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DES-7200 Configuration Guide Chapter 4 OSPF
Configuration
4-1
4 OSPF Configuration
4.1 OSPF Overview
OSPF (Open Shortest Path First) is an internal gateway routing protocol based on
link status developed by the IETF OSPF work group. OSPF, a routing protocol
specific for IP, directly runs on the IP layer. Its protocol number is 89. OSPF
packets are exchagned in multicast form using the multicast address 224.0.0.5
(for all OSPF routers) and 224.0.0.6 (for specified routers).
The link status algorithm is an algorithm totally different from the Huffman vector
algorithm (distance vector algorithm). The RIP is a traditional routing protocol that
uses the Huffman vector algorithm, while the OSPF protocol is the typical
implementation of the link status algorithm. Compared with the RIP routing
protocol, the OSPF uses a different algorithm, and also introduces the new
concepts such as route update authentication, VLSMs, and route aggregation.
Even if the RIPv2 has made great improvements, and can support the features
such as route update authentication and VLSM, the RIP protocol still has two fatal
weaknesses: 1) slow convergence; 2) limited network size, with the maximum hot
count of no more than 16. The OSPF is developed to overcome these
weaknesses of the RIP, making the IGP protocol applicable for large and
complicated network environments.
The OSPF protocol establishes and calculates the shortest path to every
destination network by using the link status algorithm. This algorithm is
complicated. The following briefly describes how the link status algorithm works:
In the initialization stage, a router will generate a link status notification
including the status of all its links.
All routers exchange the link status message in the multicast way. Upon
receiving the link status update message, each router will copy it to its local
database and then transmit it to other routers.
When every router has a complete link status database, the router uses the
Dijkstra algorithm to calculate the shortest path trees to all the target
networks. The results include destination network, next-hop address, and
cost, which are the key parts of the IP routing table.
If there is no link cost or network change, the OSPF will become silent. If any
changes occur on the network, the OSPF advertises the changes via the link
status message of only the changed links. The routers involved in the changes