Using IS-IS in Dual Environments

As specified in RFC 1195, Integrated IS-IS supports routing in dual environments with both ISO CLNS and IP services. Integrated IS-IS can be used to forward both IP and CLNP packets simultaneously in such environments. This section briefly discusses requirements for using IS-IS in such enviroments and potential issues that might arise.

A critical requirement for using IS-IS in a dual environment is to keep all routers in an area consistently configured. In other words, each area must be configured to surpport either IP only or CLNP only, or both, and all routers in the area must be configured similarly. This is necessary so that all routers in the area can build a consistent level Link-State database and obtain identical views of the areas topology. However, an IP-only router can connect to another router in an area that supports both IP and CLNP and form Level 2 adjacencies with that router for sharing of IP routing information.

Figure 8-8 shows a network topology with three separate areas. Area 1 supports only ISO CLNP, and so all routers in this area must be configured for IS-IS in ISO CLNP mode only. Area 2 supports both IP and ISO CLNP, and so routers in this area must operate in dual mode. Area 3 is an IP-only area, and therefore, all routers in this area must be configured for IP only. Also, as shown in the illustration, this requirement does not apply to the backbone where Level 2 routing is performed.

Figure 8-8. Using IS-IS in Dual IP and ISO domains.

graphics/08fig08.gif

Some key points to note about using IS-IS for IP and ISO CLNP follow:

  • An administrative distance of 110 is used for ISO CLNP routing and 115 for IP routing.

  • CLNP static routes are automatically redistributed into IS-IS, while explicit configuration is required to carry IP static routes into IS-IS. Also, the redistribute static statement requires the keyword ip for the IP setup.

  • Enabling IS-IS for IP routing automatically enables CNLS routing on the entire router. However, recent IOS releases allow CNLS to be disabled when using IS-IS for only IP routing. This can be done with the command no clns routing.

  • IS-IS routing for IP on an inteface is enabled with the interface level IOS command ip router isis <tag>, whereas clns routing is enabled similarly but with the command clns router isis <tag>.

  • Making an interface passive removes both the ip router isis <tag> and clns router isis <tag> commands from the interface configuration, and no adjacencies are formed over that interface for IP or CLNP routing. However, any IP prefixes configured on the interface are carried into the router's LSP. Because CLNP routing is completely disabled on the interface, care must be used when applying the passive-interface command in dual environments.

Examples 8-17 and 8-18 illustrate problems that arise when routers in the same area are configured inconsistently.

Case Study 1: Mixing ISO and IP

Example 8-17 shows a scenario in which a connection RTA and RTB is confiigured for both IP and CLNP routing on RTA but enabled for only IP routing on RTB. This configuration is not allowed in IOS because according to RFC 1195, a link between two routers in the same area must be configured identically at either end. Therefore, no adjacency is formed between RTA and RTB, as shown in the show clns neighbors command output.

Example 8-17 Same Area Routers with Dual (IP and CNLP) and IP-Only Configurations
 Configuration for RTA ! Int serial0 Ip address 192.168.10.9 255.255.255.252 Ip router isis ! Configuration for RTB graphics/08exa01.gif ! Int serial1 Ip address 192.168.10.10 255.255.255.252 Ip router isis clns router isis ! RTA#  show clns neighbors  System ID    Interface  SNPA      State  Holdtime  Type   Protocol RTB          Se0        *HDLC*    Up     23        L1     ES-IS 

Case Study 2: Mixing ISO and IP

Example 8-18 shows a scenario where IS-IS is enabled for CLNP routing at one end of the connection, at RTA, and IP-only routing at RTB. This is also not allowed because both routers are in the same area, so adjacency is not formed between RTA and RTB, as shown in the show clns neighbors ouput.

Example 8-18 Same Area Routers with IP-Only and CLNP-Only Configurations
 Configuration for RTA ! Int serial0 Ip address 192.168.10.9 255.255.255.252 Ip router isis clns router isis ! Configuration for RTB graphics/08exa02.gif ! Int serial1 Ip address 192.168.10.10 255.255.255.252 Ip router isis ! RTA#  show clns neighbors  System ID    Interface  SNPA      State  Holdtime  Type   Protocol RTB          Se0        *HDLC*    Init   23        L1     IS-IS 


IS-IS Network Design Solutions
IS-IS Network Design Solutions (Networking Technology)
ISBN: 1578702208
EAN: 2147483647
Year: 2005
Pages: 144
Authors: Abe Martey

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