An IP frame showing the protocol type to be EIGRP

FIGURE 4 . 5 An IP frame showing the protocol type to be EIGRP
Updates can follow two paths. If a route update contains a better metric or a new route, the
routers simply exchange the information. If the update contains information that a network is
unavailable or that the metric is worse than before, an alternate path might need to be found.
If the new metric is still better than the metric of the feasible successor, the entry will remain,
and its metric will be adjusted. When a new path must be found, the router first searches the
topology database for feasible successors. If no feasible successors are found, a query is multicast
to all adjacent routers. Each router then responds to the query. Depending on how the
Frame Header
Frame Payload
IP Header Protocol Packet Payload
88 = EIGRP
CRC
router responds, different paths will be taken. After the intermediate steps are taken, either of
two final actions can occur:
 If route information is eventually found, the route is added to the routing table, and an
update is sent.
 If the responses from the adjacent routers do not contain route information, the route is
removed from the topology and routing tables.
After the routing table has been updated, the new information is sent to all adjacent routers
via multicast.
Reliable Transport Protocol (RTP) is the key to EIGRP routing updates. RTP allows for
guaranteed delivery in sequential order of EIGRP routing updates.
EIGRP RTP multicasts Hello packets, queries, and update packets whenever possible. These
multicast packets are sent to the well-known multicast address of 224.0.0.10. Unicast packets
are always used for acknowledgments (ACKs), which are basically empty Hello packets, and
replies. Unicast packets will also be used to send Hello packets for multipoint interfaces on
X.25, Frame Relay, and ATM that have access speeds equal to or less than the speed of a T1.
Hello packets are sent unreliably and never acknowledged.
EIGRP implements a unique mechanism known as pacing in order to prevent routing
updates from consuming too much bandwidth on lower speed links. Pacing allows EIGRP to
regulate the amount of traffic it sends to a portion of the interface’s bandwidth. The traffic we’re
referring to is Hello packets, routing updates, queries, replies, and acknowledgments.
The default setting for pacing in EIGRP is 50 percent of the bandwidth on any given interface.
This default setting can be adjusted with the following command in interface configuration mode:
ip bandwidth-percent eigrp as-number percent
as-number = Autonomous System Number
percent = percent of bandwidth EIGRP may use
This is an important command to configure if you have manipulated routing by
changing the bandwidth statement. Note also that EIGRP will not sustain such
utilization, but bursts could occasionally consume 100 percent of a link’s bandwidth.
This limitation is a safeguard against such occurrences.