Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 - Manual (Page 186)

168 Brocade ICX 6650 Layer 3 Routing Configuration Guide
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OSPF overview
This chapter describes how to configure OSPF Version 2 on Brocade Layer 3 Switches using the CLI.
OSPF Version 2 is supported on devices running IPv4.
NOTE
The terms Layer 3 Switch and router are used interchangeably in this chapter and mean the same
thing.
OSPF overview
Open Shortest Path First (OSPF) is a link-state routing protocol. The protocol uses link-state
advertisements (LSAs) to update neighboring routers regarding its interfaces and information on
those interfaces. The router floods these LSAs to all neighboring routers to update them regarding
the interfaces. Each router maintains an identical database that describes its area topology to
help a router determine the shortest path between it and any neighboring router.
Brocade Layer 3 Switches support the following types of LSAs, which are described in RFC 1583:
• Router link
• Network link
• Summary link
• Autonomous system (AS) summary link
• AS external link
• Not-So-Stubby Area (NSSA) external link
• Grace LSAs
OSPF is built upon a hierarchy of network components. The highest level of the hierarchy is the
Autonomous System (AS). An autonomous system is defined as a number of networks, all of which
share the same routing and administration characteristics.
An AS can be divided into multiple areas as shown in Figure 17 on page 169. Each area represents
a collection of contiguous networks and hosts. Areas limit the area to which link-state
advertisements are broadcast, thereby limiting the amount of flooding that occurs within the
network. An area is represented in OSPF by either an IP address or a number.
You can further limit the broadcast area of flooding by defining an area range. The area range
allows you to assign an aggregate value to a range of IP addresses. This aggregate value becomes
the address that is advertised instead all of the individual addresses it represents being
advertised. You can assign up to 32 ranges in an OSPF area.
An OSPF router can be a member of multiple areas. Routers with membership in multiple areas
are known as Area Border Routers (ABRs). Each ABR maintains a separate topological database
for each area the router is in. Each topological database contains all of the LSA databases for each
router within a given area. The routers within the same area have identical topological databases.
The ABR is responsible for forwarding routing information or changes between its border areas.