- [Instructor] If we have a router with a couple
of route processors, we'll call them RP1 and RP2,
we can have a redundancy within our router chassis,
but there's a challenge.
If route processor one is currently active,
and we have a neighborship with another router,
maybe OSPF maybe EHRP,
but we've got a neighborship with another router,
if RP1 were to go down that primary route processor
and we failed over to RP2,
what might happen,
temporarily anyway,
is we're going to lose that neighborship,
and we're going to have to reform another neighborship
between our second route processor RP2 and our neighbor,
which in this case is R2.
That's the main issue.
We're trying to prevent that re-establishment of
a neighborship when we're failing over between
a primary and a secondary route processor.
To address that main issue we can use a feature called SSO,
or Stateful Switchover.
With SSO we are continually sending that
configuration and state information
between the primary and the secondary route processor,
meaning that if we did lose the primary route processor,
we would almost instantaneously have
a neighborship that's still up and available
between our backup route processor and our neighbor.
But there's a secondary issue,
even though we're maintaining our neighborship,
thanks to SSO,
we still might drop some packets initially,
and the reason is that backup route processor
does not have an IP routing table,
it doesn't know how to forward packets yet,
it has to reconstruct that routing table.
But we can add yet another feature
to prevent that from happening.
What we can do is use SSO in conjunction with NSF,
which stands for Nonstop Forwarding, here's how that works.
In addition to having the
config and state information synchronized,
we're also going to have the
Cisco Express Forwarding information, or CEF,
that information is going to live in memory in the router,
and if we did lose our primary route processor,
that information that's in memory could be used by NSF,
Nonstop Forwarding, to make forwarding decisions,
so we don't have to start with an empty routing table,
we've got that CEF database that can be active
and can be used until we can
construct our own IP routing table.
But that's the benefit of using both SSO,
Stateful Switchover, in conjunction with
Nonstop Forwarding, NSF.
Again, SSO, that's going to help us maintain adjacency's,
or neighborships between our neighbors,
and we couple that with Nonstop Forwarding,
so that we can make routing decisions
immediately after that failover.