EIGRP route selection is perhaps what
distinguishes it most from other routing protocols. Following are its key
characteristics:
- EIGRP selects primary and backup routes that are kept
in the topology table (up to six per destination). The primary routes
are then moved to a routing table.
- Like OSPF, EIGRP supports several types of routes:
internal, external (that is, non-EIGRP), and summary routes.
- EIGRP uses the same composite metric as IGRP to
determine the best path. The metric can be based on five criteria. The
default criteria used are as follows:
---The smallest
bandwidth between source and destination
Delay --- Cumulative
interface delay along the path
Following are additional criteria that can be used.
These criteria are not recommended for use because they do not result in
the path selection choices that you may think.
--- Worst
reliability between source and destination based on keepalives
Loading --- Worst load on a
link between source and destination based on bits per second
MTU --- Smallest MTU in path
Note: EIGRP doesn't sent updates unless there is a
topology change. The reliability or loading of a link could change
dramatically but this wouldn't necessarily change the topology. EIGRP,
then, would not send any updates to advise of the change. Therefore, enabling the use of reliability or load does not really provide
any additional intelligence to the path selection process.
EIGRP uses the DUAL algorithm to calculate the best
route to a destination. DUAL selects routes based on the composite metric
and ensures that the selected routes are loop free. In Figure ,
the EIGRP metric determines that the three-hop path using the T1 lines is
a better route that the single 19.2-kbps link. RIP would select the single
19.2-kbps link by selecting the path based on hop count.
EIGRP uses the following process to determine which
routes to keep in the topology and route tables:
- DUAL is run on the topology table to determine the
best and loop-free primary and backup routes to each destination.
- "Best" is the lowest metric
route that is calculated by adding the metric between the next-hop
router and the destination (referred to as advertised distance)
to the metric between the local router and the next-hop router (the
total is referred to as feasible distance). For example, in Figure
,
from router A, the advertised distance to
network 7 using router B is 21, and the feasible distance is 31
because of the additional link cost between routers A and B, which is
10.
- The next-hop router(s) selected as the best path is
referred to as the successor. Multiple successors can exist if they
have the same feasible distance. All successors are added to the route
table. In Figure
,
router B is the successor for network 7.
- The next-hop router(s) for the backup path is
referred to as the feasible successor. If the successor route is no
longer valid and a suitable feasible successor exists, this feasible
successor replaces an invalid successor in the routing table without a
recomputation. More than one feasible successor can be kept at one
time. These routes do not need to have the same feasible distance, but
their advertised distance must be less than the feasible distance of
the successor route.
- The successors and feasible successors are kept in
the topology table, along with all other routes, and referred to as
possible successors. The only routes removed are those that have a
metric of infinity (unreachable).
Note: EIGRP uses the same composite metric
as IGRP to determine the best path. The default criteria used for that
metric are bandwidth (the smallest bandwidth between source and
destination) and delay (cumulative interface delay along the path).
|