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Enhanced Interior Gateway Routing Protocol
routing information, which includes the routes learned and the advertise-
ment of topology changes.
Route redistribution, which will be covered in its own section later in this
chapter, allows routes learned by one AS EIGRP session to be shared with
another session. When route distribution occurs, the routes are tagged as
being learned from an external EIGRP session. Each type of route is assigned
its own administrative distance value.
Neighbor Relationships
Using Hello messages, EIGRP sessions establish and maintain neighbor rela-
tionships with neighboring routers. This is a quality of a link-state routing
protocol. EIGRP uses the Hello protocol just like OSPF does, as discussed in
Chapter 5, to establish and maintain the peering relationships with directly
connected routers. The Hello packets sent between EIGRP neighboring rout-
ers determine the state of the connection between them. Once the neighbor
relationship is established using the Hello protocol, the routers then
exchange route information.
Each EIGRP session running on a router establishes a neighbor table in
which each router stores information on all the routers known to be directly
connected neighbors. The neighboring routers' IP address, hold time inter-
val, smooth round-trip timer (SRTT), and queue information are all kept in
the table, which is used to help determine when there are topology changes
that need to be propagated to the neighboring routers.
The only time EIGRP advertises its entire routing table is when two neigh-
bors initiate communication. When this happens, both neighbors advertise
their entire routing tables to one another. After each has learned its neigh-
bor's directly connected or known routes, only changes to the routing table
are propagated.
When Hello messages are sent out each of the routers' interfaces, replies
to the Hello packets are sent with the neighboring router's topology table
(which is not the routing table) and include each route's metric information
with the exception of any routes that were already advertised by the router
receiving the reply. As soon as the reply is received, the receiving router sends
out what is called an ACK (acknowledgement) packet to acknowledge
receipt, and the routing table is updated if any new information is received
from the neighboring router. Once the topology table has been updated, the
originating router will then advertise its entire table to any new neighbors
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