Case Study: Migration from Narrow to Wide Metrics

With the availability of IS-IS wide metrics support in IOS, operators of many existing networks have been considering migrating their configurations from narrow metrics to take advantage of the flexibility and convenience larger metric values provide. As discussed in the previous chapter, the easiest and least painful way to do this migration is by calling a flag day, shutting down the network, making appropriate changes, and turning the routers back up. However, stringent service level agreements (SLAs) in some environments prevent this approach. Fortunately, there are other options. There are actually two other approaches that are discussed in the following sections. Narrow metrics that allow metric values up to 63 are supported by TLVs that allocate only 6 bits for the metric field. These TLVs are commonly referred to as old style TLVs. Support for new wide metrics is provided by new style TLVs that allocate 24 or 32 bits for metric. See Chapter 5, "IS-IS Link-State Database," section "IS-IS Metric Extensions," for more details.

Method 1

The first method involves advertising the same information twice each time with a different metric format: once with old-style TLVs and once with new-style TLVs. This ensures that all routers understand the advertised information.

The advantages and disadvantages of using this method is covered in Chapter 6, "The Shortest Path First Algorithm." This section focuses on the actual transition steps that need to be followed when using this method:

Step 1. All routers are presumably running old software or new software in default mode so they advertise and use only the old-style TLVs.

Step 2. Make sure the routers are running software that supports new-style TLVs and configure them to advertise both old-style and new-style metrics. Routers that have not yet been upgraded continue advertising and processing only old-style TLVs. Reconfigured routers with new software will receive both types of TLVs and process both. The configuration is shown as follows :

 router isis      metric-style transition 

Step 3. When all the routers have been upgraded as described in Step 2, configure them as follows to advertise and accept only new-style TLVs:

 router isis      metric-style wide 

Step 4. Finally, metric values greater than 63 can now be configured because all the routers can now send and interpret routing received with new-style TLVs.

Method 2

In this method, routers advertise only one style of TLV at the same time but can understand both styles of TLV during the migration.

The benefit of this method is that LSPs remain approximately the same size during migration. Also, no ambiguity exists because the same information is not advertised twice inside a single LSP.

The disadvantage is that all routers must understand the new-style TLVs before any router can start advertising them. Therefore, this transition scheme is useful when transitioning the complete network (or area) to use wide metrics. This method also involves more steps.

This method requires the following transition steps:

Step 1. All routers are running older software or new software but in default mode, so they can advertise and use only old-style TLVs. Upgrade each router to new software that supports new-style TLVs and configure it to advertise old-style TLVs but also accept both TLV styles as follows:

 router isis      metric-style narrow transition 

Step 2. Reconfigure each router in turn to advertise only new-style TLVs but also to accept both styles of TLVs:

 router isis      metric-style wide transition 

Step 3. In this step, all the routers are configured for the last time to advertise and accept only new-style TLVs as follows:

 router isis      metric-style wide 

The migration is over and metrics greater than 63 can be configured as desired.

NOTE

Summary of Cisco IOS Configuration Commands

The following subcommands are available under router isis:

• metric-style narrow (default) ” Enables the router to advertise and accept only old-style TLVs

• metric-style wide ” Enables the router to advertise and accept only new-style TLVs

• metric-style transition ” Enables the router to advertise and accept both styles

• metric-style narrow transition ” Enables the router to advertise old-style TLVs and accept both styles

• metric-style wide transition ” Enables the router to advertise new-style TLVs and accept both styles

The following is a summary, of the steps used in two metric transition schemes:

Method 1: Narrow, to transition, to wide

Method 2: Narrow, to narrow transition, to wide transition, to wide