Information Technology Reference
In-Depth Information
Example 5-3 shows the BGP and routing table for 10.2.0.0/16 before the link failure.
Network State Before the Failure
Example 5-3
R11#show ip bgp 10.2.0.0
BGP routing table entry for 10.2.0.0/16, version 3
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Not advertised to any peer
Local
172.16.4.1 (metric 2323456) from 172.16.4.1 (172.16.4.1)
Origin IGP, metric 307200, localpref 100, valid, internal
Local
172.16.6.1 (metric 2297856) from 172.16.6.1 (172.16.6.1)
Origin IGP, metric 307200, localpref 100, valid, internal, best
2323456
2297856
best
After the failure, the IGP metric in the BGP table has increased as a result of the IGP next-
hop change, as shown in Example 5-4.
Network State After the Failure
Example 5-4
R11#show ip bgp 10.2.0.0
BGP routing table entry for 10.2.0.0/16, version 3
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Not advertised to any peer
Local
172.16.4.1 (metric 2861056) from 172.16.4.1 (172.16.4.1)
Origin IGP, metric 307200, localpref 100, valid, internal
Local
172.16.6.1 (metric 2835456) from 172.16.6.1 (172.16.6.1)
Origin IGP, metric 307200, localpref 100, valid, internal, best
2861056
2835456
best
Because the iBGP peer still can be reached, the iBGP session is still intact. Also, the best
path is still from R6, because the new metric to R6 is still less than that to R4, as shown in
Example 5-4. However, if the link between R6 and R8 also fails (two simultaneous fail-
ures), the BGP best path is affected, because now R11 needs to reach R6 via R4, making
R4 the preferred BGP next hop for 10.2.0.0/16. This is shown in Example 5-5.
BGP Path Information with Multiple Failures
Example 5-5
R11#show ip bgp 10.2.0.0
BGP routing table entry for 10.2.0.0/16, version 7
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x240
Not advertised to any peer
Local
172.16.6.1 (metric 3347456) from 172.16.6.1 (172.16.6.1)
Origin IGP, metric 307200, localpref 100, valid, internal
Local
172.16.4.1 (metric 3321856) from 172.16.4.1 (172.16.4.1)
Origin IGP, metric 307200, localpref 100, valid, internal, best
3347456
3321856
best
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