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.