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DUAL can select alternate routes based on the
tables kept by EIGRP. By building these tables, every EIGRP router can
track all the routing information in an AS, not just the "best"
routes.
The following sections examine the neighbor
table, the routing table, and the topology table in detail and provide an
example of each. In addition, we will look at the various packet types
used by EIGRP to build and maintain these tables.
The Neighbor Table
The most important table in EIGRP is the neighbor table (refer to Figure ).
The neighbor relationships tracked in the neighbor table is the basis for
all the EIGRP routing update and convergence activity.
The neighbor table contains information
about adjacent neighboring EIGRP routers. Whenever a new neighbor is
discovered, the address of that neighbor and the interface used to reach
it are recorded in a new neighbor table entry.
A neighbor table is used to support
reliable, sequenced delivery of packets. One field in each row of the
table includes the sequence number of the last packet received from that
neighbor. EIGRP uses this field to acknowledge a neighbor's transmission
and to identify packets that are out of sequence.
As shown in Figure ,
an EIGRP neighbor table includes the following key elements:
- Neighbor address (Address) - The
network-layer address of the neighbor router.
- Hold time (Hold Uptime) - 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.
- Smooth Round-Trip Timer (SRTT) -
The average time that it takes to send and receive packets from a
neighbor. This timer is used to determine the retransmit interval (RTO).
- Queue count (Q Cnt) - 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.
Note that an EIGRP router can maintain
multiple neighbor tables, one for each PDM running (e.g., IP, IPX, and AppleTalk as
shown in Figure ). A router must run a unique EIGRP process for each routed
protocol.
The Routing Table
The routing table contains the routes installed by DUAL as the best
loop-free paths to a given destination as shown in Figure .
EIGRP will maintain up to four routes per destination. These routes can be
of equal or unequal cost. EIGRP routers maintain a separate routing table
for each routed protocol.
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