BGP routing table not showing all paths

[Tauer Drumond]

I have a router connected and peering EBGP with 2 differents ISPs....

I receive exactly the same prefixes from both of them, but, BGP routing table only shows one path through ISP 1

When I shutdown ISP 1, BGP routing table shows the path through neighbor 2...

Why am I not seeing two paths when both ISPs are UP?

[julian.bendix · Accepted solution]

Hi @Jon Marshall !

Sorry to hook here, but if I may ask:

I thought the ISP2 PE Router will learn the Routes announced by the CE Router,
then the ISP2 PE Router will (in this scenario) decide that the best route is through the CE Router.
Then ISP2 PE Route will announce that again back to CE Router, but CE Router will never ever learn the Routes due to the AS_PATH Loop Prevention.

So far I was not aware that we have a Split Horizon behavior in eBGP which will prevent announcing the route I learnt from a given neighbor back to the same neighbor.
Do you have any official documentation about this perhaps?

Thanks a lot and best regards!
Julian

[Jon Marshall]

Julian 

No need to apologise especially when you are right :) 

If indeed ISP2 sees this as the best path and it should do based on AS path length it will advertise back to the CE but will be dropped as you say. 

Thanks for correcting. 

Jon

[Tauer Drumond]

I found a way to achieve the needs... 

I just issued "allowas-in" command on neighbor 172.19.254.165, then now I receive the prefix back to me and bgp routing table is populated with 2 paths to the same prefix.

Thank you all for the help

Best Regards

[julian.bendix]

Hi @Jon Marshall ,

thanks for clearing that up, I wasn't sure anymore now :)

Have a nice day!
Julian