OSPF forwarding address

For S series switches supporting OSPF, the functions and setting rules of the OSPF forwarding address are as follows:
The forwarding address (FA) field in an OSPF Type 5 LSA, similar to that of RIPv2 or BGPv4, specifies information about the next hop so that devices in a routing area can forward data to the next-hop address outside the AS through the imported route described in the Type 5 LSA. This prevents a device on a broadcast network from sending data to itself as the next hop and then forwarding the data to a device in an external routing area on the same broadcast network.

For a Type 5 LSA generated by the ASBR for external routes, the FA value can be 0.0.0.0 or non-0.0.0.0.
FA setting rules are as follows:
If OSPF is not enabled on the next-hop interfaces of the external routes imported by an ASBR, the FA is 0.0.0.0.
If the FA of a Type 5 LSA is 0.0.0.0, the device that receives the LSA calculates the route using Adv Rtr (the ASBR address) as the next-hop address.
If all of the following conditions are met, the FA is a non-0.0.0.0 address.
The ASBR has a routable next-hop interface.
The next-hop interface on the ASBR is not configured as a silent interface.
The next-hop interface on the ASBR is not an OSPF P2P or P2MP interface.
The IP address of the next-hop interface on the ASBR is within the OSPF network range.
If any condition is not met, the FA is 0.0.0.0.
After the ASBR fills a non-0.0.0.0 address in the FA field of a Type 5 LSA, the device that receives the LSA calculates the route using the non-0.0.0.0 address as the next-hop address.

The FA of an NSSA LSA cannot be 0.0.0.0.
If a loopback address is configured for an interface in the NSSA, the loopback address is automatically selected as the FA.
If no loopback interface exists in the NSSA, the IP address of the first Up interface is selected as the FA.


Before calculating a route using an LSA, OSPF checks whether there are intra-area or inter-area routes to the ASBR. If not, OSPF does not use the LSA in route calculation and no route can be calculated.

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