6.2 EIGRP Technology
6.2.5 Protocol-specific modules
As indicated earlier in the chapter, one of the key design principles guiding the development of EIGRP was complete independence from routed protocols. Therefore, EIGRP implemented a modular approach to supporting routed protocols. Many other protocols are either specifically designed for a single routed protocol---such as IP, AppleTalk, and so on---or have mechanisms for supporting multiple protocols. EIGRP has such native mechanisms, but they are completely modular. In theory, EIGRP can be easily retrofitted to support any new routed protocols that may be developed by just adding another protocol-specific module.

Each protocol-specific module is responsible for all functions related to its specific routed protocol. The IP-EIGRP module is responsible for the following, for example:

  • Sending and receiving EIGRP packets that bear IP data
  • Notifying DUAL of new IP routing information that is received
  • Maintaining the results of DUAL's routing decisions in the IP routing table
  • Redistributing routing information that was learned by other IP-capable routing protocols

IP-EIGRP can redistribute routes learned from other IP-capable routing protocols, including OSPF, RIP, Integrated Intermediate System-Intermediate System (IS-IS), and Border Gateway Protocol (BGP). EIGRP has comparable modules for supporting both AppleTalk and IPX. The AppleTalk module (AT-EIGRP) can redistribute routes learned from the Routing Table Maintenance Protocol (RTMP). IPX-EIGRP can redistribute routing information from Novell's proprietary version of RIP as well as that company's Service Advertisement Protocol (SAP) and Novell Link State Protocol (NLSP).

EIGRP's IP-EIGRP module brought support for many of the updates to IP that Cisco's IGRP customer base had been clamoring for. Specifically, IP-EIGRP introduced support for VLSM as well as CIDR. IGRP did not support either of these features.