Chapter 9. Configuring IS-IS for IP Routing on Cisco Routers
| This chapter provides guidelines, procedures, and an overview of configuration commands for enabling IS-IS on Cisco routers. The Cisco implementation of the IS-IS protocol provides numerous commands for enabling various capabilities and optimization of IS-IS functionality, some of which are infrequently used in real applications. This chapter focuses on relevant capabilities, features, and related commands commonly used by network operators who run IS-IS in their networks. However, wherever possible, some less-common configuration tips are highlighted. For the complete suite of supported features and commands, check out the Cisco IOS configuration manuals, available at www.cisco.com. Note that this book primarily focuses on application of the IS-IS protocol in IP-only environments, such as those found in Internet service provider networks. This focus is certainly reflected in this chapter as can be discerned from the examples and related discussions. This chapter discusses the following:
The following specialized topics and relevant configuration examples are covered:
The following two basic steps are required to enable IS-IS routing on Cisco routers running appropriate Cisco IOS releases that support IS-IS:
The Cisco IOS configuration command router isis [tag] enables the IS-IS routing process. The tag is an optional keyword for labeling the routing process, and it has only local significance in recent versions of IOS. This means that it doesn't have to be consistent across all routers. Most service providers use the same tag on all routers in their domain for consistency. Be aware that in some older versions of IS-IS, routers might be unable to form adjacencies when the tags are mismatched. Adding an NSAP address in the router configuration level completes basic configuration for the routing process. The command clns routing is automatically added to the configuration upon activationof the routing process. However, recent changes in IOS (for example, 12.0S and 12.0ST release trains) make this command relevant only in environments where ISO CLNP forwarding is required. For pure IP environments, CLNP forwarding is not required, and the entry can be removed using the negation command, no clns routing. When basic IS-IS routing is enabled, a Cisco router functions as both a Level 1 and Level 2 router unless either Level 1 or Level 2 mode of operation is manually disabled. The IOS router-level command no is-type <level-1 level-2-only> disables Level 1 or Level 2 routing on all interfaces. The next step after configuring the routing process is to enable IS-IS routing on the interfaces of interest with the command ip router isis [tag]. As indicated previously, CLNP routing is enabled by default when the IS-IS process is activated. If the router has to operate in dual mode, routing both IP and CLNP, the interface-level command clns router [tag] needs to be configured. If only IP routing support is required, the ip router [tag] command is sufficient and the CLNP routing might be disabled globally, as explained earlier. The interface command [no] isis circuit type <level-1 level-2> can be used to specify the desired level of routing (Level 1 or Level 2) depending on the global configuration at the router level. It is interesting to note that in the IS-IS confiuration, no IP network statements are used as in the case of other IP routing protocols. The IP subnets on IS-IS enabled interfaces are automatically placed in the IP reachability TLVs used in LSPs, which are then flooded to adjacent neighbors. |
EAN: 2147483647
Pages: 144