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Software Configuration Guide—Release 15.0(2)SG
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Chapter 9 Configuring Cisco NSF with SSO Supervisor Engine Redundancy About NSF with SSO Supervisor Engine Redundancy
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 pa cket
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.
When the restarting router has received all EOT indications from its neighbors or when the NSF converge
timer expires, EIGRP notifies the RIB of convergence. EIGRP waits for the RIB convergence signal and
then floods its topology table to all awaiting NSF-aware peers.
NSF Guidelines and Restrictions
NSF with SSO has these restrictions:
• With aggressive protocol timers (such as, when the default exceeds the timer value), upon
switchover, the protocol software running on the new active supervisor engine might not initialize
in time to send "hello" packets to its neighboring switches or routers. If the protocol takes longer
time to initialize because of other CPU-demanding tasks, then the protocol encounters state
transitions and causes a loss in traffic on the order of seconds. We recommend that you do not
configure aggressive timers in conjunction with SSO/NSF.
• For NSF operation, you must have SSO configured on the device.
• NSF with SSO supports IP Version 4 traffic and protocols only; NSF with SSO does not support IPv6
traffic.
• The Virtual Redundancy Routing Protocols (VRRP) is not SSO-aware, meaning state information is
not maintained between the active and standby supervisor engine during normal operation. VRRP
and SSO can coexist but both features work independently. Traffic that relies on VRRP may switch
to the VRRP standby in the event of a supervisor engine switchover.
• All neighboring devices participating in BGP NSF must be NSF-capable and configured for BGP
graceful restart.
• OSPF NSF for virtual links is not supported.