path selection between ibgp and ebgp

[networkinblood]

if same prefix learning via ibgp and ebgp ...will it follow bgp path selection algorithm or i t ll select b est path by AD value directly

[Harold Ritter · Accepted solution]

Hi @networkinblood ,

It will follow the BGP best path algorithm. The AD is assigned to the path only after it is selected by BGP and only serves to prefer which routes from different sources (static, bgp, ospf, isis, rip, connected, etc) will be installed in the RIB.

Regards,

[networkinblood]

Thnks for revert Sir ...I ll come back to you with much more doubts ..

[balaji.bandi]

As per the BGP path selection algorithm, higher local preference IBGP route will be preferred over lower local preference EBGP route.

Check some path selection documents :

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/8651-21.html

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

[networkinblood]

Hi Sir , thank for revert  but I m not confused with bgp path selection algorithm...I was confused whether it ll directly go for Ad value or it ll follow path selection algorithm 

[balaji.bandi]

The BGP decision process is applied to the routes contained in the Routing Information Base, Incoming (RIB-In) which contains routes learned from inbound update messages. The output of the decision process is the set of routes that will be advertised to BGP speakers in local or remote autonomous systems and are stored in the Adjacency RIB, Outgoing (RIB-Out). 

[networkinblood]

Sir still u hvnot got my q , I m aware how bgp best path get elected . I m
asking if prefix is already present in RIB table which was learned via IBGP
and new update packet of bgp comes for same NRLI which is EBGP now , so
will it select best path by following path algorithm or route get selected
in bgp table on the basis of AD value ( 20 vs 200 ).

[balaji.bandi]

If a router learns about a route to a destination from more than one routing protocol, and they all have the same prefix length, AD is compared. The preference is given to the route with the lower AD.