepalive - 60 seconds Hold-down - 180 seconds (3 multiplied by Keepalive) ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
69
In Multihoming scenario if primary link gets fail, after how long traffic will be shifted to secondary link. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
By default, fast external failover is enabled for eBGP neighbor. So when the egress interface fails, it immediately bring down the BGP session and next best path will be selected. If you have it disabled, it will rely on the BGP hold timer. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
70
Explain the term BGP Graceful Restart? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
Initially, to control this un-stability, GR (Graceful Restart) principle was proposed, where on router’s control plane switchover, router doesn’t report the switchover information immediately to its own neighbor rather it wait for certain period of time (which is called grace interval) .If the router’s control plane comes back up and re-establishes its peering sessions before the grace period expires, as would be the case during a control plane switchover, the temporarily broken peering sessions do not effect the network beyond the neighbors. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
71
Can we tune BGP timers to improve BGP convergence? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
It is possible to tune the BGP keepalive timers to be as low as 1/ 3 seconds, but the risk of peering session flapping become significant with such settings. Hence, as a good practice, BGP timers of 5 and 15 seconds can be considered and has been seen to work efficiently in customer environments. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 27). Kindle Edition.
72
What is difference between eBGP and iBGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 28). Kindle Edition.
73
What is the advantage of using BGP AS Prepend? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 28). Kindle Edition.
The AS-Path comes pretty early in the BGP path selection process and hence preferred attribute to use in BGP. The shorter the AS-PATH the more preferred a route becomes. AS-PATH Prepending is used to make a route less desirable, along a specific path, by making the AS-PATH longer. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 28). Kindle Edition.
74
What is cluster id in BGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 28). Kindle Edition.
Route Reflector Cluster ID is a four-byte BGP attribute, and, by default, it is taken from the Route Reflector’s BGP router ID.If two routers share the same BGP cluster ID, they belong to the same cluster. Before reflecting a route, route reflectors append its cluster ID to the cluster list. If the route is originated from the route reflector itself, then route reflector does not create a cluster list. If the route is sent to EBGP peer, RR removes the cluster list information. If the route is received from EBGP peer, RR does not create a cluster list attribute. Cluster list hence is used for loop prevention by only the route reflectors. Route reflector clients do not use cluster list attribute, so they do not know to which cluster they belong. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 28-29). Kindle Edition.
75
What is the meaning of update source loopback? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 29). Kindle Edition.
BGP Command “neighbor update-source” Configures the router to force BGP to use the IP address of the loopback interface when talking to a neighbor. Syntax: [no] neighbor {ip-address | peer-group-name} update-source loopback identifier ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 29). Kindle Edition.
76
Can we use local preference outside the autonomous system? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 29). Kindle Edition.
No, since Local preference has local significance. Further, Local preference is not attached to eBGP updates, only to iBGP updates. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 29). Kindle Edition.
77
What is confederation? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 29). Kindle Edition.
BGP confederations allows us to divide a single AS into one or more ASs and assign the whole group to a single AS. Each smaller AS will need to have a full mesh inside its own AS. Even though these ASs will have EBGP peers to ASs within the confederation, they exchange routing as if they were using IBGP; next hop, metric and local preference information are preserved. To the outside world, the confederation (the group of ASs) will look as a single AS. Below is sample topology to configure the BGP confederation where AS100 has been divided into two smaller AS 10 and 20 with two routers each. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 29). Kindle Edition.
78
Router R2 is getting following log message – “% BGP-3-NOTIFICATION: received from neighbor 192.168.12.1 2/ 2 (peer in wrong AS) 2 bytes 0014”. Diagram below. What could be the probable reason for this error log? What is mitigation approach? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 30). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 30). Kindle Edition.
Reason for error message – “On Router R2, BGP AS number 20 is configured instead of AS 2.” Any of below 3 options may be used to address above shared problem - Option 1 - Change the BGP configuration on R2 ie remove “Router BGP 20” and replace whole configuration on “Router BGP 2” Option 2 - On R1, configure neighbor command with “Remote-as” of R2 being 20 instead of 2 Option 3 - Configure “local-AS 2” on R2 to show to R1 that it belongs to AS 2 and not AS 20 Below link further elaborates the overall scenario and detailed configuration to resolve the problem condition - https:// ipwithease.com/ troubleshooting-scenario-on-incorrect-bgp-as-number/ ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 30-31). Kindle Edition.
79
What will the BGP first check to see if a prefix is accessible? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
To see if a prefix is accessible, 1st check BGP does is whether Next Hop Router is reachable. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
80
What are the two methods for reducing the number of IBGP connection in a network? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
2 methods of reducing number of iBGP connections are – Route Reflectors BGP Confederation ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
81
What makes a neighbor internal BGP (iBGP)? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
When two BGP-enabled devices are in the same autonomous system (AS), the BGP session is called an internal BGP session, or IBGP session ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
82
What the command "neighbor update-source" do? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
BGP Command “neighbor update-source” Configures the router to force BGP to use the IP address of the loopback interface when talking to a neighbor. Syntax: [no] neighbor {ip-address | peer-group-name} update-source loopback identifier ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
83
Explain the term RIB in BGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 31). Kindle Edition.
In order for BGP to be able to perform its functions it stores this information is a special type of database called the BGP Routing Information Base (RIB). For BGP RIB to learn BGP routes through its neighbors, store the best BGP routes and then advertise the best routes to neighbors/ peers, there is need for multiple BGP tables. BGP Routing Information Base consists of three parts as explained below – Adj-RIBs-In – This BGP information refers to routing prefix received from neighbor without applying any filtering or attribute manipulation. Attribute modifications or route filtering is applied after Adj-RIB-In. Loc-RIB – BGP maintains its own master routing table called the Local Routing Information Base (Loc-RIB). Best routes is selected after applying routing policies on the routes available in Adj-RIBs-In. Whenever an Adj-RIB-In changes, the main BGP process decides if any of the neighbour’s new routes are preferred to routes already in the Loc-RIB and it replaces as required. Adj-RIBs-Out – This table refers to Routes/ NLRI selected from Loc-RIB after applying outbound routing/ filtering policies. This table stores the routing information that was selected by the local BGP router and the advertised to its peers through BGP update messages. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 31-32). Kindle Edition.
84
Two BGP peers connected through a routed firewall are unable to establish a peering relationship. What could be the most likely cause? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 32). Kindle Edition.
Primary reason is that EBGP multihop is not configured between the 2 BGP peers. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 32). Kindle Edition.
85
What is the order of preference of attributes (route-map, filter-list, prefix-list, distribute-list ) when some or all are applied to one neighbor in BGP? Kindly share for inbound updates ? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 32). Kindle Edition.
This is the order of operation: Route map Filter list IP prefix list Distribute list ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 32). Kindle Edition.
86
What is the order of preference of attributes (route-map, filter-list ,prefix-list, distribute-list ) when some or all are applied to one neighbor in BGP? Kindly share for outbound updates? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
This is the order of operation: Distribute list IP prefix list Filter list Route map ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
87
How can I verify if a BGP router announces its BGP networks and propagates them? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
Use below commands in order to check if the IPprefix are announced to the directly connected BGP neighbor - “show ip bgp neighbors [address] advertised-routes” command shows which messages are being sent. “show ip bgp neighbors [address] routes” command shows which messages are being received. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
88
Does the route reflector change the next hop attribute of a reflected prefix? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
By default, the next hop attribute is not changed when a prefix is reflected by route reflector. However, you can issue the neighbor next-hop-self command in order to change the attribute of the next hop for prefixes reflected from an eBGP peer to any route reflector client. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
89
How much of minimal RAM is required to learn complete BGP routing table from one BGP peer? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
Cisco typically recommends a minimum of 512 MB of RAM in the router to store a complete global BGP routing table from one BGP peer. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
90
What are the benefits of configuring BGP peer groups? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 33). Kindle Edition.
Below are the key benefits which can be reaped out from BGP peer groups – Reduction of the resource of BGP devices when it makes the updates to the BGP neighbors. Reduction in amount of configuration which is requires to be done on BGP enabled device. Configuration becomes simple and easy ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 33-34). Kindle Edition.
91
Is it possible to change BGP ConnectRetry timer? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
Presently, the Cisco IOS ConnectRetry timer cannot be changed from its default of 120 seconds. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
92
What is the version of BGP that first supported CIDR? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
In the BGP the current versions i.e. BGP-4 version supports CIDR. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
93
In the global routing table, can the same AS number show up more than once in a path. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
Yes, this is pretty much possible. This happens because some AS inject their AS number in the path more than once. However, when AS Path Prepending is not used across AS, every AS number will only show up once, since same AS can’t because BGP doesn't allow looping AS paths. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
94
Can I run two BGP process on single router? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
No, we can't run multiple BGP processes on the single router. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
95
What is the cost of external and internal BGP routes? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
eBGP (External) = 20 iBGP (Internal) = 200 ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
96
Which parameters and attributes have to be equal before MED is compared to select the best path? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
attributes need to be equal before MED is compared - WEIGHT LOCAL_PREF AS_PATH Origin ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
97
What is the purpose of route dampening? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 34). Kindle Edition.
Route Dampening is a way to suppress flapping routes so that they are "suppressed" instead of being advertised. An unstable network can cause BGP routes to flap, which can cause other BGP routers in the network to constantly reconverge. This wastes valuable CPU cycles and can cause severe problems in the network. Henceforth, ISPs use route dampening to mitigate these issues. Some of terms related to Route dampening include - Penalty - Half life time Suppress limit Reuse limit Max suppress limit Suppressed route History entry ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (pp. 34-35). Kindle Edition.
98
In general which routes will affect by route dampening? eBGP or iBGP? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 35). Kindle Edition.
The main functionality is to stop the rippling effect of route updates or withdraws. It only applies to routes learn with eBGP. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 35). Kindle Edition.
99
What is the default value of MED? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 35). Kindle Edition.
Default value of MED is 0 ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 35). Kindle Edition.
100
How do I debug routes for a particular vrf in the Cisco IOS-XR environment? ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 35). Kindle Edition.
Use the debug bgp keepalive [vrf [vrf-name | all]] vpnv4 unicast command in order to debug routes for a given vrf in the Cisco IOS-XR environment. ipwithease, ipwithease. BGP Top 100 Interview Questions and Answers (p. 35). Kindle Edition.

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