Understanding the network.pdf - Back to Home

EIGRP). In addition, like IGRP and EIGRP, it is possible to run multiple OSPF
instances. This should only be done in special circumstances and generally avoided.
Each OSPF process builds its own topological database and has its own routing
algorithm process, so running multiple OSPF instances adds unneeded load to the
router.
Where OSPF differs from a configuration standpoint is in the way the network
announcements are entered. OSPF is a true classless protocol. When configuring
RIPv2, EIGRP, and so on, networks are entered (and listed in the configuration)
using their natural classful boundaries. OSPF provides the capability to enter the
network announcements in their classless form, along with a network mask to
interpret the network address. One thing about the netmask: It is entered in reverse
form, just like access lists. Also, like access lists, the same calculation formula is
used to calculate the reverse mask:
255.255.255.255
-255.255.252.0 (Subnet mask)
—————————
0. 0. 3.255 (Reverse mask)
The capability to enter the network and mask gives you a lot of flexibility in terms of
how networks are announced. With other IGP protocols, network summarization
can present problems, especially in large internetworks where classless addressing
is used extensively. To address this problem, summarization is disabled with the
router configuration subcommand . OSPF does not summarize
unless you configure it to.
In large LANs where subnetting is used and the possibility of discontinuous subnets
exists, auto-summarization can be a pain. In a large internetwork, however,
summarization is an efficiency gain, since summarization reduces the number of
routes required in the routing table. On large internetworks, whenever possible, you
want to deploy your network address space using classless boundaries that can be
treated as CIDR supernets. This makes it easy to use CIDR addressing to summarize
network announcements. Establishing an effective network addressing hierarchy is
essential for OSPF to function efficiently.
NOTE
A situation where multiple OSPF processes might be desirable is when the router is
acting as gateway between two separate internetworks. A common example would
be if OSPF was being used as an EGP by a network service provider and as an IGP by
a client. The client gateway router would run both an internal and external OSPF