OSPF Overview
22-2 Configuring OSPFv2
The OSPF protocol is designed expressly for the TCP/IP internet environment. It provides for the
authentication of routing updates, and utilizes IP multicast when sending and receiving the
updates.
OSPF routes IP packets based solely on the destination IP address found in the IP packet header.
IP packets are not encapsulated in any further protocol headers as they transit the Autonomous
System (AS). OSPF is a dynamic routing protocol in that it quickly detects topological changes in
the AS, such as router interface failures, and calculates new loop-free routes after a period of
convergence. This period of convergence is short and involves a minimum of routing traffic.
In a link-state routing protocol, each router maintains a database describing the AS’s topology.
This database is referred to as the link-state database. Each participating router has an identical
database. Each individual piece of this database is a particular router’s local state made up of such
information as the router’s usable interfaces and reachable neighbors. The router distributes its
local state throughout the AS by flooding.
OSPF Areas
OSPF allows sets of networks to be grouped together. Such a grouping is called an area. The
topology of an area is hidden from the rest of the AS. This information hiding enables a significant
reduction in routing traffic. Also, routing within the area is determined only by the area’s own
topology, lending the area protection against bad routing data.
Areas are labeled with 32-bit identifiers that typically are written in the dotted decimal notation of
an IPv4 address, although this notation is not required. Area IDs are NOT IP addresses, however,
and may duplicate any IPv4 address without conflict.
OSPF area types include:
• Backbone Area
The backbone area (area 0, or 0.0.0.0) is the core of the OSPF network. All other areas must be
connected to it. Inter-area routing is carried out by routers connected to the backbone area and
to their own areas.
•Stub Area
A stub area does not receive route advertisements external to the AS, and routing from within
the area is based entirely on a default route.
• Not-So-Stubby Area (NSSA)
A not-so-stubby area is an extension of the stub area type that allows the injection of external
routes in a limited way. A NSSA can import AS external routes and send them to other areas,
but cannot receive AS external routes from other areas.
•Transit Area
A transit area is used to pass network traffic from one adjacent area to another. The transit
area does not originate the traffic nor is it the destination of the traffic.
An area is a generalization of an IP subnetted network. OSPF enables the flexible configuration of
IP subnets. Each route distributed by OSPF has a destination and mask. Two different subnets of
the same IP network number may have different masks providing a different range of addresses
for that subnet. This is commonly referred to as Variable Length Subnet Masking (VLSM). A
packet is routed to the longest or most specific match. Host routes are considered to be subnets
whose masks are “all ones” (0xffffffff).
Note: Refer to the Release Notes for your fixed switch platform for the number of areas supported
by your device.
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