Routing solution

[Arnaud.ravazzola]

Good afternoon,

First, sorry for my bad english. Next my problem, I try to configure BGP on cisco routers. All configurations files are in attachment.

I have 3 routers, please see in the atchment the schema of the network. I try to make communication between router 0 and 2. They are in the same Autonomous system but no link between them. The router in the AS 200 make the link between them. With eBGP the communication work between router 0 and 1 and between 1 and 2 but not between 0 and 2.

In full OSPF configuration its work and with 3 AS and BGP its works too. But i want router0 and 2 in the same AS and different with 1.

Have you got a solution ?

Thank you

Best Regards

Arnaud

[chrihussey · Accepted solution]

What you are trying to do won't work. By default BGP does not accept its own AS coming in from an EBGP peer.

You could possibly create a tunnel to connect the two routers via AS 200 and IBGP across them that way.

[Arnaud.ravazzola]

Thank you for you answer.

To understand well, I have design a new network just for understand. If the link between 10 and 7 is broken you can't speak between them without VPN ?

[Julio E. Moisa · Accepted solution]

Hi

As router 0 and 2 are an iBGP they need to be directly connected to work, now if R1 is the interconnection router, you can configure on R0 and R2 the command: 

neighbor <neighbor IP> allowas-in 

To enable the communication between these routers, the rule says the iBGP routers need to be interconnected (full mesh) in order to avoid loops, now in order to avoid a full mesh you can configure route-reflector or create a BGP confederation. Try the below command.

:-)

[Julio E. Moisa]

There is 2 ways to allow the same AS:

1) Usually used on ISP routers facing to the customer routers and it is configuring: as-override

2) Usually used on Customer routers facing to the other routers (usually to ISP) and it is configuring: allowas-in

[Arnaud.ravazzola]

I thank you for all these details. I am not sure I understood everything. I don't what ISP means.

I am downloading GNS3 to test.

[Arnaud.ravazzola]

Thank you Julio.

I can't try in Cisco Packet Tracer. I need to test your command in real equipment. 

But I can't make a link betweek them so i need to do 3 AS no ?

[Julio E. Moisa]

Hi

Yes, packet tracer has some limitations. You could use a tunnel as mentioned the previous forum member. But if you really want to use Router 1 (eBGP), It will not work. 

Have you tried in GNS3?

This is the explanation:

https://ccieblog.co.uk/bgp/bgp-as-override-vs-allow-as-in