Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

Previous Table of Contents Next


Chapter 115
Open Shortest Path First (OSPF, RFC 2178)

Open Shortest Path First (OSPF, RFC 2178)

•  Shortest-path routes based on true metrics, not just a hop count.
•  Computes the routes only when triggered to or every 30 minutes (whichever is less).
•  Pairs a network address entry with a subnet mask.
•  Allows for routing across equal paths.
•  Supports ToS.
•  Permits the injection of external routes (other ASs).
•  Authenticates route exchanges.
•  Quick convergence.
•  Direct support for multicast in both the IP header and the MAC header.

The major shortcomings of the RIP protocol are:

•  The maximum distance between two stations (the metric, measured in router hops) is 15 hops. A destination (network ID) whose hop count is 16 is considered unreachable.
•  The cost to a destination network is measured in hops. RIP determines a route based on a hop count that does not take into consideration any other measurements except for the number of routers between the source and destination networks. A two-hop high-speed network will be bypassed for a one-hop low-speed link. A router can be tricked into taking a better path by adjusting the hop-count metric on the router port, but this reduces the available diameter.
•  RIP updates it entire table on a periodic basis consuming bandwidth using the broadcast address. (RIPv1; RIPv2 uses multicast or broadcast).
•  RIP sends its update in a 576-byte datagram. If there are more entries than 512 bytes, multiple datagrams must be sent. For example, 300 entries require 12 back-to-back 512-byte datagrams.
•  RIP suffers from slow convergence. In the worse case, a RIP update can take over 15 minutes end to end. This can cause blackholes, loops, etc.
•  RIPv1 does not support VLSM.

The first shortest-path-first routing protocol was developed and used in the ARPAnet packet switching network all the way back in 1978. This research work was developed and used in many other routing protocol types and prototypes. One of those is OSPF.

OSPF Features

•  Shortest-path routes are based on true metrics, not just a hop count.
•  The routing tables are updated only when needed, or every 30 minutes using a multicast address.
•  A network address entry is paired with a subnet mask.
•  Routing across equal paths is allowed, performing load balancing.
•  Type of Service (ToS) routing is supported.
•  The injection of external routes is permitted (routes from other autonomous systems).
•  Route exchanges are authenticated.
•  Quick convergence is realized.
•  Multicast is directly supported.


Previous Table of Contents Next