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Saturday, May 1, 2010

Question 71

How can you determine the BGP AS of a backbone router?

In this case we do not know the AS # for R2 (Backbone). The backbone is configured to neighbor with R1. On R1 configure the neighbor using any AS#

Example on R1:
router bgp 100
 neighbor 10.0.0.2 remote-as 300

The following message appears:

R1#
*Apr 23 13:23:41.335: %BGP-3-NOTIFICATION: sent to neighbor 10.0.0.2 passive 2/2 (peer in wrong AS) 2 bytes 0190

Convert the hexadecimal value of 0190 to decimal which equals 400. This is the AS number of the backbone router. Modify the neighbor statement with AS#400


router bgp 100
 neighbor 10.0.0.2 remote-as 400

*Apr 23 13:25:47.635: %BGP-5-ADJCHANGE: neighbor 10.0.0.2 Up 

R1(config-router)#do sh ip bgp summary
BGP router identifier 10.0.0.1, local AS number 100
BGP table version is 3, main routing table version 3
2 network entries using 240 bytes of memory
3 path entries using 156 bytes of memory
2/2 BGP path/bestpath attribute entries using 248 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 668 total bytes of memory
BGP activity 2/0 prefixes, 3/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.0.0.2        4          400       6       6        3    0    0 00:01:13        2


R1(config-router)#do sh ip bgp        
BGP table version is 3, local router ID is 10.0.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  10.0.0.0/24      10.0.0.2                 0             0 400 i
*>                  0.0.0.0                  0         32768 i
*> 192.168.0.0      10.0.0.2                 0             0 400 i

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