plies are always sent in response to queries to indicate to the originator that it does not need to go into Active state because it has feasible successors. Replies are unicast to the originator of the query. Both queries and replies are transmitted reliably.
Request packets are used to get specific information from one or more neighbors. Request packets are used in route server applications. They can be multicast or unicast. Requests are transmitted unreliably.
EIGRP has the notion of internal and external routes. Internal routes are ones that have been originated within an EIGRP autonomous system (AS). Therefore, a directly attached network that is configured to run EIGRP is considered an internal route and is propagated with this information throughout the EIGRP AS. External routes are ones that have been learned by another routing protocol or reside in the routing table as static routes. These routes are tagged individually with the identity of their origination.
External routes are tagged with the following information:
The router ID of the EIGRP router that redistributed the route.
The AS number where the destination resides.
A configurable administrator tag.
Protocol ID of the external protocol.
The metric from the external protocol.
Bit flags for default routing.
As an example, suppose there is an AS with three border routers. A border router is one that runs more than one routing protocol. The AS uses EIGRP as the routing protocol. Let's say two of the border routers, BR1 and BR2, use Open Shortest Path First (OSPF) and the other, BR3, uses Routing Information Protocol (RIP).
Routes learned by one of the OSPF border routers, BR1, can be conditionally redistributed into EIGRP. This means that EIGRP running in BR1 advertises the OSPF routes within its own AS. When it does so, it advertises the route and tags it as an OSPF learned route with a metric equal to the routing table metric of the OSPF route. The router-id is set to BR1. The EIGRP route propagates to the other border routers. Let's say that BR3, the RIP border router, also advertises the same destinations as BR1. Therefore BR3, redistributes the RIP routes into the EIGRP AS. BR2, then, has enough information to determine the AS entry point for the route, the original routing protocol used, and the metric. Further, the network administrator could assign tag values to specific destinations when redistributing the route. BR2 can use any of this information to use the route or re-advertise it back out into OSPF.
Using EIGRP route tagging can give a network administrator flexible policy controls and help customize routing. Route tagging is particularly useful in transit ASes where EIGRP would typically interact with an inter-domain routing protocol that implements more global policies. This combines for very scalable policy based routing.
EIGRP provides compatibility and seamless inter-operation with IGRP routers. This is important so users can take advantage of the benefits of both protocols. The compatibility features do not require users to have a flag day to enable EIGRP. EIGRP can be enabled in strategic places carefully without disruption to IGRP performance.
There is an automatic redistribution mechanism used so IGRP routes are imported into EIGRP and vice versa. Since the metrics for both protocols are directly translatable, they are easily comparable as if they were routes that originated in their own AS. In addition, IGRP routes are treated as external routes in EIGRP so the tagging capabilities are available for custom tuning.
IGRP routes take precedence over EIGRP routes by default. This can be changed with a configuration command that does not require the routing processes to restart.
The following network diagram illustrates how DUAL converges. The example focuses on destination N only. Each node shows its cost to N (in hops). The arrows show the node's successor. So, for example, C uses A to reach N and the cost is 2.
If the link between A and B fails, B sends a query informing its neighbors that it has lost its feasible successor. D receives th