Connected: An Internet Encyclopedia
E.3 Obsoleting LSInfinity in router links advertisements
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E.3 Obsoleting LSInfinity in router links advertisements
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 These justifications are no longer valid.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.
Next: E.4 TOS encoding updated
Connected: An Internet Encyclopedia
E.3 Obsoleting LSInfinity in router links advertisements