Route Tagging

Enhanced IGRP supports internal and external routes. Internal routes originate within
an Enhanced IGRP AS. Therefore, a directly attached network that is configured to run Enhanced IGRP is considered an internal route and is propagated with this information throughout the Enhanced IGRP AS. External routes are learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origin.

External routes are tagged with the following information:

• Router ID of the Enhanced IGRP router that redistributed the route
• AS number of the destination
• Configurable administrator tag
• ID of the external protocol
• Metric from the external protocol
• Bit flags for default routing

Route tagging allows the network administrator to customize routing and maintain flexible policy controls. Route tagging is particularly useful in transit ASs, where Enhanced IGRP typically interacts with an interdomain routing protocol that implements more global policies, resulting in a very scalable, policy-based routing.

Enhanced IGRP Packet Types

Enhanced IGRP uses the following packet types: hello and acknowledgment, update, and query and reply.

Hello packets are multicast for neighbor discovery/recovery and do not require acknowledgment. An acknowledgment packet is a hello packet that has no data. Acknowledgment packets contain a nonzero acknowledgment number and always are
sent by using a unicast address.

Update packets are used to convey reachability of destinations. When a new neighbor is discovered, unicast update packets are sent so that the neighbor can build up its topology table. In other cases, such as a link-cost change, updates are multicast. Updates always are transmitted reliably.

Query and reply packets are sent when a destination has no feasible successors. Query packets are always multicast. Reply packets are sent in response to query packets to instruct the originator not to recompute the route because feasible successors exist. Reply packets are unicast to the originator of the query. Both query and reply packets are transmitted reliably.