Chapter 16: Dynamic Routing Protocols
STANDARD Revision 1.0 C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved. 495
Open Shortest Path First Version 2
Open Shortest Path First (OSPF) is a dynamic link state routing protocol developed by the Internet Engineering Task Force
(IETF) that:
Supports Classless Inter-Domain Routing (CIDR)
Provides for routing update authentication, both simple and MD5
Uses IP multicast when sending/receiving the updates
Responds quickly to topology changes with a smaller amount of routing protocol traffic.
The OSPF specification is published as Request For Comments (RFC) 2328.
Link State Routing Protocol Description
The OSPF routing protocol maintains a link state database of all subnets available on the network. This includes details
about which routers are attached to the links.
If a link goes down, the router that is directly attached to it immediately sends a Link State Advertisement (LSA) to its
neighbor routers. Information about the link state propagates throughout the network. Each router reviews its database
and re-calculates the routing table independently.
Routing Metrics
A router learns multiple paths to a particular destination network, and chooses the path with the best metric in its routing
table.
Types of Metrics
Different routing protocols use different types of metrics:
Link States — Rather than counting the number of hops as a metric, OSPF bases its path descriptions on link states that
take into account additional network information.
Cost Metrics — OSPF also lets the user assign cost metrics to each interface so that some paths are given preference.
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