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Implementing IS-IS on Cisco IOS XR Software
Information About Implementing IS-IS on Cisco IOS XR Software
RC-85
Cisco IOS XR Routing Configuration Guide
IS-IS Functional Overview
Small IS-IS networks are typically built as a single area that includes all routers in the network. As the
network grows larger, it may be reorganized into a backbone area made up of the connected set of all
Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know
how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone
routers know how to reach other areas.
The IS-IS routing protocol supports the configuration of backbone Level 2 and Level 1 areas and the
necessary support for moving routing information between the areas. Routers establish Level 1
adjacencies to perform routing within a local area (intra-area routing). Routers establish Level 2
adjacencies to perform routing between Level 1 areas (interarea routing).
For Cisco IOS XR software, each IS-IS instance can support either a single Level 1 or Level 2 area, or
one of each. By default, all IS-IS instances automatically support Level 1 and Level 2 routing. You can
change the level of routing to be performed by a particular routing instance using the is-type command.
Key Features Supported in the Cisco IOS XR IS-IS Implementation
The Cisco IOS XR implementation of IS-IS conforms to the IS-IS Version 2 specifications detailed in
RFC 1195 and the IPv6 IS-IS functionality based on the Internet Engineering Task Force (IETF) IS-IS
Working Group draft-ietf-isis-ipv6.txt document.
The following list outlines key features supported in the Cisco IOS XR implementation:
• Improved configuration syntax and enhanced show commands
• Single topology IPv6
• Multitopology
• Nonstop forwarding (NSF), both Cisco proprietary and IETF
• Three-way handshake
• Mesh groups
• Multiple IS-IS instances
• Configuration of a broadcast medium connecting two networking devices as a point-to-point link
IS-IS Configuration Grouping
Cisco IOS XR groups all of the IS-IS configuration in router configuration mode, including the portion
of the interface configurations associated with IS-IS. The grouping makes the configuration process
clearer, and eliminates some of the clutter in the global interface stanza. To display the IS-IS
configuration in its entirety, use the show isis interface command.
The command output displays the running configuration for all configured IS-IS instances, including the
interface assignments and interface attributes.