HP 600 Series, Services And Applications

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DECnet nodes establish adjacencies as part of their initialization sequence.
End nodes multicast “Endnode Hello Messages” to establish adjacencies
with routers on the same LAN. Routers multicast “Router Hello Messages” to
establish adjacencies with one another and to elect the designated router.
Additionally, the designated router multicasts “Router Hello Messages” to all
end nodes to inform them that it is the designated router.
Hierarchical Routing
DECnet routing is hierarchical. Level 1 routing occurs when packets are
routed within an area. Level 2 routing occurs when packets are routed
between areas. An HP router always performs as both level 1 and level 2
routers, even if it is not adjacent to a router in another area.
A DECnet router maintains routing tables for both a level 1 and a level 2. The
level 1 routing table contains information on routes to destination nodes
within the area. The level 2 routing table contains information on routes to
destination areas.
Figure 3 shows a two-area DECnet internetwork. Each area has two routers.
When end node 1.10 sends a packet to end node 2.10, it is routed through all
four routers in the internetwork. When router 1.1 receives the packet des-
tined for node 2.10, the router must refer to its level 2 routing table to deter-
mine where to send the packet because the destination node is in a different
area. It sends the packet to router 1.2. This router must also refer to its level
2 routing table to determine where to send the packet. It sends the packet to
router 2.1. Since the destination node 2.10 is in the same area as router 2.1,
the router refers to its level 1 routing table to determine where to send the
packet. It sends the packet to router 2.2, which forwards it to end node 2.10.
Figure 3. Hierarchical Routing of a Packet Sent from Node 1.10 to Node 2.10
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DECnet Routing Service
Hierarchical Routing
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