From: Arun Arumuganainar (aarumuga@hotmail.com)
Date: Tue May 16 2006 - 11:38:45 ART
I have a interesting BGP route selection problem . It would be great if any
body can explain me this .
Setup description :-
~~~~~~~~~~~~~~
1) I have BGP Synchronisation enabled and I am redistributing my BGP routes
in to IGP . Here the IGP is RIP .
2) I learn a same prefix learned via two sources as follows
a) First route via iBGP with Local-Pref set to 500
b) Second one via eBGP with Local pref set to 300
Actually I see route A is slected by the virtue of superior local pref .This
is as expected.
****************************************
Router Traces
****************************************
R4#sh ip bgp
BGP table version is 7, local router ID is 172.16.104.1
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
r 172.16.107.0/24 172.16.124.3 300 0 100 700 ?
r>i 172.16.103.1 0 500 0 100 700 ?
****************************************
Pls Note : Routes are tagged as 'r' as the SYNC is enabled and routes are
also learned via IGP .
Step to reproduce the problem :-
I shut doen the iBGP peer using neighbor shut down command and now the eBGP
routes are getting comfortably installed . as given in the router trace .
****************************************
Router Traces
*************************************
R4(config-router)#neighbor 172.16.103.1 shutdown
R4(config-router)#do sh ip bgp
BGP table version is 9, local router ID is 172.16.104.1
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
*> 172.16.107.0/24 172.16.124.3 300 0 100 700 ?
****************************************
Now I bring up the BGP Peer again . Now my expectation would be iBGP route
should get re-installed right ??? But to my surprise it is not getting
installed again.
****************************************
Router Traces
*************************************
R4(config-router)#no neighbor 172.16.103.1 shutdown
R4(config-router)#
*May 16 14:16:40.449: %BGP-5-ADJCHANGE: neighbor 172.16.103.1 Up
R4(config-router)#
R4(config-router)#do sh ip bgp
BGP table version is 9, local router ID is 172.16.104.1
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
* i172.16.107.0/24 172.16.103.1 0 500 0 100 700 ?
*> 172.16.124.3 300 0 100 700 ?
****************************************
To get back to old route I need to reset the peer .
Just wondering if any body has seen this behaviour.
Thanks and Regards
Arun
This archive was generated by hypermail 2.1.4 : Thu Jun 01 2006 - 06:33:21 ART