OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

10.3. Route Tagging

Both OSPF and IS-IS allow the association of a value called a route tag with external prefixes. You can think of route tags as a protocol extensibility feature in that they enable you to arbitrarily assign some administratively meaningful value to external prefixes to associate them with some group that you define. You can then create routing policies that act on the external prefixes according to their assigned tags. In this, route tags for IGPs^[3] serve the same purpose as BGP community attributes.

^[3] RIPv2 and Cisco EIGRP also support route tags.

The Route Tag field is provided in OSPF types 5 and 7 LSAs (Figure 10.10). A Route Tag field is also provided by an IS-IS type 1 sub-TLV (Figure 10.11), which is in turn carried ...