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Understanding EIGRP Concepts
Computer Networking
11th Grade

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was Cisco proprietary until 2013.Now its informational RFC.It is a advanced distance vector protocol.It is an IGP.
Why to use EIGRP
■ EIGRP uses a robust metric based on both link bandwidth and link delay, so routers
make good choices about the best route to use (see Figure 9-2).
■ EIGRP converges quickly, meaning that when something changes in the internetwork,
EIGRP quickly finds the currently best loop-free routes to use.
The Concept of a Distance and a Vector
The term distance vector describes what a router knows about each route. At the end of
the process, when a router learns about a route to a subnet, all the router knows is some
measurement of distance (the metric) and the next-hop router and outgoing interface to use
for that route (a vector, or direction).Distance: The metric for a possible route
Vector: The direction, based on the next-hop router for a possible route
Route poisoning
DV protocols help prevent routing loops by ensuring that every router learns that the route
has failed, through every means possible, as quickly as possible. One of these features,
route poisoning, helps all routers know for sure that a route has failed.
Route poisoning refers to the practice of advertising a failed route, but with a special metric
value called infinity.
EIGRP neighbor requirements
■ It must pass the authentication process if used.
■ It must use the same configured autonomous system number (which is a configuration
■ The source IP address used by the neighbor’s Hello must be in the same subnet as the
local router’s interface IP address/mask.
■ The routers' EIGRP K-values must match. (However, Cisco recommends to not change
these values.)
Reliable Transport Protocol
EIGRP sends update messages without UDP or TCP, but it does use a protocol called
Reliable Transport Protocol (RTP). RTP provides a mechanism to resend any EIGRP
messages that are not received by a neighbor. By using RTP, EIGRP can better avoid loops
because a router knows for sure that the neighboring router has received any updated routing
information. (The use of RTP is just another example of a difference between basic DV
protocols like RIP, which have no mechanism to know whether neighbors receive update
messages, and the more advanced EIGRP.)
EIGRP update messages
EIGRP uses EIGRP update messages to send topology information to neighbors. These
update messages can be sent to multicast IP address if the sending router needs
to update multiple routers on the same subnet; otherwise, the updates are sent to the unicast
IP address of the particular neighbor. (Hello messages are always sent to the
multicast address.) The use of multicast packets on LANs allows EIGRP to exchange routing
information with all neighbors on the LAN efficiently.
Feasible distance (FD):
The local router’s composite metric of the best route to reach a
subnet, as calculated on the local router
Reported distance (RD):
The next-hop router’s best composite metric for that same subnet
Feasibility condition rule
A router determines whether a route is a feasible successor based on the feasibility condition:
If a nonsuccessor route’s RD is less than the FD, the route is a feasible successor route.
DUAL -Diffusing Update Algorithm
When a route fails, and the route has no feasible successor, EIGRP uses a distributed algorithm
called Diffusing Update Algorithm (DUAL) to choose a replacement route. DUAL
sends queries looking for a loop-free route to the subnet in question. When the new route
is found, DUAL adds it to the routing table.
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