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CCNP-ENCOR > BGP > Flashcards

BGP Flashcards

1
Q

What is BGP

A

BGP is a dynamic routing protocol that exchanges routes based on a variety of attributes. It is an EGP. Its purpose is to route different AS’s together.

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2
Q

Why does BGP not advertise incremental updates or refresh network advertisements

A

BGP prefers stability and was designed for neighbours not to change very often. if there was a link flap it would cause recomputation of thousands of routes which would be chaotic.

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3
Q

What is a BGP AS

A

The autonomous system (AS) within BGP is a collection of network devices under the same network administration. there are 4,294,967,295 ASNs are available.

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4
Q

What is a private BGP AS and what are the range

A

A private BGP AS is an AS that is not routable on the internet, similar to private addresses.

64,512 - 65,535
4,200,000,000 - 4,294,967,294

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5
Q

Who provides the AS to a company and what should you do if you can’t get one

A

IANA provides the AS, if they can’t provide one use the same one as your ISP.

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6
Q

What would happen if you use another companies AS on the internet

A

It would cause packet loss and cause chaos on the internet.

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7
Q

What is a BGP path attribute (PA)

A

BGP PA’s are associated with each route path, they allow BGP with more control and granularity.

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8
Q

What are the different types of BGP PA’s

A

Well known mandatory
Well known discretionary
Optional transitive
Optional Non-transitive

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9
Q

What is: well known mandatory PA

A

a PA that is recognised by all BGP implementations and is required for BGP to work.

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10
Q

What is: well known discretionary PA

A

A PA that may or may not be included in BGP process.

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11
Q

What is a optional PA and what is the difference between transitive and non-transitive

A

Optional PA’s are optional in the BGP implementation.

Transitive PA’s stay with the NLRI from AS to AS. Non-transitive are dropped between AS’s.

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12
Q

What is NLRI

A

Network Layer Reachability Information: Is the BGP routing update that specifies:

Network prefix
prefix length
Any PA’s

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13
Q

How does BGP prevent loops

A

BGP uses a well known mandatory PA called PA_Path this records all AS numbers the route passes through.

When a BGP router receives a NLRI routing update with their own AS number already listed within the PA_Path the router knows that this is in a loop and therefore drops the packet.

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14
Q

What is multi-protocol BGP

A

MP BGP adds granularity to the NLRI to define an address family and a sub address family.

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15
Q

What is an AF and SAF in BGP

A

AFI: address family is IPv4/IPv6

SAFI: sub AF unicast or multicast traffic

Each AF + SAF has its own configuration and respective database.

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16
Q

BGP uses hello packets to discover neighbours dynamically.

True or False

A

False - BGP can not discover neighbours dynamically it uses IP addresses to define neighbours.

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17
Q

What port does BGP use and does it use TCP/UDP mechanism

A

TCP port 179.

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18
Q

BGP neighbours connected to the same network use ARP tables to ID IP addresses.

True or False

A

True.

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19
Q

What is multi-hop BGP

A

Forming a neighbour relationship with a BGP router more than 1 hop away.

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20
Q

What is the requirement for multi-hop BGP to work

A

There must be a route installed in the RIB to the remote BGP router.

Either statically or dynamically.

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21
Q

What are the 2 BGP session types

A

iBGP - internal BGP neighbours established within the same AS (AD200)

eBGP - External BGP, neighbours established within different AS (AD20)

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22
Q

What is the use for iBGP

A

When transit connectivity is required within an organisation between ISPs etc.. also with multiple routing policies are used.

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23
Q

What is transit connectivity

A

When all BGP routers in the AS allow inbound traffic on the one side of the AS and output on the other side of the network.

Best case is full neighbour relationships making it a full-mesh.

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24
Q

What is eBGP used for

A

Connectivity between AS’s. Most commonly used to exchange routes on the internet.

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25
Q

What is the difference in TTL between iBGP and eBGP

A

iBGP - TTL default is 255 (to allow multi hop)

eBGP - TTL is 1 by default

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26
Q

How many message types does BGP use to communicate and what are they

A
  1. OPEN
  2. UPDATE
  3. NOTIFICATION
  4. KEEPALIVE
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27
Q

What is the purpose of each BGP message type

A

OPEN - Establishes neighbour adjacencies.

UPDATE - Advertises, updates and withdraws routes from RIB.

NOTIFICATION - indicates an error to BGP neighbour.

KEEPALIVE - used to maintain neighbour adjacency. default every 60 seconds.

28
Q

What is the Hold timer used for

A

BGP uses the hold timer to maintain the adjacencies if the timer reaches zero the route is removed. default value 180 seconds.

resets from receiving a KEEPALIVE or NOTIFICATION.

29
Q

what is the BGP ID

A

It is used as a identifier in BGP process it is either assigned statically or dynamically (same process as OSPF).

30
Q

BGP uses a TCP session to form a relationship with a peer.

What are the states of the session.

A

Idle - Listening for BGP sessions to initiate.

Connect - 3 way hand shakes is initiating, peer with higher IP manages session. if success - moved to open sent.

Active - if the connect retry timer reaches zero before handshake complete states moves to active then idle to restart.

Open Sent - Open message has been sent and waiting for Open message from peer.

Open Confirm - on receipt of a KEEPALIVE or NOTFICATION. KEEPALIVE > Established if NOTIFICATION > Idle.

Established - BGP TCP session established, routes exchanged via UPDATE message.

31
Q

How to configure basic BGP

A
  1. initialise BGP process
    router bgp 65000
  2. Under bgp process form neighbours
    neighbour 10.1.1.1 remote-as 65100
  3. Activate AF (IPv4 unicast is up by default)
    address family IPv4 neighbour 10.1.1.1 activate
  4. advertise routes to BGP process under AF
    network 192.168.1.0 mask 255.255.255.0
32
Q

What are the different BGP tables

A

Loc-RIB table- contains all NLRI following validity check

Adj-RIB-IN - contains NLRI before it is processed by routing polices

Adj-RIB-OUT- contains all NLRI after it has been processed by routing polices.

33
Q

What command is used to view BGP table and BGP routes

A

show bgp [afi] [safi]

show ip bgp route

34
Q

Which routes from the Loc-RIB are installed into the global RIB

A

the best paths.

35
Q

What is the warning with redistributing routes in BGP

A

Be careful when redistributing internet BGP routes into IGP as there is millions of routes and this could cause chaos and damage to resources.

36
Q

What is the benefit of summarising routes in BGP

A

Conserves resources and improves path calculation as the size of the routing tables are significantly smaller with summarisation.

37
Q

What is the 2 ways to configure BGP summarisation

A

Static - static route to null0 interface and summary prefix advertised.

dynamic - use a aggregate network prefix that summarises the networks.

38
Q

What is the command and optional PAs for configuring BGP summarisation

A

under the AF:
aggregate-address 10.10.10.0 mask 255.255.254.0 [summary-only][as-set]

39
Q

What does the summary-only PA do

A

suppresses the prefixes within the summary prefix from been shown in the routing table.

The prefixes are still available to view in the BGP table (Loc-RIB).

40
Q

What does the AS_SET PA do

A

Used to keep the AS path history for the aggregate history

41
Q

IPv6 BGP configuration is the same as IPv4, with the exception of a different address family

True or False.

A

True

42
Q

What is BGP multihoming

A

It is a method of providing resilience in BGP sessions.

43
Q

What is the most resilient method of bgp multihoming

A

2 BGP router connected via iBGP, which are connected to 2 separate SP routers via eBGP

44
Q

What is internet transit routing

A

When an enterprise connects to 2 different service providers it runs the risk of there AS becoming a transit connecting for internet traffic.

45
Q

How do you avoid internet transit routing

A

Apply outbound BGP route policy that only allows for local BGP routes to be advertised to other AS’s.

46
Q

What is symmetric and asymmetric traffic flow?

A

symmetric: traffic flows the same path in both directions

asymmetric: traffic flows in different routes on the out and in directions.

47
Q

What is branch transit routing and what is the issue with it

A

When a link failure occurs and instead of the traffic flowing from router to MPLS SP router the traffic traverses another MPLS branch router then to the MPLS SP.

this causes extra usage on the branch router.

48
Q

What is conditional matching in BGP

A

Is a method of matching network prefixes

49
Q

ACLs react differently when matching against IGP compared to BGP.

True or False

A

True.

50
Q

What is prefix matching

A

It allows the selection of multiple networks with a variety of prefix lengths that can be specified.

51
Q

192.168.0.0/16 ge 24

the following prefix’s do or don’t match:

  1. 192.168.0.0/16
  2. 192.168.7.0/24
  3. 192.168.4.0/26
A
  1. No match
  2. Match
  3. Match
52
Q

in prefix matching what does (le) and (ge) refer to

A

less than equal to

greater than equal to

53
Q

what is a prefix list

A

Contains multiple prefix matching statements with either a permit or deny

54
Q

What is the sequence increment default value for a prefix list

A

5

55
Q

the logic for IPv6 prefix lists are the same as Ipv4 prefix lists.

True or False

A

True

56
Q

What is regex commonly used for with regards to BGP

A

Parse through BGP tables

57
Q

What is the regex expression for:

Local originating routes
only routes from neighbour AS200

A

^$

permit ^200_

58
Q

What is a route map and what is the benefit of route maps over ACLs

A

Can filter networks similar to ACLs.

As well as that they can add/modify/remove network attributes to influence routing path decisions.

59
Q

What is the components of a route map

A

Sequence number - incremented by 10

Conditional matching criteria

permit or deny - default is permit

optional action - allows for addition/modification/removal of characteristics

60
Q

in a route map following a permit statement with multiple match statements what happens if both statements are not matched

A

it is not permitted as both need to match

61
Q

What are the 2 methods of clearing a BGP connection

A

Hard reset - tears down BGP session, this removes the BGP routes from the peer and is the most disruptive.

Soft reset - invalidates the BGP cache and requests a full advertisement from its BGP peer.

62
Q

What is the purpose of BGP communities

A

provides additional capability for tagging/modifying GP routing polices on upstream and downstream routers.

63
Q

BGP communities are optional non transitive PA

True or False

A

False BGP communities are an optional transitive PA

64
Q

What are the 3 well known BGP communities

A

Internet - standard community for identifying routes that should be advertised to the internet.

No advertise - routes should not be advertised to the internet.

No export - iBGP peers only.

65
Q

Are BGP communities advertised by default

A

No

66
Q

what does the additive keyword do in BGP community config

A

prevents overwriting of the BGP community

67
Q

After the longest prefix match and AD of the routing protocol what does BGP prefer for install a route.

A
  1. Weight
  2. Local preference
  3. Local originated
  4. AIGP
  5. Shortest AS_Path
  6. Origin type
  7. Lowest MED
  8. eBGP over iBGP
  9. Lowest IGP next hop address
  10. if both eBGP use oldest neighbour
  11. Route that comes from BGP peer with lowest RID
  12. minimum cluster list length
  13. neighbour with lowest address.

CCNP-ENCOR (17 decks)

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