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DECnet Overview

308619-14.00 Rev 00

1-5

When a Nortel Networks router receives a packet, it examines the destination

address contained in the packet header. If the destination address is within the

same area as the router, the router forwards the packet toward the destination

system using the least-cost path. If the address is to another area, the router

behaves differently depending on whether the interface performs Level 1 and

Level 2 routing or Level 1 Only routing:

• If the router interface performs both Level 1 and Level 2 routing, the router

forwards the packet to the specified area using the least-cost path.

• If the router interface performs Level 1 Only routing, the router forwards the

packet to the nearest Level 2 router.

• If the router and all of its interfaces perform Level 1 Only routing, the router

forwards the packet to the nearest Level 2 router.

The Routing Process

Routing a DECnet packet through the network involves

• Updating adjacent routers on the availability of surrounding nodes and

circuits

• Listening for messages from adjacent routers identifying available nodes and

circuits

• Determining the routing path with the least cost

• Forwarding the packet to the specified adjacent node

Update Process

The Nortel Networks router software for DECnet services continually monitors

the circuits directly attached to it. It periodically receives routing control messages

from its adjacent routers. These routing updates inform the router of the current

network topology. For example, if a circuit on a router fails, or another circuit is

added, the network topology changes. The router then generates and transmits

routing updates to all adjacent routers informing them of the changes. Timers

control how often the router sends out updates.

Routing updates describe which nodes in the local area are reachable (called node

or Level 1 topology updates), and which other areas in the network are reachable

(called area or Level 2 topology updates). Routers use this information to update

their routing tables.

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