Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 251)

‐ A second ASBR comes on-line
‐ A second ASBR that is already on-line begins advertising an equivalent route to the same
destination.
In either case above, the router with the higher router ID floods the AS External LSAs and the other
router flushes its equivalent AS External LSAs. For example, if Router D is offline, Router E is the only
source for a route to the external routing domain. When Router D comes on-line, it takes over flooding
of the AS External LSAs to Router F, while Router E flushes its equivalent AS External LSAs to Router
F.
• One of the ASBRs starts advertising a route that is no longer equivalent to the route the other ASBR
is advertising. In this case, the ASBRs each flood AS External LSAs. Since the LSAs either no longer
have the same cost or no longer have the same next-hop router, the LSAs are no longer 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 no longer an
ASBR. In this case, the other ASBR floods the AS External LSAs. For example, if Router D goes off-
line, then Router E starts flooding the AS with AS External LSAs for the route to Router F.
Support for OSPF RFC 2328 Appendix E
Brocade devices support Appendix E in OSPF RFC 2328. Appendix E describes a method to ensure
that an OSPF router generates unique link state IDs for type-5 (External) link state advertisements
(LSAs) in cases where two networks have the same network address but different network masks.
NOTE
Support for Appendix E of RFC 2328 is enabled automatically and cannot be disabled. No user
configuration is required.
Normally, an OSPF router uses the network address alone for the link state ID of the link state
advertisement (LSA) for the network. For example, if the router needs to generate an LSA for network
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
• 10.0.0.0 255.255.255.0
All three networks have the same network address, 10.0.0.0. Without support for RFC 2328 Appendix
E, an OSPF router uses the same link state ID, 10.0.0.0, for the LSAs for all three networks. For
example, if the router generates an LSA with ID 10.0.0.0 for network 10.0.0.0 255.0.0.0, this LSA
conflicts with the LSA generated for network 10.0.0.0 255.255.0.0 or 10.0.0.0 255.255.255.0. The result
is multiple LSAs that have the same ID but that contain different route information.
When appendix E is supported, the router generates the link state ID for a network as the following
steps.
1. Does an LSA with the network address as its ID already exist?
• ‐ No - Use the network address as the ID.
‐ Yes - Go to "Support for OSPF RFC 2328 Appendix E".
2. Compare the networks that have the same network address, to determine which network is more
specific. The more specific network is the one that has more contiguous one bits in its network mask.
For example, network 10.0.0.0 255.255.0.0 is more specific than network 10.0.0.0 255.0.0.0,
Support for OSPF RFC 2328 Appendix E
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