EIGRP PDM, Protocol Dependent Module

EIGRP supports different Network Layer Protocols, i.e IPv4, IPv6, IPX and AppleTalk (Though Last two are useless now). To support all of the protocols at a time EIGRP uses PDM for each of the protocols separately. Each PDM will maintain a separate series of tables containing the routing information that applies to a specific protocol, means there be separate IPv4/EIGRP table and IPv6/EIGRP table in the router.
This way EIGRP can add PDMs to easily adapt to new or revised routed protocols such as IPv6. Each PDM is responsible for all functions related to its specific routed protocol.
The EIGRP module sends and receives packets but passes received information to DUAL, which makes routing decision, i.e chooses the best route from Local Topology Tables and put into the Routing Table.

The main function of IPv4/EIGRP module are:
  • Send and receive EIGRP packets that bear IP data.
  • Notify DUAL (Diffusing Update Algorithm) 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.”