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EIGRP data structure


EIGRP data structure
3.1.5

Like OSPF, EIGRP relies on different types of packets to maintain its tables and establish relationships with neighbor routers. This page will describe these packet types.
The following are the five types of EIGRP packets:
  • Hello
  • Acknowledgment
  • Update
  • Query
  • Reply
EIGRP relies on hello packets to discover, verify, and rediscover neighbor routers. Rediscovery occurs if EIGRP routers do not receive hellos from each other for a hold time interval but then re-establish communication.
EIGRP routers send hellos at a fixed, but configurable interval called the hello interval. The default hello interval depends on the bandwidth of the interface. On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10.
EIGRP routers store information about neighbors in the neighbor table. The neighbor table includes the Sequence Number (Seq No) field to record the number of the last received EIGRP packet that each neighbor sent. The neighbor table also includes a Hold Time field which records the time the last packet was received. Packets should be received within the Hold Time interval period to maintain a Passive state. The Passive state is a reachable and operational status.
If EIGRP does not receive a packet from a neighbor within the hold time, EIGRP considers that neighbor down. DUAL then steps in to re-evaluate the routing table. By default, the hold time is three times the hello interval, but an administrator can configure both timers as desired.
OSPF requires neighbor routers to have the same hello and dead intervals to communicate. EIGRP has no such restriction. Neighbor routers learn about each of the other respective timers through the exchange of hello packets. They then use that information to forge a stable relationship regardless of unlike timers.
Hello packets are always sent unreliably. This means that no acknowledgment is transmitted.
EIGRP routers use acknowledgment packets to indicate receipt of any EIGRP packet during a reliable exchange. RTP provides reliable communication between EIGRP hosts. A message that is received must be acknowledged by the recipient to be reliable. Acknowledgment packets, which are hello packets without data, are used for this purpose. Unlike multicast hellos, acknowledgment packets are unicast. Acknowledgments can be attached to other kinds of EIGRP packets, such as reply packets.
Update packets are used when a router discovers a new neighbor. EIGRP routers send unicast update packets to that new neighbor so that it 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, which alerts them to the change. All update packets are sent reliably.
An EIGRP router uses 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. A query is then multicasted to all neighbors in an attempt to locate a successor to the destination network. Neighbors must send replies that either provide information on successors or indicate that no information is available. Queries can be multicast or unicast, while replies are always unicast. Both packet types are sent reliably.
The next page will describe the EIGRP algorithm

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