Route is not advertised to iBGP Peer

[isatis1231]

Hello,

I made a lab with GNS and there is something that I don't understand.

I would like to know why router R3 doesn't advertise network 2.2.2.2 to its iBGP peer R4.

Same question for R4. R4 doesn't advertise network 1.1.1.1 to R3. Is it normal and why?

Ios Version: 3600 Software (C3640-IK9O3S-M), Version 12.4(25c), RELEASE SOFTWARE (fc2)

Thank you for your help!

[Joseph Nelson · Accepted solution]

The reason R3 doesn't tell R4 about the 2.2.2.2/32 network is because R3 choose this path through R4 as the best path. Looking at the BGP table on R3, you see that .3.2 (i.e. R4) is chosen as the best path because it has a shorter AS_PATH value. When R3 picks the R4 path, it will not turn around and tell R4 about that that it knows a path to 2.2.2.2---this would cause a forwarding loop. This is bgp-split horizon operation.

If you go to R3 and increase the Local Preference ( > 100 ) for the R1 peer, you would see that R4 would see R3 as the best path for 2.2.2.2/32. 

Try it out and let me know...

Joe

** Rate if Correct**

[isatis1231]

Hello Joe,

Perfect, I understand now.

Thank you for your help!