Protocol-dependent modules (PDMs) are used by the

routing protocol A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packet ...

EIGRP Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that is used on a computer network for automating routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary pr ...

to make decisions about adding routes learned from other sources; for example other routers or

routing protocols A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packet ...

to the

routing table In computer networking, a routing table, or routing information base (RIB), is a data table stored in a router or a network host that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with t ...

. In fact EIGRP has the capability for routing several different protocols including IPv4 and IPv6 using protocol-dependent modules (PDMs). The PDM is also capable of carrying information from the routing table to the topology table. EIGRP offers support for various routed protocols (e.g. Internet Protocol Version 6 (IPv6), IP,

IPX Internetwork Packet Exchange (IPX) is the network layer protocol in the IPX/SPX protocol suite. IPX is derived from Xerox Network Systems' IDP. It also has the ability to act as a transport layer protocol. The IPX/SPX protocol suite was very po ...

AppleTalk AppleTalk is a discontinued proprietary suite of networking protocols developed by Apple Computer for their Macintosh computers. AppleTalk includes a number of features that allow local area networks to be connected with no prior setup or the ...

), and has added support for Service Routing ( SAF) PDMs. The only other routing protocol that comes with support for multiple network layer protocols is Intermediate System-to-Intermediate System (IS-IS). "In theory, EIGRP can add PDMs to easily adapt to new or revised routed protocols such as

IPv6 Internet Protocol version 6 (IPv6) is the most recent version of the Internet Protocol (IP), the communications protocol that provides an identification and location system for computers on networks and routes traffic across the Internet. I ...

. Each PDM is responsible for all functions related to its specific routed protocol. The IP-EIGRP module is responsible for the following functions: # Send and receive EIGRP packets that bear IP data. # Notify DUAL (

Diffusing Update Algorithm The diffusing update algorithm (DUAL) is the algorithm used by Cisco's EIGRP routing protocol to ensure that a given route is recalculated globally whenever it might cause a routing loop. It was developed by J.J. Garcia-Luna-Aceves at SRI Internat ...

) of new IP routing information that is received. # Maintain the results of DUAL routing decisions in the IP routing table. # Redistribute routing information that was learned by other IP-capable routing protocols." When a newly discovered neighbor is learned, the address and interface of the neighbor are recorded, and this information is held in the neighbor table, stored in RAM. There is one neighbor table per each protocol-dependent module.

References

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