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BGP passes different default route IP other than neighbor's IP





Sample Diagram here.

The dual ISPs network that I am working has an interesting setup.

In Router 1, here is a partial config

ip route

partial show ip route

Gateway of last resort is to network
B* [200/0] via, 1w5d

Questions, what is the purpose of this? I am trying to duplicate this in lab for testing, how do you pass different default route IP from "ISP1 Router1" and "ISP1 Routers" to neighbor Router1?

Could you provide a sample config?


2 Replies 2

Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Hello @rhap4boyz ,

>> B* [200/0] via, 1w5d

it looks like that you have iBGP session between router1 and ISP1 Router1 and this would be not correct. Using iBGP makes the BGP next hop to be propagated unchanged and this is why you need the static route

ip route

otherwise the BGP prefix with next-hop = cannot be installed in your Router 1  ( BGP next-hop unknown no route for it in the IP routing table)

Or the commands are taken from ISP1 Router1 ?

In this second case all you need is an eBGP session between Router1 and ISP1 Router1 this will provide for BGP next-hop rewrite to so that Router1 can accept the default route .

To be noted in iBGP the BGP next-hop is left unchanged and the AS path attribute is also unchanged.

In eBGP the BGP next-hop is rewritten to the address used for the eBGP session and ISP1 AS number is added on the left of the AS path attribute.


Hope to help



MHM Cisco World
VIP Mentor VIP Mentor
VIP Mentor

the edge router AND if you have multi homed same ISP use iBGP between router as backup path, 
for example if the traffic point to edge router1 and edge router1 loss it connection to ISP, it can use iBGP to forward traffic to edge router2

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