BGP

Question 1

BGP ASN Ranges

test - test ack

Answer 1

• 16-bit integers btwn 1-65535
• 32-bit AS numbers created recently when 16-bit was full
• 1-64495 - Public ASNs
• 64496-64511 - reserved for documentation
• 64512-65535 - Private ASNs

Question 2

BGP Traits

(5 points)

Answer 2

1) uses TCP 179
2) incremental, triggered updates
3) slow to converge
4) exchange network reachability info, called path vectors
5) advertises path of ASNs and the networks at the end of that path

Question 3

BGP Data Structures

(Table Names)

Answer 3

1) BGP Neighbor Table
2) BGP Table
3) IP Routing Table

Question 4

BGP Administrative Distance

Answer 4

1) EBGP AD = 20
2) IBGP AD = 200

Question 5

BGP Message Types

Answer 5

1) Open
2) Keepalive
3) Update
4) Notification

Question 6

BGP Open Message Contents

Answer 6

1) Version number - highest common version is used; most use BGPv4 now
2) AS Number - local router’s ASN. if it’s not what the peer expects, the BGP session is ended
3) Hold Time - time to wait between the successive keepalive and update msg from the sender. smaller hold time is accepted
4) BGP Router ID - 32-bit field that ID’s the sender. Determined the same way as in OSPF
5) Optional Parameters - Type, Length, Value (TLV) encoded.

Question 7

BGP Update Message Contents

Answer 7

1) Withdrawn routes
2) Path Attributes
3) Network Layer Reachability Information (NLRI)

Question 8

BGP Keepalive Messages

Answer 8

• If the negotiated hold time is 0, then periodic keepalives aren’t sent
• consists only of a message header

Question 9

EBGP Neighbor Requirements

Answer 9

1) Different ASNs
2) Defined neighbors - a TCP session must be estabilished before starting BGP routing update exchanges
3) Reachability - by default, EBGP neighbors must be directly connected, and the IPs on that link must be reachable from each AS

Question 10

BGP Route Selection Process
(High level)

Answer 10

1) Highest weight attribute (local to router)
2) Highest local preference attribute (global with AS)
3) Route originated by the local router (next hop = 0.0.0.0)
4) Shortest AS Path (Least number of AS in AS-Path attribute)
5) Lowest Origin attribute (IGP < EGP < incomplete)
6) Lowest MED attribute (exchanged between AS’s)
7) EBGP path over an IBGP path
8) (IBGP route) the path through the closest IGP neighbor (best IGP metric)
9) (EBGP route) Oldest EBGP path (neighbor with the longest uptime)
10) Lowest neighbor BGP router ID
11) Lowest neighbor IP address (multiple paths to the same neighbor)

Question 11

BGP Path Attribute Types

Answer 11

• Well-known Mandatory
• Well-known Discretionary
• Optional Transitive
• Optional Non-Transitive

Question 12

BGP Well-Known Mandatory Path Attributes

Answer 12

• required to be present for every route in every update
• Origin
• AS-Path
• Next-Hop

Question 13

BGP Path Attribute - Origin

Answer 13

1) Well-Known Mandatory
2) set to “i” for IGP if injected with network command or via aggregation
3) set to “?” if injected with redistribution
4) “e” is deprecated

Question 14

BGP Path Attribute - AS-Path

Answer 14

1) Well-Known Mandatory
2) sequence of AS numbers through which the network is accessed
3) local AS number is prepended each time the route crosses an AS boundary
4) the AS that originally injected the route into BGP is found at the right-most end of the AS path
5) router chooses the path with the least number of AS hops
6) used to avoid loops - the route is ignored if the router sees its own AS in the AS Path

Question 15

BGP Path Attribute - Next-Hop

Answer 15

1) Well-Known Mandatory
2) indicates the IP address of the next-hop router
3) each router modifies the next-hop attribute as the route passes through the network

Question 16

BGP Well-Known Discretionary Path Attributes

Answer 16

• must be supported by device, but doesn’t have to be used
• Local preference
• Atomic aggregate

Question 17

BGP Path Attribute - Local Preference

Answer 17

1) Well-known Discretionary
2) used to achieve a consistent routing policy for exiting an AS
3) second most preferred criteria
4) higher value is preferred
5) used when two or more routers provide exit points
6) propagated throughout the AS, but stripped out of updates to EBGP sessions
7) default value of 100, which is applied to updates from external neighbors

Question 18

BGP Path Attribute - Atomic Aggregate

Answer 18

1) Well-known discretionary
2) attached to a route that is created as a result of route summarization; signals that info may have been lost when original entries were summarized

Question 19

BGP Optional Transitive Path Attributes

Answer 19

1) might not be recognized by the router, but are still propagated
2) if a router propagates an unknown transitive attribute, it sets an extra bit in the attribute to indicate at least one of the routers in the path didn’t understand
3) Aggregator
4) Community

Question 20

BGP Path Attribute - Aggregator

Answer 20

1) Optional Transitive
2) identifies the AS and the router within that AS that created a route summarization or aggregate

Question 21

BGP Path Attribute - Community

Answer 21

1) Optional Transitive
2) numerical value that can be used to tag certain routes and make decisions based on the tag

Question 22

BGP Optional Nontransitive Path Attributes

Answer 22

1) might not be recognized by the router, and only locally significant
2) Multi-Exit Discriminator

Question 23

BGP Path Attribute - Weight

Answer 23

1) first parameter that is analyzed in the path selection process
2) configured locally and not propagated
3) values range from 0 - 65535. defaults to 32768 if path originated locally, and other paths have 0 by default
4) higher weights are preferred
5) Cisco proprietary

Question 24

BGP Path Attribute - Multi-Exit Discriminator

Answer 24

1) Optional Non-Transitive
2) set to influence inbound traffic on multiple entry points from another AS
3) initially exchanged btwn EBGP peers, but the neighbor doesn’t propagate it to other AS’s
4) default is 0 and it resets when the update is passed to another AS
5) mostly considered as a tie-break but no guarantee it get used b/c it’s optional