IP Routing

Route Summarization

RIP-1 and IGRP automatically summarize subnets into a major network number when crossing a network-number boundary. OSPF does not automatically summarize routes. Route summarization in OSPF must be manually configured on an ABR or an ASBR. Further, OSPF allows route summarization on any bit boundary (unlike RIP and IGRP, which summarize only classful network numbers).

Summarizing routes keeps the routing tables smaller and easier to troubleshoot. However, route summarization in OSPF is not just a nice thing to do -- it is necessary to reduce the size of the OSPF topology database, especially in a large network. A large topology database requires a large amount of router memory, which slows down all processes, including SPF calculations.

To allow summarization at ABRs and ASBRs, IP addresses must be carefully assigned. First, allocate enough addresses to each area to allow for expansion. Then set a bit boundary on which to summarize routes. This is easier said than done. Most network engineers inherit a network with a haphazard mess of addresses and changing requirements.

Summarizing at the ABR (Inter-Area Summarization)

Consider TraderMary’s network in Figure 6-10. Network 10.0.0.0 exists in area 1, and network 172.16.0.0 exists in area 0. Let’s see how we can summarize on these area boundaries.

The command to summarize at an ABR is:

area area-id range address mask

where area-id is the area whose routes are to be summarized, address is a network number, and mask ...