notes: EIGRP Operation

Enhanced Interior Gateway Routing Protocol (EIGRP) is a hybrid routing protocol—combining features of both distance vector and link-state routing protocols.

- VLSM support.
- Hybrid IGP using DUAL (Diffusing Update Algorithm). - rapid convergence
- Uses own transport protocol: 88.
- Multicasts to destination 224.0.0.10 (ttl=0) using RTP, the receiving neighbor unicasts an acknowledgment.
- Unequal-cost load sharing up to 16 links.
- EIGRP does not form neighbors over secondary networks/IP's.
- Low CPU utilization—with typically only hellos and partial updates being sent on a link
- Incremental updates
- Scalability
- Ease of configuration
- Automatic route summarization, or manual route summarization
- MD5 route authentication


- Route entries are classified into one of three categories:

1. Interior routes: Is a path to a subnet of the network address of the data link on which the update is being broadcast.

>> Interior route is "local" to the major network to which the advertising and receiving router are commonly connected.

>> 192.168.2.192/26 is advertised to 192.168.2.64/26 within same AS as interior route because it falls within the same major network.

2. System routes: Is a path to a network address, which has been summarized by a network boundary router.

>> 192.168.3.0 is advertised to 192.168.2.0, within the same AS as a system route.

3. Exterior routes: Is a path to a default network, or a network in another autonomous system

>> 196.12.1.0 is advertised to 64.32.0.0 in a separate AS as a exterior route.


eigrp decision making process - uses composite metric

- internal routes are preferred first
- calculated metric used to select best

- External routes are last
- calculated metric used to select best