iBGP neighborship using Loopback

[Karan Belani]

Hello, 
I want to know how iBGP manipulates OSPF routes to R2 loopback to go via R4 when the link between R1 and R2 goes down.
Here I am establishing iBGP neighborship using loopbacks. I have shut down the fa0/0 interface of R1.

 

 

 

[M02@rt37]

Hello @Karan Belani,

I perhaps not understand your need so sorry.

You seem to have configured OSPF to route Routers' loopback and you configure iBGP based on these loopback.

Then when the link between R1 and R2 goes down, R1 loopback is known via R4. IBGP does not manipulate OSPF routes.

[Karan Belani]

Hello, 
I have established neighborship between all routers using their loopback and for the connectivity to loopback, I have used OSPF. 
Now when I shut down the link between R1 and R2 the routes to R2 loopback from R1 via R4. I want to know how did this happen?

[M02@rt37 · Accepted solution]

@Karan Belani 

Thanks to your ospf, acting as an underlay, you still get path to R2's loopback via R4. Then iBGP session beween R1 and R2, based on loopback routed via ospf, is always established.

[Karan Belani]

Hello M02@rt37 , 
Thanks, I understood it now, because of OSPF the neighborship is UP.

[M02@rt37]

You're right @Karan Belani 

[MHM Cisco World]

the BGP dont care about the path IGP use direct or via R4

[Karan Belani]

Hello, 
So here even if I didn't use BGP, I would still get path to R2's loopback via R4 because of OSPF?

[MHM Cisco World]

Yes' correct

Here you can configure in each router two LO' one advertise via ospf and use as update source of bgp 

Other LO advertise via bgp.

[Karan Belani]

Hello, 
Thank you for helping.

[Giuseppe Larosa]

Hello @Karan Belani ,

iBGP sessions are not limited to TTL =1 like a standard eBGP session does. By using iBGP sessions on loopbacks the iBGP session is able to survive the shutdown of R1 LAN port by using OSPF to reroute.

You can see iBGP like an application using OSPF IGP routing services.

Hope to help

Giuseppe

[Karan Belani]

Hello, 
Thank you for explaining, so here BGP neighborship is maintained with the help of OSPF reroute am I right?

[Giuseppe Larosa]

Hello @Karan Belani ,

>> so here BGP neighborship is maintained with the help of OSPF reroute am I right?

Yes you are right

Hope to help

Giuseppe

[Karan Belani]

Hello, 
Thank you, I understood it now, I was so focused on BGP that I forgot about OSPF.