E.3 Obsoleting LSInfinity in router links advertisements (original) (raw)

Connected: An Internet Encyclopedia
E.3 Obsoleting LSInfinity in router links advertisements

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E.3 Obsoleting LSInfinity in router links advertisements

The metric of LSInfinity can no longer be used in router links advertisements to indicate unusable links. This is being done for several reasons:

It removes any possible confusion in an OSPF area as to just which routers/networks are reachable in the area. For example, the above virtual link fix relies on detecting the existence of virtual links when running the Dijkstra. However, when one-directional links (i.e., cost of LSInfinity in one direction, but not the other) are possible, some routers may detect the existence of virtual links while others may not. This may defeat the fix for the virtual link problem.
It also helps OSPF's Multicast routing extensions (MOSPF), because one-way reachability can lead to places that are reachable via unicast but not multicast, or vice versa.

The two prior justifications for using LSInfinity in router links advertisements were 1) it was a way to not support TOS before TOS was optional and 2) it went along with strong TOS interpretations. These justifications are no longer valid. However, LSInfinity will continue to mean "unreachable" in summary link advertisements and AS external link advertisements, as some implementations use this as an alternative to the premature aging procedure specified in Section 14.1.

This change has one other side effect. When two routers are connected via a virtual link whose underlying path is non-TOS- capable, they must now revert to being non-TOS-capable routers themselves, instead of the previous behavior of advertising the non-zero TOS costs of the virtual link as LSInfinity. See Section 15 for details.

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Connected: An Internet Encyclopedia
E.3 Obsoleting LSInfinity in router links advertisements