Cisco Express Forwarding; Multicast Mls Nsf With Sso - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Chapter 5 Configuring NSF with SSO Supervisor Engine Redundancy
A networking device is NSF-aware if it is running NSF-compatible software. A device is NSF-capable
if it has been configured to support NSF; it will rebuild routing information from NSF-aware or
NSF-capable neighbors.
Each protocol depends on CEF to continue forwarding packets during switchover while the routing
protocols rebuild the Routing Information Base (RIB) tables. After the routing protocols have converged,
CEF updates the FIB table and removes stale route entries. CEF then updates the line cards with the new
FIB information.

Cisco Express Forwarding

A key element of NSF is packet forwarding. In a Cisco networking device, packet forwarding is provided
by Cisco Express Forwarding (CEF). CEF maintains the FIB, and uses the FIB information that was
current at the time of the switchover to continue forwarding packets during a switchover. This feature
reduces traffic interruption during the switchover.
During normal NSF operation, CEF on the active supervisor engine synchronizes its current FIB and
adjacency databases with the FIB and adjacency databases on the redundant supervisor engine. Upon
switchover of the active supervisor engine, the redundant supervisor engine initially has FIB and
adjacency databases that are mirror images of those that were current on the active supervisor engine.
For platforms with intelligent line cards, the line cards will maintain the current forwarding information
over a switchover. For platforms with forwarding engines, CEF will keep the forwarding engine on the
redundant supervisor engine current with changes that are sent to it by CEF on the active supervisor
engine. The line cards or forwarding engines will be able to continue forwarding after a switchover as
soon as the interfaces and a data path are available.
As the routing protocols start to repopulate the RIB on a prefix-by-prefix basis, the updates will cause
prefix-by-prefix updates to CEF, which it uses to update the FIB and adjacency databases. Existing and
new entries will receive the new version ("epoch") number, indicating that they have been refreshed. The
forwarding information is updated on the line cards or forwarding engine during convergence. The
supervisor engine signals when the RIB has converged. The software removes all FIB and adjacency
entries that have an epoch older than the current switchover epoch. The FIB now represents the newest
routing protocol forwarding information.

Multicast MLS NSF with SSO

NSF with SSO does not support IPv6 multicast traffic. If you configure support for IPv6 multicast traffic,
Note
configure RPR redundancy.
Multicast multilayer switching (MMLS) NSF with SSO is required so that Layer 3 multicast traffic that
is switched by the router is not dropped during switchover. Without MMLS NSF with SSO, the Layer 3
multicast traffic is dropped until the multicast protocols converge.
During the switchover process, traffic is forwarded using the old database (from the previously active
supervisor engine). After multicast routing protocol convergence has taken place, the shortcuts
downloaded by the newly active PISA will be merged with the existing flows and marked as new
shortcuts. Stale entries will slowly be purged from the database allowing NSF to function during the
switchover while ensuring a smooth transition to the new cache.
Because multicast routing protocols such as Protocol Independent Multicast (PIM) sparse mode and PIM
dense mode are data driven, multicast packets are leaked to the router during switchover so that the
protocols can converge.
OL-11439-03
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
Understanding NSF with SSO Supervisor Engine Redundancy
5-3