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| 5.3 |  | EIGRP Components |
| 5.3.1 |  | EIGRP packet types |

Like OSPF, EIGRP relies on several different kinds of packets to maintain its various tables and establish complex relationships with neighbor routers. The five EIGRP types are listed as follows:

- Hello
- Acknowledgment
- Update
- Query
- Reply

The following sections describe these packet types in detail.

Hello Packets
EIGRP relies on hello packets to discover, verify, and rediscover neighbor routers. Rediscovery occurs if EIGRP routers do not receive each other's hellos for a hold time interval but then reestablish communication.

EIGRP routers send hellos at a fixed and configurable interval, called the hello interval. The default hello interval depends on the bandwidth of the interface.

EIGRP hello packets are multicast. On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10.

An EIGRP router stores information about neighbors in the neighbor table, including the last time that each neighbor responded. The information will only be stored if any of its EIGRP packets, hello or otherwise, is received. If a neighbor is not heard from for the duration of the hold time, EIGRP considers that neighbor down, and DUAL must step in to reevaluate the routing table. By default, the hold time is three times the hello interval, but an administrator can configure both timers as desired.

Recall that OSPF requires neighbor routers to have the same hello and dead intervals to communicate. EIGRP has no such restriction. Neighbor routers learn about each other's respective timers through the exchange of hello packets. They use that information to forge a stable relationship, unlike timers.

Acknowledgment Packets
An EIGRP router uses acknowledgment packets to indicate receipt of any EIGRP packet during a reliable exchange. Recall that RTP can provide reliable communication between EIGRP hosts. To be reliable, a sender's message must be acknowledged by the recipient. Acknowledgment packets, which are dataless hello packets, are used for this purpose. Unlike multicast hellos, acknowledgment packets are unicast. Acknowledgments can also be made by piggybacking on other kinds of EIGRP packets, such as reply packets.

Hello packets are always sent unreliably and therefore do not require acknowledgment.

Update Packets
Update packets are used when a router discovers a new neighbor. An EIGRP router sends unicast update packets to that new neighbor so that the new router can add to its topology table. More than one update packet may be needed to convey all the topology information to the newly discovered neighbor.

Update packets are also used when a router detects a topology change. In this case, the EIGRP router sends a multicast update packet to all neighbors, alerting them about the change.

All update packets are sent reliably.

Query and Reply Packets
EIGRP routers use query packets whenever it needs specific information from one or all of its neighbors. A reply packet is used to respond to a query.

If an EIGRP router loses its successor and cannot find a feasible successor for a route, DUAL places the route in the active state. At this point, the router multicasts a query to all neighbors, searching for a successor to the destination network. Neighbors must send replies that either provide information on successors or indicate that no successor information is available.

Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.

| Interactive Media Activity Drag and Drop: EIGRP Timer Basics Upon completion of this activity, the student will be able to identify the default hello intervals and hold times for EIGRP.

| Interactive Media Activity Drag and Drop: EIGRP Packet Types Upon completion of this activity, the student will be able to identify the different EIGRP packet types.

| Web Links Enhanced Interior Gateway Routing Protocol http://www.cisco.com/warp/public/103/eigrp-toc.html