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 device 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 device uses the network address alone for the link state ID of the link state advertisement (LSA) for the network. For example, if the device needs to generate an LSA for network 10.1.2.3 255.0.0.0, the device generates ID 10.1.2.3 for the LSA.

However, suppose that an OSPF device 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 device uses the same link state ID, 10.0.0.0, for the LSAs for all three networks. For example, if the device 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 device 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, because the first network has 16 ones bits (255.255.0.0) whereas the second network has only 8 ones bits (255.0.0.0).
      • For the less specific network, use the networks address as the ID.
      • For the more specific network, use the network’s broadcast address as the ID. The broadcast address is the network address, with all ones bits in the host portion of the address. For example, the broadcast address for network 10.0.0.0 255.255.0.0 is 10.0.255.255.

    If this comparison results in a change to the ID of an LSA that has already been generated, the device generates a new LSA to replace the previous one. For example, if the device has already generated an LSA for network with ID 10.0.0.0 for network 10.0.0.0 255.255.255.0, the device must generate a new LSA for the network, if the device needs to generate an LSA for network 10.0.0.0 255.255.0.0 or 10.0.0.0 255.0.0.0.

Parent topic: OSPFv2