Neighbor Relationships

Neighbor Relationships
Using Hello messages, EIGRP establishes and maintains neighbor relationships with neighboring
routers. This is a quality of a link-state routing protocol. EIGRP uses the Hello protocol, just like
OSPF, to establish and maintain the peering relationships with directly connected routers. OSPF
will be discussed in Chapter 5, “OSPF Operation in a Single Area,” and Chapter 6, “Interconnecting
OSPF Areas.” The Hello packets sent between EIGRP neighboring routers determine the
state of the connection between them. Once the neighbor relationship is established, the routers
then exchange route information.
120 Chapter 4  IGRP and EIGRP
Each EIGRP session running on a router maintains a neighbor table in which each router
stores information on all the routers known to be directly connected neighbors. The neighboring
router’s IP address, hold time interval, smooth round-trip timer (SRTT), and queue information
are all kept in this table, which is used to help determine when a neighbor is acquired
or lost, resulting in topology changes that need to be propagated to neighboring routers.
The only time EIGRP advertises its entire routing table is when two neighbors initiate communication.
When this happens, both neighbors advertise their entire routing tables to each
other. After each has learned its neighbor’s directly connected or known routes, only changes
to the routing table are propagated.
One thing to keep in mind about EIGRP is that it doesn’t broadcast Hello packets. Instead,
EIGRP will send multicast Hellos to the well-known multicast address of 224.0.0.10. Sending
a multicast Hello instead of a broadcast allows any device not running EIGRP to filter the
packet on the network interface card (NIC). Doing this cuts down on needless processing of
packets. Remember, if this had been a broadcast packet, all of the devices on the network would
need to process the packet, causing undue CPU load on end stations attached to this network.
Hello packets are multicast at intervals of five seconds. For multipoint interfaces of X.25,
Frame Relay, and ATM, as well as non-packet-switched networks like BRI ISDN that have access
speeds equal to or less than the speed of a T1, the Hello packet will be unicast every 60 seconds.
Each Hello packet sent contains the EIGRP version number, the AS number, the K-values,
and the hold time. In order for neighboring routers to form adjacencies, they must be using the
same AS number and K-values.
When initial Hello messages are sent out, replies to the Hello packets are sent with the neighboring
router’s topology table (which is different from 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
(acknowledgment) 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 that come online. Then when
the originating router receives information from its neighbors, the route calculation process begins.
Now that you have a good understanding of how EIGRP neighbors form adjacencies, let’s
take a look at how EIGRP chooses the best routes.