When OSPF Imports Routes as Type 5 LSAs, What Is the Function of the Input Forwarding Address?

Created: Apr 18, 2017 08:35:38Latest reply: Apr 18, 2017 17:15:21 695 2 0 0
When OSPF Imports Routes as Type 5 LSAs, What Is the Function of the Input Forwarding Address?
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user_2790689     Created Apr 18, 2017 12:00:39 Helpful(0) Helpful(0)

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Adamcolob     Created Apr 18, 2017 17:15:21 Helpful(0) Helpful(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 following conditions are met, the forwarding address is set to non-zero:
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 router 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.
In any other case, the forwarding address is 0.0.0.0.
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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