LevelOne GTL-2691 - Manual (Page 669)

C HAPTER 20 | Unicast Routing
Configuring the Open Shortest Path First Protocol (Version 2)
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Figure 421: Configuring OSPF
C OMMAND U SAGE
◆ OSPF looks at more than just the simple hop count. When adding the
shortest path to any node into the tree, the optimal path is chosen on
the basis of delay, throughput and connectivity. OSPF utilizes IP
multicast to reduce the amount of routing traffic required when sending
or receiving routing path updates. The separate routing area scheme
used by OSPF further reduces the amount of routing traffic, and thus
inherently provides another level of routing protection. In addition, all
routing protocol exchanges can be authenticated. Finally, the OSPF
algorithms have been tailored for efficient operation in TCP/IP
Internets.
◆ OSPFv2 is a compatible upgrade to OSPF. It involves enhancements to
protocol message authentication, and the addition of a point-to-
multipoint interface which allows OSPF to run over non-broadcast
networks, as well as support for overlapping area ranges.
◆ When using OSPF, you must organize your network (i.e., autonomous
system) into normal, stub, or not-so-stubby areas; configure the
ranges of subnet addresses that can be aggregated by link state
advertisements; and configure virtual links for areas that do not have
direct physical access to the OSFP backbone.
■
To implement OSPF for a large network, you must first organize the
network into logical areas to limit the number of OSPF routers that
actively exchange Link State Advertisements (LSAs). You can then
define an OSPF interface by assigning an IP interface configured on
this router to one of these areas. This OSPF interface will send and
receive OSPF traffic to neighboring OSPF routers.
backbone
normal
area
NSSA
isolated
area
stub
Autonomous System A
Autonomous System B
external network
Router
ABR
ABR
ABR
ABR
ASBR
ASBR
ASBR
virtual
link