6-2
Stinger®
IP2000 Configuration Guide
OSPF Configuration
Overview of OSPF features supported by the IP2000
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Hierarchical routing via normal areas, stub areas, and not-so-stubby-areas
(NSSAs)
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Shortest path first link-state routing algorithm
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Diagnostics and traps
Authentication
All OSPF protocol exchanges are authenticated by simple authentication by default.
Only trusted routers can participate in the autonomous system’s routing. A variety of
authentication schemes can be used. In fact, different authentication types can be
configured for each area. For a discussion of areas, see “Hierarchical routing (areas)”
on page 6-5.
Authentication provides added security for the routers that are on the network.
Routers that do not have the password are unable to gain access to the routing
information, because authentication failure prevents a router from forming
adjacencies. (For a discussion of adjacencies, see “Exchange of routing information”
on page 6-2.) If both sides of a connection do not support the same authentication
method, packet error messages can result.
In addition to null and simple password authentication, Stinger units support the
MD5 cryptographic authentication method for OSPF as described RFC 2328. For
details about MD5 encryption, see RFC 2328.
Support for variable-length subnet masks
OSPF routers handle variable-length subnet masks (VLSMs). Each route distributed
by OSPF has a destination address and subnet mask, and two different subnets of the
same IP network can use different size subnet masks. A packet is routed to the best
(longest or most specific) match. Host routes are considered to be subnets whose
masks are all ones (0xFFFFFFFF).
Note
OSPF is useful for networks that use VLSMs. However, to prevent excessive
link-state calculations by all OSPF routers on the network, make every effort to
assign subnets that are as continuous as possible.
Exchange of routing information
An OSPF router stores its information about the network in a topological database
and propagates only changes to the database. Selected neighboring routers form
relationships, referred to as adjacencies, for the purpose of exchanging routing
information. Not every pair of neighboring routers become adjacent. Routers
connected by point-to-point networks and virtual links always become adjacent. On
multiaccess networks, all routers become adjacent to routers identified as the
designated router (DR) and the backup designated router (BDR).
As the adjacency is established, the neighbors exchange databases and build a
consistent, synchronized database between them. When an OSPF router detects a
change on one of its interfaces, it modifies its topological database and multicasts the
change to its adjacent neighbors, which in turn propagate the change to their
adjacent neighbors, until all routers within an area have synchronized topological
databases. This process provides quick convergence among routers.
A link state advertisement (LSA) is a packet that describes various aspects of an OSPF
route. Each LSA is flooded throughout a routing domain. The collected LSAs of all
