eBGP or iBGP - Route Selection?

[snarayanaraju]

Hello Everybody - This is about the Route Selection logic used in Cisco Routers.  (https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html)

Scenario: A router is connected to two Links. The prefix 69.32.1.0/24 is learned using iBGP (AD:200) and eBGP (AD:20) from those Links. Which Route will be populated in the RIB in this case? The Route learned from iBGP peering or eBGP peering?

regards,Sairam

[Jaderson Pessoa]

Hello,

routes learned from EBGP are prefereble.

look more here: https://books.google.com.br/books?id=OfA8BQAAQBAJ&pg=PA30&dq=ccie+bgp+origin&hl=pt-BR&sa=X&ved=0ahUKEwjypNSXtLDmAhUXJrkGHSvzDroQ6wEILTAA#v=onepage&q=ccie%20bgp%20origin&f=false

[Jon Marshall]

All other things beings equal EBGP routes are preferred over IBGP routes. 

Jon 

[snarayanaraju]

Hello Everybody - Thanks for sharing your thoughts.

I did this in a quick lab. This is my finding. Let us discuss

 iBGP is taking the preference over eBGP. I am receiving 69.32.1.0/24 from both iBGP and eBGP and iBGP learned path get into Routing table. 

By the output, I understood iBGP is preferred because of lowest AS-PATH when compared to eBGP (which adds the ASN in the path attribute). Am i right?

R4#show ip bgp

Network Next Hop Metric LocPrf Weight Path

* 69.32.1.0/24    10.1.4.1    409600   0   20 ?
*>i               10.1.2.2    409600   100 0 ?

R4#sho ip bgp 69.32.1.0/24
BGP routing table entry for 69.32.1.0/24, version 4
Paths: (2 available, best #2, table Default-IP-Routing-Table)
20
10.1.4.1 from 10.1.4.1 (199.199.199.1)
Origin incomplete, metric 409600, localpref 100, valid, external
Local
10.1.2.2 from 10.1.2.2 (200.200.200.2)
Origin incomplete, metric 409600, localpref 100, valid, internal, best
R4#

[Jon Marshall · Accepted solution]

Yes, it is choosing the IBGP route because the AS path is shorter and that rule comes before the EBGP vs IBGP rule in the BGP best path algorithm. 

Jon

[rais]

Usually, the RIB should contain both the routes. Which route gets installed in the routing-table depends upon multiple attributes.

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

HTH.