In this section, you learn how EIGRP discovers
neighbors, discovers routes, chooses routes, and maintain routes when
there is a change in the network. This section focuses on the following
elements of EIGRP router performance:
- Building the neighbor table
- Discovering routes
- Choosing routes
- Maintaining routes
Recall that a neighbor table is a
table that is maintained by the EIGRP router and that lists adjacent
routers. Its purpose is to ensure communication between each
of the directly connected neighbors.
Like OSPF, EIGRP routers multicast hello
packets to discover neighbor routers. If you
recall, adjacent routers are the only ones that can exchange routing
information. Each router builds a neighbor table from hello packets that
it receives from adjacent EIGRP routers running the same network-layer
protocol.
Hello packets are sent out periodically to verify an EIGRP neighbor's availability. On a
multi-access, point-to-point connection, and multi-point configurations with bandwidth greater than T1, hellos are sent every five seconds, by default. On an NBMA network, such as multipoint circuits with bandwidth
T1 or less, hellos are sent every 60 seconds, by default.
EIGRP maintains a neighbor table for each
configured network-layer protocol. Use the show ip eigrp neighbors
command to read the table, as shown in the main figure.
The neighbor table includes the following
key elements:
--- The
network-layer address of the neighbor router.
Queue
count --- Indicates
the number of packets waiting in queue to be sent. If this value is
constantly higher than zero, then there may be a congestion problem at the
router. A zero means that there are no EIGRP packets in the queue.
Smooth Round Trip Timer (SRTT) ---
Indicates
the average time it takes to send and receive packets from a neighbor.
This timer is used to determine the retransmit interval (RTO).
Hold Time ---
The
interval to wait without receiving anything from a neighbor before
considering the link unavailable. Originally, the expected packet was a
hello packet, but in current Cisco IOS software releases, any EIGRP
packets received after the first hello will reset the timer.
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