OSPF: Prefix Suppression

Maintaining transit network routes between two endpoints can clutter LSDBs and cause unnecessary overhead in maintaining LSDBs.  These prefix’s are merely used for transport between two sites and knowledge of them is largely not needed as most traffic will be generated by end hosts on non-transit networks.

OSPF combines topology and addressing information in type 1 and type 2 LSAs, meaning that the addressing of these transit links would be maintained in these LSAs.  The goal is to suppress the prefix only, not necessarily the link state information.  Type 1 LSAs describe a router and its adjacencies to its neighboring objects, there are four possible link types described by the type 1 LSA:

Point to point link – This is a transit link pointing to another routers RID, it contains no addressing information and will not be influenced by prefix suppression.

Link to a transit network – This is a transit link pointing toward the transit networks DR IP address, it contains no further IP information outside of the DR IP and will not be influenced by prefix suppression.

Stub network – This is the IP prefix used in a true stub network or a prefix used on a point to point link to another router, a router can suppress all stub network entries in type 1 LSAs that correspond to IP prefixes used on point to point links.

Virtual link – This is a virtual transit point to point link pointing to a virtually adjacent router RID, it contains no addressing information and will not be influenced by prefix suppression.

So for type 1 LSAs only stub network entries that contain prefixes for point to point interfaces will be suppressed.

Type 2 LSAs describe  transit multiaccess networks and all connected routers, and they include information from which the IP prefix used in the network can be calculated.

Specifically the link state ID ofa type 2 LSA is set to the IP of the DR in the multiaccess network, and the LSa body contains the network  and subnet mask, among other things.

The IP prefix of a network can be computed by bitwise ANDing the Link State ID of the LSA and the netmask carried in its payload.  Therefore these can’t be removed without making the format of the LSA incompatible. So RFC 6860 uses a different approach, it sets the netmask field to the value of 255.255.255.255, which is an invalid mask for a multiaccess network.

routers implementing RFC 6860 will recognize the netmask as a signal that the LSA contains no IP prefix information.  This will allow routers not implementing RFC to install a host route toward that networks DR, while the advantage of saving routing table space is lost on these routers no interoperability issues are introduced.

Prefix suppression can be activated by entering in the prefix-suppression command in router ospf mode.  This will suppress the router from advertising any prefixes except loopbacks, secondary IP addresses and prefixes on passive interfaces, these are considered nontransit prefixes.

It can also be configured per interfaces using the ip ospf prefix-suppression command.

You can exempt an interface from being suppressed by using the ip ospf prefix-suppression disable command.

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