Why BGP need IGP?

[ricaela]

Hi Guys, just wanted to know why BGP need IGP like ospf , eigrp etc. ?

also, if BGP already have a lot of attributes in selecting best path, why is it that i still see (IGP like) ospf cost or isis cost used together with BGP? (see attached picture for reference).

kindly explain to me.. thank you!

[marce1000]

 - FYI : https://community.cisco.com/t5/routing/why-bgp-need-an-igp/td-p/2350946

 M.

[MHM Cisco World]

if two peer inside same AS is tie then the tie breaker is IGP cost.

[pman · Accepted solution]

Hi,

According to list provides the rules that are used to determine BGP best path:
rule number 7 says this:
Prefer eBGP over iBGP paths.

This means that if a router sees the network from 2 routers in the same ASN (iBGP) then it will continue according to the list and examine rule number 8 which says this:
Prefer the path with the lowest IGP metric to the BGP next hop.

Example:

 

When using a single routing protocol, BGP selects the closer AS exit point. BGP table on PE1 has two (almost) identical entries, the only difference being the IGP metric toward the BGP next hop. The path with the lower IGP metric is selected.

And how you actually see it happening

BGP paths for IP prefix 172.16.0.0/16

BGP routing table entry for 172.16.0.0/16, version 4
Paths: (2 available, best #2, table default)
  Not advertised to any peer
  Refresh Epoch 1
  Local
    10.0.0.2 (metric 101) from 10.0.0.2 (10.0.0.2)
      Origin IGP, metric 0, localpref 100, valid, internal
      rx pathid: 0, tx pathid: 0
  Refresh Epoch 1
  Local
    10.0.0.1 (metric 2) from 10.0.0.1 (10.0.0.1)
      Origin IGP, metric 0, localpref 100, valid, internal,best
      rx pathid: 0, tx pathid: 0x0

https://blog.ipspace.net/2021/01/bgp-igp-metric-admin-distance.html