to match acknowledgments with data packets. Figure
displays the output of a sample show ip eigrp neighbors
command. This command reveals various key elements that will be
discussed later in this module. Interactive Media
Activity Point and Click: EIGRP Neighbor Table
Upon completion of this activity, the student will be able
to identify the key elements of the EIGRP neighbor table.
Content 2.2 EIGRP Components and Operation
2.2.3 EIGRP Topology Table When the router
dynamically discovers a new neighbor, it sends an update about
the routes that it knows to its new neighbor. The new neighbor
then sends an update about all the routes that it knows to the
router. These updates populate the topology table, which
contains all the destinations advertised by neighboring
routers. It is important to note that if a neighbor is
advertising a destination, it must be using that route to
forward packets. This rule must be strictly followed by all
distance vector protocols. The topology table also maintains
the advertised distance (AD) that each neighbor advertises for
each destination, and the feasible distance (FD) that this
router would use to reach the destination via that neighbor.
The topology table is updated when a directly connected route
or interface changes or when a neighboring router reports a
change to a route. A destination entry in the topology table
can be in one of two states: active or passive. A destination
is in passive state when the router is not performing a
recomputation. It is in active state when the router is
performing a recomputation. If feasible successors are always
available, a destination never has to go into the active state
and avoids a recomputation. The desired state is passive. A
recomputation occurs when a destination has no feasible
successors. The router initiates the recomputation by sending a
query packet to each of its neighboring routers. If the
neighboring router has a route for the destination, it sends a
reply packet; if it does not have a route, it sends a query
packet to its neighbors. In this case, the route is also in
active state in the neighboring router. When a destination is
in active state, a router cannot change the destination’s
routing table information. After a router has received a reply
from each neighboring router, the destination entry returns to
the passive state, and the router can select a successor.
Figure displays the output of a sample show ip eigrp
topology command.
Content 2.2 EIGRP
Components and Operation 2.2.4 EIGRP Routing
Table The routing table is compiled from information in the
topology table. A router compares all FDs to reach a specific
network and then selects the route with the lowest FD, which is
the successor route, and places it in the routing table. The FD
for the chosen route becomes the EIGRP routing metric to reach
that network in the routing table. Figure displays the output
of a sample show ip route eigrp command. Interactive
Media Activity Matching: EIGRP Terminology
Upon completion of this activity, the student will be able to
identify EIGRP terminology.
Content 2.2 EIGRP
Components and Operation 2.2.5 EIGRP Packet
Formats Class EIGRP uses five generic packet types . The
following sections describe these packet types in detail.
Hello Packets
EIGRP relies on hello packets to
discover and verify neighbor routers. EIGRP routers send hello
packets at a fixed and configurable interval, called the hello
interval. The default hello interval depends on the bandwidth
of the interface. EIGRP hello packets are multicast. On IP
networks, EIGRP routers send hello packets 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. The information is stored only if an
EIGRP packet is received. If a neighbor is not heard from
during the hold time, EIGRP considers that neighbor to be down
and DUAL steps in to reevaluate the routing table. By default,
the hold time is three times the hello interval, but both
timers can be configured as desired. Unlike OSPF, which
requires that neighbor routers have the same hello and dead
intervals to communicate, EIGRP has no such restriction.
Neighbor routers learn about each other’s respective timers
through the exchange of hello packets. They use that
information to forge a stable relationship.
Update
Packets
Updates convey which destinations are
reachable. When a new neighbor is discovered, update packets
are unicasted so that the neighbor can build up 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 about the change. All
update packets are sent reliably.
Query Packets
An EIGRP router uses query packets whenever it needs
specific information from one or all of its neighbors. Queries
and replies are used when an EIGRP router loses its successor
and cannot find a feasible successor for a route. When this
occurs, DUAL places the route in active state, and the router
multicasts a query to all neighbors searching for a successor.
Queries are transmitted reliably.
Reply
Packets
A reply packet is used to respond to a query.
Replies are always unicasted to indicate to the originator that
it does not need to go into active state because it has
feasible successors. Replies are transmitted reliably.
Acknowledgment Packets
To be reliable, a
sender’s message must be acknowledged by the recipient. An
EIGRP router uses an acknowledgment packet, which is a dataless
hello packet, to indicate that it received an EIGRP packet
during a reliable exchange. Unlike multicast hello packets,
acknowledgment packets are unicast. Acknowledgments can also be
bundled with other types of EIGRP packets, such as reply
packets. Hello packets are always sent unreliably and therefore
do not require acknowledgment. Note
EIGRP also uses
request packets to get specific route information from one or
more neighbors. A request packet can be multicast or unicast.
Requests are transmitted unreliably.
Interactive Media
Activity Drag and Drop: EIGRP Timer Basics
Upon completion of this activity, the student will be able to
identify the default hello intervals and hold times for EIGRP.
Interactive Media Activity Drag and Drop: EIGRP
Packet Types
Upon completion of this activity, the
student will be able to identify the different EIGRP packet
types.
Content 2.2 EIGRP Components and
Operation 2.2.6 EIGRP Packet Exchange
Example Discovering and establishing neighbor routes occurs
simultaneously in EIGRP. The following is a high-level
description of the process, using the topology in Figure as an
example: - Router A comes up on the link and sends a
hello packet through all of its EIGRP-configured
interfaces.
- Router B receives the hello packet
and sends a hello packet in return to router A. This hello
packet contains the router IDs of the neighbors that router B
knows about, including the new entry for router A.
Neighbor adjacency is established when router A sees its router
ID in the neighbor field of the hello packet that it received
from router B. - Router B then forwards an update
packet containing all the routes and metrics in its routing
table (except those learned through that interface because of
the split horizon rule).
- Router A replies with
an acknowledgement packet, indicating that it received the
update information.
- Router A assimilates all
update packets into its topology table. The topology table
includes all destinations advertised by neighboring routers,