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next-hop path attribute bgp question

Hello,

One thing about next-hop path attribute is not clear to me. In slides 35 and 80 (solutions for 35) edge router changes next-hop attribute to self (2.2.20.1). Is this necessary for the whole scheme to work?

External router's (R5) address (2.2.2.2) is also reachable by all internal routers from AS x as this router has an IP address on a network shared by AS z and AS x. If this is true, no change in next-hop attribute is required.

Also, RFC 1771 (page 22, last paragraph) says:

"When a BGP speaker advertises the route to a BGP speaker located in
its own autonomous system, the advertising speaker shall not modify
the NEXT_HOP attribute associated with the route. When a BGP speaker
receives the route via an internal link, it may forward packets to
the NEXT_HOP address if the address contained in the attribute is on
a common subnet with the local and remote BGP speakers."

Thank you for helping,
Aleksandar
Posted by Aleksandar Dragojevic on Tuesday 5 December 2006 at 23:42
Comments
One thing about next-hop path attribute is not clear to me. In slides 35 and 80 (solutions for 35) edge router changes next-hop attribute to self (2.2.20.1). Is this necessary for the whole scheme to work?

A: NO

External router's (R5) address (2.2.2.2) is also reachable by all internal routers from AS x as this router has an IP address on a network shared by AS z and AS x. If this is true, no change in next-hop attribute is required.

A: CORRECT

Also, RFC 1771 (page 22, last paragraph) says:

"When a BGP speaker advertises the route to a BGP speaker located in
its own autonomous system, the advertising speaker shall not modify
the NEXT_HOP attribute associated with the route.

A: this is how should be done according to RFC, but in practice if you are not in control of the external network over which you learn advertisments (Interconnection Point room), you migh want not to advertise this as next hop within your AS, so you put your internal interface. (But it will work with both approaches.) /end A

When a BGP speaker
receives the route via an internal link, it may forward packets to
the NEXT_HOP address if the address contained in the attribute is on
a common subnet with the local and remote BGP speakers."

(A: this is why it can be usefull to do as RFC says)
Posted by Slavisa Sarafijanovic on Wednesday 6 December 2006 at 10:28