Cisco® CCNA Exam Cram Notes : VLSM

I. Networking Fundamentals

4. IPv4 Addressing

4.2 VLSM

VLSM (Variable Length Subnet Masking) allows efficient use of IP addresses. Networks implemented with VLSM can be summarized more efficiently due to manual control. With a distance vector protocol such as RIP , only one subnet mask value can be used on a network, as subnet mask values are not sent in routing updates. Variable Length Subnet Masking or VLSM gives the router the ability to announce a series of addresses with a single routing table entry. The way that is does this, is it looks at the addresses and ascertains which of those addresses are similar out to a designated bit pattern, also known as "high-order bits". It then looks at the subnet mask accompanying the update and decides which series of addresses are considered matches.

Variable-Length Subnet Masking: Variable-length subnet masking (VLSM) means implementing more than one mask on the same major class of a network. It allows for a more efficient use of IP address space both in terms of hosts and subnets, and can be essential on a network that does not have an abundance of IP address space. The following routing protocols support VLSM:

1. RIPv2


3. IS-IS


5. BGP


EIGRP supports aggregation and variable length subnet masks (VLSM). Unlike Open Shortest Path First (OSPF), EIGRP allows summarization and aggregation at any point in the network. EIGRP supports aggregation to any bit. This allows properly designed EIGRP networks to scale exceptionally well without the use of areas. EIGRP also supports automatic summarization of network addresses at major network borders.

EIGRP is an advanced routing protocol that combines many of the features of both link-state and distance-vector routing protocols, EIGRP's DUAL algorithm contains many features which make it more of a distance vector routing protocol than a link-state routing protocol.

Classful routing protocols: RIPv1, IGRP are examples of classful routing protocols. It is important to know that classful routing protocols do not exchange subnet information during routing information exchanges. The summarization is always done automatically at major network boundaries.

Classless routing protocols: RIPv2, EIGRP, OSPF, BGPv4, and IS-IS are examples of classless routing protocols. In classless routing protocols, subnet information is exchanged during routing updates. This results in more efficient utilization of IP addresses. The summarization in classless networks is manually controlled.

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