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OSPF Forwarding Address

Dear All,

 

Please advise on the below.

In OSPF, route will not be installed in the routing table if the forwarding address is not learnt via OSPF. So if I have created a static route for the forwarding address on the router, the route will be missing though it will be in database.

I understand it is RFC specification. But what is the reason for designing that way?

 

Thanks

 

 

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1 ACCEPTED SOLUTION

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Engager

Hi,the purpose of a non-zero

Hi,

the purpose of a non-zero forwarding-address is to avoid unnecessary hops if a third-party next-hop exists for an external route.

Several checks to prevent routing-loops and ensure reachability must be passed before an ASBR sets the forwarding-address to an non-zero IP-address:

The third-party next-hop has to be directly connected on an ASBR's interface, which is

  • (non-passive) OSPF enabled
  • no OSPF point-to-point or point-to-multipoint interface

When this conditions are met, the corresponding network must be learned as an OSPF internal route. Before another router installs the external route in its routing-table, it just checks if this is the case; otherwise something went obviously wrong and it doesn't install the route.

Cisco Document 13682: Common Routing Problem with OSPF Forwarding Address

 

HTH

Rolf

 

P.S.: I focussed on standard area types, the usage of the FA in NSSAs is slightly different.

View solution in original post

1 REPLY 1
Highlighted
Engager

Hi,the purpose of a non-zero

Hi,

the purpose of a non-zero forwarding-address is to avoid unnecessary hops if a third-party next-hop exists for an external route.

Several checks to prevent routing-loops and ensure reachability must be passed before an ASBR sets the forwarding-address to an non-zero IP-address:

The third-party next-hop has to be directly connected on an ASBR's interface, which is

  • (non-passive) OSPF enabled
  • no OSPF point-to-point or point-to-multipoint interface

When this conditions are met, the corresponding network must be learned as an OSPF internal route. Before another router installs the external route in its routing-table, it just checks if this is the case; otherwise something went obviously wrong and it doesn't install the route.

Cisco Document 13682: Common Routing Problem with OSPF Forwarding Address

 

HTH

Rolf

 

P.S.: I focussed on standard area types, the usage of the FA in NSSAs is slightly different.

View solution in original post

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