Is-Is Functional Overview - Cisco NCS 6000 Series Configuration Manual

IS-IS Functional Overview

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).
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.
Restrictions
When multiple instances of IS-IS are being run, an interface can be associated with only one instance (process).
Instances may not share an interface.
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:
• 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
• Fast-flooding with different threads handling flooding and shortest path first (SPF).
Note For information on IS-IS support for Bidirectional Forwarding Detection (BFD), see Interface and Hardware
Component Configuration Guide for Cisco NCS 6000 Series Routers and Interface and Hardware Component
Command Reference for Cisco NCS 6000 Series RoutersInterface and Hardware Component Command
Reference for Cisco NCS 5000 Series Routers .
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
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Implementing IS-IS