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4.1 | ![]() |
RIP v2 Overview | |
4.1.6 | ![]() |
Classless routing protocols |
The true defining characteristic of classless routing protocols is the
capability to carry subnet masks in their route advertisements. One
benefit of having a mask associated with each route is that the
all-zeros and all-ones subnets are now available for use. Classful
routing protocols cannot distinguish between an all-zeros subnet, for
example 172.16.0.0, and the major network number 172.16.0.0.
Likewise, they cannot distinguish between
a broadcast on the all-ones subnet 172.16.255.255, and an all-subnets
broadcast 172.16.255.255. If the subnet masks are included, this difficulty disappears. In this example 172.16.0.0/16 is the major network number and 172.16.0.0/24 is an all-zeros subnet. 172.16.255.255/16 and 172.16.255.255/24 are just as distinguishable. By default the Cisco IOS rejects any attempt to configure an all-zeros subnet as an invalid address/mask combination even if a classless routing protocol is running. To override this default behavior, enter the global command ip subnet-zero. A much greater benefit of having a subnet mask associated with each route is being able to use variable-length subnet masking (VLSM) and to summarize a group of major network addresses with a single aggregate address. Variable-length subnet masks, and address aggregation or supernetting, are examined with greater detail in Module 5, EIGRP.
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