Is-Is Operation - Cisco Catalyst 4500 Series Configuration Manual

Chapter 11 Configuring Cisco NSF with SSO Supervisor Engine Redundancy

IS-IS Operation

When an IS-IS NSF-capable router performs a supervisor engine switchover, it must perform the
following tasks in order to resynchronize its link state database with its IS-IS neighbors:
•
•
The IS-IS NSF feature offers two options when you configure NSF:
•
•
If neighbor routers on a network segment are running a software version that supports the IETF Internet
draft for router restartability, they assist an IETF NSF router that is restarting. With IETF, neighbor
routers provide adjacency and link-state information to help rebuild the routing information following a
switchover. A benefit of IETF IS-IS configuration is operation between peer devices based on a proposed
standard.
If you configure IETF on the networking device, but neighbor routers are not IETF-compatible, NSF
Note
aborts following a switchover.
If the neighbor routers on a network segment are not NSF-aware, you must use the Cisco configuration
option. The Cisco IS-IS configuration transfers both protocol adjacency and link-state information from
the active to the redundant supervisor engine. An advantage of Cisco configuration is that it does not rely
on NSF-aware neighbors.
IETF IS-IS Configuration
As quickly as possible after a supervisor engine switchover, the NSF-capable router sends IS-IS NSF
restart requests to neighboring NSF-aware devices using the IETF IS-IS configuration. Neighbor
networking devices recognize this restart request as an indicator that the neighbor relationship with this
router should not be reset, but that they should initiate database resynchronization with the restarting
router. As the restarting router receives restart request responses from routers on the network, it can
begin to rebuild its neighbor list.
After this exchange is complete, the NSF-capable device uses the link-state information to remove stale
routes, update the RIB, and update the FIB with the new forwarding information; IS-IS is then fully
converged.
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. The
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.
OL-25340-01
Relearn the available IS-IS neighbors on the network without causing a reset of the neighbor
relationship
Reacquire the contents of the link state database for the network
Internet Engineering Task Force (IETF) IS-IS
Cisco IS-IS
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
About NSF with SSO Supervisor Engine Redundancy
11-7