In the below scenario (GNS3), IBGP peering enabled between R1-R2, R1-R3, R2-R3 and EBGP peering enabled between R2-R4,R3-R5,R4-R6,R5-R7. OSPF enabled as IGP. Scenario attached for reference.
The problem I've observed in R1 is not getting entire BGP routing table for destinations 30.x.x.x/40.x.x.x.
I'm able to see only best routes in R1 BGP routing table, but alternate valid routes are not visible in its topology table.
R1#sh ip bgp BGP table version is 81, local router ID is 100.100.2.1 *>i188.8.131.52/24 10.10.1.2 0 100 0 200 300 ? *>i184.108.40.206/24 10.10.1.2 0 100 0 200 300 ? *>i220.127.116.11/24 10.10.2.2 0 100 0 200 400 i *>i18.104.22.168/24 10.10.2.2 0 100 0 200 400 i *> 100.100.1.0/24 0.0.0.0 0 32768 i *> 100.100.2.0/24 0.0.0.0 0 32768 i
More confusing part to me is when I disable IBGP peering between R2-R3 or shutdown interface between R2-R3 or else if I disable ospf in R1,R2 & R3 routers , I'm able to see both best route and alternate valid route in BGP topology table.
When you have the R2-R3 link enabled, 40.x.x.x prefixes will be preferred by R3 and 30.x.x.x will be preferred by R2 due to AS_PATH. So R2 will install 40.x.x.x prefixes pointing to R3 and vice versa to 30.x.x.x routes.
I would need to check it to make sure, but let's take 40.x.x.x routes as an example, R2 would install it pointing to R3, it would send it to R1, but although it is the originator, the NH would still be R3 as it is an iBGP. That's what I think it is happening, what you can do is to enable R2-R3 link and check the advertised routes to R1 "show ip bgp neighbor 10.10.1.1 advertised-routes" You can also enable in R1 the command "neighbor A.B.C.D soft-reconfiguration inbound" to R2 and then check the received routes with "show ip bgp neighbor x.x.x.x received-routes"
Let me know if you can confirm that on your lab as I just changed my mac and I'm too lazy to install gns3 :-)
If it does not answer your question I'll go ahead and install it.
BGP only advertises the preferred network, not all the learned ones, so R3 learns 30.30.X.X network through R2 and R5, it chooses R2 because of a shorter AS_PATH, there's no reason to advertise the route back to it's originator, now, if you go to R2 and shut the neighbor to R4, R3 will start advertising the prefix it learned from R5.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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