676 BigIron RX Series Configuration Guide53-1001986-01Overview of OSPF (Open Shortest Path First)25• A second ASBR comes on-line• A second ASBR that is already on-line begins advertising an equivalent route to the samedestination.In either case above, the router with the higher router ID floods the AS External LSAs and theother router flushes its equivalent AS External LSAs. For example, if Router D is offline, RouterE is the only source for a route to the external routing domain. When Router D comes on-line, ittakes over flooding of the AS External LSAs to Router F, while Router E flushes its equivalent ASExternal LSAs to Router F.• One of the ASBRs starts advertising a route that is no longer equivalent to the route the otherASBR is advertising. In this case, the ASBRs each flood AS External LSAs. Since the LSAs eitherno longer have the same cost or no longer have the same next-hop router, the LSAs are nolonger equivalent, and the LSA reduction feature no longer applies.• The ASBR with the higher router ID becomes unavailable or is reconfigured so that it is nolonger an ASBR. In this case, the other ASBR floods the AS External LSAs. For example, ifRouter D goes off-line, then Router E starts flooding the AS with AS External LSAs for the routeto Router F.Support for OSPF RFC 2328 appendix EBigIron RX provides support for Appendix E in OSPF RFC 2328. Appendix E describes a method toensure that an OSPF router generates unique link state IDs for type-5 (External) link stateadvertisements (LSAs) in cases where two networks have the same network address but differentnetwork masks.NOTESupport for Appendix E of RFC 2328 is enabled automatically and cannot be disabled. No userconfiguration is required.Normally, an OSPF router uses the network address alone for the link state ID of the link stateadvertisement (LSA) for the network. For example, if the router needs to generate an LSA fornetwork 10.1.2.3 255.0.0.0, the router generates ID 10.1.2.3 for the LSA.However, suppose that an OSPF router needs to generate LSAs for all the following networks:• 10.0.0.0 255.0.0.0• 10.0.0.0 255.255.0.0• 10.0.0.0 255.255.255.0All three networks have the same network address, 10.0.0.0. Without support for RFC 2328Appendix E, an OSPF router uses the same link state ID, 10.0.0.0, for the LSAs for all threenetworks. For example, if the router generates an LSA with ID 10.0.0.0 for network 10.0.0.0255.0.0.0, this LSA conflicts with the LSA generated for network 10.0.0.0 255.255.0.0 or 10.0.0.0255.255.255.0. The result is multiple LSAs that have the same ID but that contain different routeinformation.When appendix E is supported, the router generates the link state ID for a network as follows.1. Does an LSA with the network address as its ID already exist?• No – Use the network address as the ID.• Yes – Go to step 2.
