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208 Summit 200 Series Switch Installation and User Guide
Interior Gateway Routing Protocols
OSPF is a link-state protocol, based on the Dijkstra link-state algorithm. OSPF is a newer Interior
Gateway Protocol (IGP), and solves a number of problems associated with using RIP on today’s
complex networks.
NOTE
Both RIP and OSPF can be enabled on a single VLAN.
RIP Versus OSPF
The distinction between RIP and OSPF lies in the fundamental differences between distance-vector
protocols and link-state protocols. Using a distance-vector protocol, each router creates a unique routing
table from summarized information obtained from neighboring routers. Using a link-state protocol,
every router maintains an identical routing table created from information obtained from all routers in
the autonomous system. Each router builds a shortest path tree, using itself as the root. The link-state
protocol ensures that updates sent to neighboring routers are acknowledged by the neighbors, verifying
that all routers have a consistent network map.
The biggest advantage of using RIP is that it is relatively simple to understand and implement, and it
has been the de facto routing standard for many years.
RIP has a number of limitations that can cause problems in large networks, including:
• A limit of 15 hops between the source and destination networks.
• A large amount of bandwidth taken up by periodic broadcasts of the entire routing table.
• Slow convergence.
• Routing decisions based on hop count; no concept of link costs or delay.
• Flat networks; no concept of areas or boundaries.
OSPF offers many advantages over RIP, including:
• No limitation on hop count.
• Route updates multicast only when changes occur.
• Faster convergence.
• Support for load balancing to multiple routers based on the actual cost of the link.
• Support for hierarchical topologies where the network is divided into areas.
The details of RIP and OSPF are explained later in this chapter.
Overview of RIP
RIP is an Interior Gateway Protocol (IGP) first used in computer routing in the Advanced Research
Projects Agency Network (ARPAnet) as early as 1969. It is primarily intended for use in homogeneous
networks of moderate size.
To determine the best path to a distant network, a router using RIP always selects the path that has the
least number of hops. Each router that data must traverse is considered to be one hop.