Sidewinder G2 6.1.2 Administration Guide - Glossary of Technical ...

Appendix C: Configuring Dynamic Routing with OSPF
Overview of OSPF routing
Overview of
OSPF routing
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OSPF is a routing protocol in that it provides information used to figure out
routes in a portion of a network. Unfortunately, it is not a routing protocol in that
it does not actually pass routes, but information about links each router has.
Based upon this link information, each router runs the same algorithm and
comes up with the same "picture" of the network.
Note: OSPF runs as its own protocol (protocol 89) on top of IP.
OSPF uses a fair amount of multicasting. When a host detects a change to a
routing table or a change in the network topology, it immediately multicasts the
information to all other hosts in the network. Unlike the RIP in which the entire
routing table is sent, the host using OSPF sends only the part that has
changed. With RIP, the routing table is sent to neighboring hosts every 30
seconds. OSPF multicasts updated information only when a change occurs.
Tip: You should read this appendix only if you have identified that your routing
topology is too complicated to use only static routing or the Routing Information
Protocol (RIP). OSPF is a complex IP routing protocol and deploying OSPF should
involve discussions between routing subject matter experts and security subject
matter experts.
A closer look at OSPF
Rather than counting the number of hops, OSPF bases its path descriptions on
link states that factor in additional network information. Also, OSPF lets you
assign cost metrics to a given host router so that some paths are given
preference.
There are three phases to the OSPF protocol:
1 Routers "discover" neighboring OSPF routers by exchanging Hello
messages. The Hello messages also determine which routers will act as
the Designated Router (DR) and Backup Designated Router (BDR). These
messages are periodically exchanged to ensure connectivity between
neighbors still exists.
2 Routers exchange their "link state databases." Link state means the
information about a system's interfaces (IP address, network mask, cost for
using that interface, and whether it is up or down).
3 Finally, the routers exchange additional information via a number of
different type of Link State Advertisements (LSAs). These "fill out" the
information needed to calculate routes. Some reasons for generating LSAs
are interfaces going up or down, distant routes changing, static routes
being added or deleted, etc.