Information Technology Reference
In-Depth Information
Classless Routing Protocols
What feature makes a routing protocol a
classless
routing protocol?
Classless routing protocols include the subnet mask with the network address in routing updates.
Classless routing protocols are required in most networks today because of their support for
VLSM
and
discontiguous
networks, among other features.
Classless routing protocols are
RIPv2
,
EIGRP
,
OSPF
,
IS-IS
, and
BGP
.
Convergence
Briefly describe convergence and explain why is it important?
Convergence is when all routers' routing tables are at a state of consistency. The network has con-
verged when all routers have complete and accurate information about the network. Convergence time
is the time it takes routers to share information, calculate best paths, and update their routing tables. A
network is not completely operable until the network has converged; therefore, most networks require
short convergence times.
Convergence properties include the
speed
of propagation of routing information and the
calculation
of
optimal paths. Routing protocols can be rated based on the speed to convergence; the faster the con-
vergence, the better the routing protocol. Generally,
RIP
and
IGRP
are slow to converge, whereas
EIGRP
and
OSPF
are faster to converge.
Metrics
There are cases when a routing protocol learns of more than one route to the same destination. To
select the best path, the routing protocol must be able to evaluate and differentiate among the available
paths. For this purpose, a metric is used. The exercise in the following section reinforces your knowl-
edge of metrics.
Metric Parameters Exercise
Two different routing protocols might choose different paths to the same destination because of using
different metrics.
In the following list, fill in the missing metric.
Hop count:
A simple metric that counts the number of routers a packet must traverse
■
Bandwidth:
Influences path selection by preferring the path with the highest bandwidth
■
Load:
Considers the traffic utilization of a certain link
■
Delay:
Considers the time a packet takes to traverse a path
■
Reliability:
Assesses the probability of a link failure, calculated from the interface error count
or previous link failures
■
Cost:
A value determined either by the IOS or by the network administrator to indicate a pref-
erence for a route
■
