Ip Routing Protocols; Ospf - Nortel Optical Multiservice Edge 6130 Planning Manual

Appendix A: Data communications planning 10-97
the learned network and data link layer address pairing in their ARP caches for
future use, thus avoiding the use of the gateway altogether. The rest of the hosts
on the LAN build up similar caches, thus reducing LAN traffic.

IP routing protocols

The primary function of IP, which resides at the network layer (3) of the OSI
(Open Systems Interconnect) model, is to receive data from the higher layer
protocols (TCP [Transmission Control Protocol] or UDP [User Datagram
Protocol] layers) on a source host, create a datagram and route the datagram
through a network to a destination host. Secondary functions of IP include
fragmentation and reassembly of the datagram, and packet lifetime control.
The most important IP routing protocols are explained in the following
sections.

OSPF

OSPF (Open Shortest Path First) is an open protocol, as defined in Request
For Comments (RFC) 1247. It is based on the Dijkstra's 'Shortest Path First'
algorithm, which is a link state routing mechanism.
In an OSPF network, each router maintains a link-state database that describes
the topology of the autonomous system (AS). The database contains the local
state for each router in the AS, including the router's usable interfaces and
reachable neighbors. Each router periodically checks for changes in its local
state and shares any changes detected by flooding link-state advertisements
(LSAs) throughout the AS. Routers synchronize their topological databases
based on the sharing of information from LSAs.
From the topological database, each router constructs a shortest-path tree, with
itself as the root. The shortest-path tree gives the optimal route to each
destination in the AS. Routing information from outside the AS appears on the
tree as leaves. OSPF routes IP traffic based solely on the destination IP address
and subnet mask, and IP Type of Service (TOS) contained in the IP packet
header.
OSPF is designed specifically for use on larger networks, providing several
features to reduce the amount of traffic and processing overhead associated
with the routing protocol. Amongst these features are the ability to segment a
large network into a number of areas, and route summarization, a technique
which greatly reduces the volume of route advertisement traffic where the
addressing scheme is hierarchical.
The manner in which a network is segmented into areas is governed by strict
rules. There is a core area—known as the backbone area—which is area
0.0.0.0, sometimes referred to as area 0. All other areas are connected directly
to the backbone area, or, where this is not possible, connected indirectly using
a 'virtual link' or tunnel. See Figure 10-28 on page 10-101.
Planning Guide NT6Q92MA Rel 1.0 Iss 1 Standard September 2006