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The Function of the Forwarding Address

Latest reply: Mar 4, 2016 10:03:50 1548 1 0 0 0

Like in IPv4 RIP version 2 (RIPv2) and Border Gateway Protocol version 4 (BGPv4), the forwarding address in OSPF Type 5 link-state advertisements (LSAs) is used to notify the devices in the local routing domain of the faster next hop to the imported AS external network described by Type 5 LSAs. This prevents an extra hop from being generated when an internal device routes to itself with itself as the next hop and forwards the traffic to the devices in an external routing domain on the same broadcast network.

For the Type-5 LSAs generated by an Autonomous System Boundary Router (ASBR), the forwarding address can be 0 or non-zero.

The rules of inputting the forwarding address are as follows:

  • If an ASBR redistributes routes but OSPF is disabled on the next hop  interface of these routes, the forwarding address is set to 0.0.0.0.

    When the value of the FA field of a Type 5 LSA is 0.0.0.0, the device that  receives the LSA knows that the device sending the LSA is an advertising router  (that is, an ASBR), and calculates the next hop.

  • When the following conditions are met, the forwarding address is set to  non-zero:

    In any other case, the forwarding address is 0.0.0.0.

    When all of the conditions are met, an ASBR fills in an address other than  0.0.0.0 in the FA field of a Type 5 LSA, and the device that receives the LSA  calculates the next hop based on the value of the FA field.

    • The route to the interface of the ASBR's next hop is reachable.

    • The next hop interface of the ASBR is not configured as a passive interface    (known as a silent interface on the switch and a passive interface under IOS);

    • The type of the next hop interface of the ASBR is not OSPF P2P or P2MP;

    • The next hop interface address of the ASBR is on the network segment    configured with the network command in OSPF.

When calculating routes, OSPF needs to check whether any intra-area route or inter-area route arrives at the ASBR. If there is no intra-area route or inter-area route destined for the forwarding address, the LSA does not take part in route calculation or calculate routes.


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user_2790689
Created Mar 4, 2016 10:03:50

Thank you.
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