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| 5.10 |  | Troubleshooting Redistribution |
| 5.10.4 |  | Redistribution problems with IS-IS |

Cisco IOS Software allows IP routes to be imported into IS-IS. Examples of the external sources are static routes and other dynamic routing protocols such as RIP and OSPF. The IP external reachability TLV is used for adding external routes into the IS-IS domain. Even though RFC 1195 specifies the IP external reachability for only Level 2 LSPs, Cisco IOS Software provides a special capability for using them in Level 1 LSPs, which allows external routes into a Level 1 area.

Most service provider networks use IS-IS as the IGP in large single-area Level 1-only or Level 2-only domains. For those with Level 1-only backbones, the capability to redistribute into Level 1 provides flexibility to import external routes into the IS-IS domain. Even though this behavior is not standardized, it should not pose interoperability issues with other vendors’ routers because both existing IS-IS standards, ISO 10589 and RFC 1195, require IS-IS implementations to ignore unsupported or unknown optional TLVs encountered while parsing IS-IS-packets.

The IOS router-level command redistribute enables redistribution. This command takes on other options, such as metric value, metric type, route map, and so on. In the Cisco implementation of IS-IS, CLNS static routes are automatically distributed into IS-IS. However, IP static routes are redistributed only by manual configuration.

When static IP routes need to be redistributed, the redistribute command requires the keyword ip to go with it, in addition to the other arguments previously mentioned. The metric type for external routes can be either internal or external. Internal metrics are comparable to metrics used for internal routes. External metrics require the I/E bit (bit 7) of the metric field to be set in addition to the actual metric, resulting in higher metric values. In current Cisco IOS Software releases, when using narrow metrics, bit 8 of the default metric field is set for external metrics, resulting in an increase of the metric value by 128.

By default, the internal metric type is assigned if nothing is specified in the configuration. Also, the external routes are added into Level 2 unless Level 1 is explicitly stated in the configuration. Figure illustrates basic examples of redistribution in IS-IS. In Figure , only the ip keyword is used with the redistribute command.

Note that below show running-config, the internal metric type has been assigned by default and the metric applied is 0. The output of show isis database on RT1 shows that the external static route has been added to only the Level 2 LSP.

The metric type is explicitly set to external in this configuration, but no metric value is applied. As explained previously, the I/E bit needs to then be set for the external metric type, effectively increasing the metric value by 64. However, Cisco IOS Software set bit 8 of the narrow metric instead of bit 7, consequently adding 128 instead to the original value of 0. The Level 2 LSP displayed shows 128 as the metric value for the external route, 172.16.1.0/24.

The IP routing table output from RT2 shows the external route, 172.16.1.0/24, which was redistributed from a static source into IS-IS on router RT1. The metric entered for this route, 138, is the total of the metric on the outgoing interface from RT2 to RT1 (10) plus the metric of 128 advertised by RT1. Other routes received from RT1 (10.0.0.1/32 and 10.1.1.0/24) are registered with a metric 20 (10 advertised by RT1 and additional 10 for the metric from RT2 to RT1).

The route-map option of the redistribute command provides more flexibility for configuring redistribution, such as selective importation of external routes into the IS-IS environment, applying special tags, and even setting the metric of redistributed routes. When used for selective importation of routes into IS-IS, route maps provide a filtering effect by controlling which elements from an external source are allowed or denied into IS-IS.

In Figure , static routes are redistributed into IS-IS while filtering through the route map TEST. Route map TEST matches the static routes against access list 1, which permits only 172.16.2.0/24 into the IS-IS environment. RT1 LSP is shown from RT2. Also shown is the routing table of RT2.

In Figure , the route map approach is used to set the metric for routes imported into IS-IS.