Extreme Networks ExtremeWare XOS 10.1, Manual

Page: 186 / 256

186 ExtremeWare XOS 10.1 Concepts Guide
Interior Gateway Protocols
Overview
The switch supports the use of two interior gateway protocols (IGPs); the Routing Information Protocol
(RIP), and the Open Shortest Path First (OSPF) protocol.
RIP is a distance-vector protocol, based on the Bellman-Ford (or distance-vector) algorithm. The
distance-vector algorithm has been in use for many years, and is widely deployed and understood.
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
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.