Eigrp Operation - Cisco Catalyst 4500 Series Software Configuration Manual

About NSF with SSO Supervisor Engine Redundancy
IS-IS NSF operation waits for a specified interval to ensure that connections are stable before attempting
another restart of IS-IS NSF. This functionality prevents IS-IS from attempting back-to-back NSF
restarts with stale information.
Cisco IS-IS Configuration
Using the Cisco configuration option, full adjacency and LSP information is saved, or checkpointed, to
the redundant supervisor engine. Following a switchover, the newly active supervisor engine maintains
its adjacencies using the check-pointed data, and can quickly rebuild its routing tables.
Note Following a switchover, Cisco IS-IS NSF has complete neighbor adjacency and LSP information;
however, it must wait for all interfaces to come on line that had adjacencies prior to the switchover. If
an interface does not come on line within the allocated interface wait time, the routes learned from these
neighbor devices are not considered in routing table recalculation. IS-IS NSF provides a command to
extend the wait time for interfaces that, for whatever reason, do not come on line in a timely fashion.
The switchover from one supervisor engine to the other happens within seconds. IS-IS reestablishes its
routing table and resynchronizes with the network within a few additional seconds. At this point, IS-IS
waits for a specified interval before it attempts a second NSF restart. During this time, the new redundant
supervisor engine boots up and synchronizes its configuration with the active supervisor engine. After
this synchronization is completed, IS-IS adjacency and LSP data is check-pointed to the redundant
supervisor engine; however, a new NSF restart is not attempted by IS-IS until the interval time expires.
This functionality prevents IS-IS from attempting back-to-back NSF restarts.

EIGRP Operation

When an EIGRP NSF-capable router initially re-boots after an NSF restart, it has no neighbor and its
topology table is empty. The router is notified by the redundant (now active) supervisor engine when it
needs to bring up the interfaces, reacquire neighbors, and rebuild the topology and routing tables. The
restarting router and its peers must accomplish these tasks without interrupting the data traffic directed
toward the restarting router. EIGRP peer routers maintain the routes learned from the restarting router
and continue forwarding traffic through the NSF restart process.
To prevent an adjacency reset by the neighbors, the restarting router uses a new Restart (RS) bit in the
EIGRP packet header to indicate a restart. The RS bit is set in the hello packets and in the initial INIT
update packets during the NSF restart period. The RS bit in the hello packets allows the neighbors to be
quickly notified of the NSF restart. Without seeing the RS bit, the neighbor can only detect an adjacency
reset by receiving an INIT update or by the expiration of the hello hold timer. Without the RS bit, a
neighbor does not know if the adjacency reset should be handled using NSF or the normal startup
method.
When the neighbor receives the restart indication, either by receiving the hello packet or the INIT packet,
it recognizes the restarting peer in its peer list and maintains the adjacency with the restarting router. The
neighbor then sends it topology table to the restarting router with the RS bit set in the first update packet
indicating that it is NSF-aware and is helping out the restarting router. The neighbor does not set the RS
bit in their hello packets, unless it is also a NSF restarting neighbor.
A router may be NSF-aware but may not be helping the NSF restarting neighbor because booting from
Note
a cold start.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
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Chapter 13 Configuring Cisco NSF with SSO Supervisor Engine Redundancy