Like OSPF, EIGRP relies on several different
kinds of packets to maintain its various tables and establish complex
relationships with neighbor routers Figure .
The five EIGRP packet types are listed here:
- 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 re-establish 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, as shown in Figure .
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. That is, 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 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.
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 via the exchange of hello packets, and they use that information to
forge a stable relationship, despite 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. Note
also that acknowledgments can be made by piggybacking on other kinds of
EIGRP packets, such as reply packets.
Hello packets are always sent unreliably
and thus 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 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, alerting them to 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.
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