The protocol used by the source router determines which set of tables is used to store the incoming data.

The AppleTalk implementation learns its routes from the routing table maintenance protocol.

The Novell IPX implementation receives its information from the Novell routing information protocol (RIP) or service advertisement protocol (SAP).

There are a variety of sources from which the IP implementation can gather its routing information. These include open shortest path first (OSPF), RIP, intermediate system-to-intermediate system (IS-IS), exterior gateway protocol (EGP), and border gateway protocol (BGP).
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Neighbor tables record the address and interface of each directly connected router.

They also record information required by RTP.

RTP uses sequence numbers in order to match packets
with data.

The last sequence number transmitted by a neighbor is recorded so that out-of-sequence packets can be detected.

Neighbor tables are also used to store round-trip timers.

The timers are used in connection with the possible retransmission of packets on a per-neighbor basis.
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Enhanced IGRP uses the same vector of metrics as IGRP.

These metrics are

    • bandwidth

    • delay

    • reliability

    • load

The bandwidth value is based on the type of interface.

This value can be modified by using the bandwidth router command.

Delay refers to propagation delay.

You can use the delay command to modify this value,
which can be useful when optimizing routing in networks
with satellite links.

Reliability and load are both dynamically computed as a rolling weighted average over five seconds.

By default, Enhanced IGRP computes the metric for a
route by using the minimum bandwidth of each hop in
the path and adding a media-specific delay for each hop.
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DUAL uses the following method to compute a feasible successor:

• DUAL determines which neighbors advertise a distance 
  to the destination that is less than the best metric 
  during an active to passive transition

• DUAL calculates the minimum computed cost to the
  destination

• DUAL determines which neighbor(s) offer a computed
  cost to the destination that is equal to the minimum
  cost as calculated by DUAL
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