Information Technology Reference
In-Depth Information
Classless Versus Classful Routing Protocols
Routing protocols can be classified based on their support for classful or classless routing.
Classful routing protocols do not advertise subnet masks in their routing updates; therefore, the
entire internetwork uses the configured subnet mask for the IP network. For example, if you use
a classful routing protocol for network 130.170.0.0, the chosen mask (i.e., 255.255.255.0) has
to be used on all router interfaces using the 130.170.0.0 network. Serial links and local-area
networks are configured with the same mask of 255.255.255.0. RIPv1 and IGRP are classful
routing protocols.
Classless routing protocols advertise the subnet mask with each route. Subnetworks of a given
IP network number can be configured with different subnet masks. Large LANs can be
configured with a smaller subnet mask, and serial links can be configured with larger subnet
mask, thereby conserving IP address space. Classless routing protocols also allow flexible route
summarization and superneting. Supernets are created by the aggregation of classful networks.
For example, 200.100.100.0/23 is a supernet of 200.100.100.0/24 and 200.100.101.0/24.
RIPv2, OSPF, EIGRP, IS-IS, and BGP are classless routing protocols.
The use of the
ip classless
command does not make routing protocol classless or classful. The
command permits packets destined for an unrecognized subnet to be forwarded to the best
supernet possible. If it is not enabled, the packets are discarded.
NOTE
Routing Metrics
Routing protocols use one or more metrics to determine the best routes to a destination. Some
routing metrics are
•
Hop count
•
Bandwidth
•
Cost
•
Load
•
Delay
•
Reliability
Hop Count
The hop count metric counts only the number of routers that the packet must take to reach a
destination. If links are the same bandwidth, this metric works well. The problem with routing
protocols that use only this metric is that the shortest hop count isn't always the quickest path.
For example, if there are two paths to a destination, one with two 56 k links and another with
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