Eigrp Operation - Cisco Catalyst 4500 Series Configuration Manual

About NSF with SSO Supervisor Engine Redundancy
Following a switchover, Cisco IS-IS NSF has complete neighbor adjacency and LSP information;
Note
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.
Note A router may be NSF-aware but may not be helping the NSF restarting neighbor because booting from
a cold start.
If at least one of the peer routers is NSF-aware, the restarting router then receives updates and rebuilds
its database. The restarting router must then find out if it had converged so that it can notify the routing
information base (RIB). Each NSF-aware router is required to send an end of table (EOT) marker in the
last update packet to indicate the end of the table content. The restarting router knows it has converged
when it receives the EOT marker. The restarting router can then begin sending updates.
An NSF-aware peer knows when the restarting router had converged when it receives an EOT indication
from the restarting router. The peer then scans its topology table to search for the routes with the restarted
neighbor as the source. The peer compares the route timestamp with the restart event timestamp to
determine if the route is still available. The peer then goes active to find alternate paths for the routes
that are no longer available through the restarted router.
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
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Chapter 11 Configuring Cisco NSF with SSO Supervisor Engine Redundancy
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