Content

| 5.3 |  | EIGRP Components |
| 5.3.4 |  | Route tagging with EIGRP |

Not only does the topology table track information regarding route states, but it also can record special information about each route. EIGRP classifies routes as either internal or external. EIGRP uses a process called route tagging to add special tags to each route. These tags identify a route as internal or external, and may include other information as well.

Internal routes originate from within the EIGRP AS. External routes originate from outside the system. Routes learned (redistributed) from other routing protocols, such as RIP, OSPF, and IGRP are external. Static routes originating from outside the EIGRP AS and redistributed inside are also external routes.

All external routes are included in the topology table and are tagged with the following information:

- The identification number, known as router ID, of the EIGRP router that redistributed the route into the EIGRP network
- The AS number of the destination
- The protocol used in that external network
- The cost or metric received from that external protocol
- The configurable administrator tag

Figure shows a specific topology table entry for an external route.

Using route tagging, particularly the administrator tag, gives an advantage when developing a precise routing policy. The administrator tag can be any number between 0 and 255. This is a custom tag that can be used to implement a special routing policy. External routes can be accepted, rejected, or propagated based on any of the route tags, including the administrator tag. Configuring an administrator tag in this way affords a high degree of control. This level of precision and flexibility proves especially useful when EIGRP networks interact with BGP networks, which themselves are policy-based. BGP will be covered in depth in Module 9.