Information Technology Reference
In-Depth Information
When a BGP session is torn down, the BGP paths received from that peer are removed from
the Adj-RIB-In, and the path-selection process is run. When the selection process is com-
plete, the BGP speaker updates its peers with the new BGP reachability information in the
form of withdrawals and advertisements. This process continues hop by hop until all BGP
speakers have been updated in all BGP autonomous systems that are affected by the failure.
Consider Figure 5-8, in which 10.2.0.0/16 is advertised from AS 65102 to AS 65103 and
AS 65101. When the link between R6 and R8 fails, the eBGP session between AS 65102
and AS 65103 is torn down, and 10.2.0.0/16 is removed from the BGP RIB on R8.
Figure 5-8
Path Updates on Link Failure Between R8 and R6
EIGRP 102
BGP 65102
R4
R6
10.2.0.0/16
10.2.0.0/16
R8
EIGRP 101
BGP 65101
EIGRP 103
BGP 65103
R3
R9
R11
EIGRP 104
BGP 65104
A withdrawal message is sent to R9, which in turn sends a withdrawal for 10.2.0.0/16 to R3
in AS 65101 and R11 in AS 65104. The best path for 10.2.0.0/16 on R9 is now from R3,
because its AS_PATH is shorter than the path from R11. The new path is advertised to R8
via iBGP and to R11 via eBGP.
The section, “External BGP Core Architecture,” examined the potential for suboptimal
routing through the use of a route map setting local preference. If this same route map were
in place, to reach 10.2.0.0/16, the traffic would traverse R9 to reach R3 and then R4, which
in the failure scenario might not be optimal. It is important to keep in mind what additional
effects might be seen in failure scenarios.
The as-path list used in Example 5-12 matches all prefixes that are originated by AS 65102.
If connectivity between AS 65102 and AS 65103 becomes severed, AS 65104 still prefers
AS 65103 for reachability to AS 65102, as shown in Example 5-13. This undesirable traffic
pattern can be resolved by making the as-path list more specific, in that it modifies only the
