The Ospf Screen - ZyXEL Communications ZyWall ATP series User Manual

10.7 The OSPF Screen

OSPF (Open Shortest Path First, RFC 2328) is a link-state protocol designed to distribute routing
information within a group of networks, called an Autonomous System (AS). OSPF offers some
advantages over vector-space routing protocols like RIP.
• OSPF supports variable-length subnet masks, which can be set up to use available IP addresses more
efficiently.
• OSPF filters and summarizes routing information, which reduces the size of routing tables throughout
the network.
• OSPF responds to changes in the network, such as the loss of a router, more quickly.
• OSPF considers several factors, including bandwidth, hop count, throughput, round trip time, and
reliability, when it calculates the shortest path.
• OSPF converges more quickly than RIP.
Naturally, OSPF is also more complicated than RIP, so OSPF is usually more suitable for large networks.
OSPF uses IP protocol 89.
OSPF Areas
An OSPF Autonomous System (AS) is divided into one or more areas. Each area represents a group of
adjacent networks and is identified by a 32-bit ID. In OSPF, this number may be expressed as an integer
or as an IP address.
There are several types of areas.
• The backbone is the transit area that routes packets between other areas. All other areas are
connected to the backbone.
• A normal area is a group of adjacent networks. A normal area has routing information about the
OSPF AS, any networks outside the OSPF AS to which it is directly connected, and any networks
outside the OSPF AS that provide routing information to any area in the OSPF AS.
• A stub area has routing information about the OSPF AS. It does not have any routing information
about any networks outside the OSPF AS, including networks to which it is directly connected. It relies
on a default route to send information outside the OSPF AS.
• A Not So Stubby Area (NSSA, RFC 1587) has routing information about the OSPF AS and networks
outside the OSPF AS to which the NSSA is directly connected. It does not have any routing information
about other networks outside the OSPF AS.
Each type of area is illustrated in the following figure.
Chapter 10 Routing
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