Requirements for Deploying VLSM

The successful deployment of VLSM has three prerequisites:

  The routing protocols must carry extended network prefix informa 

tion with each route advertisement.

  All routers must implement a consistent forwarding algorithm based

on the  longest match. 

  For route aggregation to occur, addresses must be assigned so that

they have topological significance. 

Routing Protocols Must Carry Extended Network Prefix Lengths

Routing protocols, such as OSPF and I IS IS, enable the deployment of

VLSM by providing the extended network prefix length or mask value

along with each route advertisement. This permits each subnetwork to

be advertised with its corresponding prefix length or mask. If the rout 

ing protocols did not carry prefix information, a router would have to

either assume that the locally configured prefix length should be

applied, or perform a look up in a statically configured prefix table that

contains all of the required masking information. The first alternative

cannot guarantee that the correct prefix is applied, and static tables do

not scale since they are difficult to maintain and subject to human error. 

To deploy VLSM in a complex topology, the administrator must select

OSPF or I IS IS as the Interior Gateway Protocol (IGP) rather than RIP 1.

Note that RIP 2, defined in RFC 1388, improves the RIP protocol by

allowing it to carry extended network prefix information. Therefore,

RIP 2 supports the deployment of VLSM. 

Forwarding Algorithm Based on the Longest Match

All routers must implement a consistent forwarding algorithm based on

the longest match algorithm. The deployment of VLSM means that the

set of networks associated with extended network prefixes may mani 

fest a subset relationship. A route with a longer extended network pre 

fix describes a smaller set of destinations than the same route with a

shorter extended network prefix. As a result, a route with a longer

extended network prefix is more specific while a route with a shorter

extended network prefix is less specific. Routers must use the route

with the longest matching extended network prefix (most specific

matching route) when forwarding traffic. 

U N D E R S TA N D I N G   I P   A D D R E S S I N G

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