Areas and Area Configuration Guidelines

DECnet-RSX Phase IV implementations can support networks consisting of many more nodes than Phase III implementations could support. Networks with thousands of nodes can now be supported using the concept of areas. If your network consists of more than 1023 nodes you must use the area network features of Phase IV. Even if your network consists of fewer than 1023 nodes, you can, in many cases, benefit by splitting your network into areas. A network containing three areas is shown in Figure 2-2.

• Level 2 Routing Node

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D ^{End Node}

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Figure 2-2: A Multiarea Network

In designing a network that implements the area concept, the network manager parti-tions the network into smaller networks called areas. Partitioning can be based on phys-ical proximity, commonality of function, or both. If you choose to partition your net-work into areas, you must view your entire netnet-work on two levels. Level 1 is equivalent in many ways to older Phase III/Phase IV networks. Levell routing nodes in Phase IV networks with area support, however, must maintain a routing database only the size of the area of which they are a member. All nodes in the same area have the same area parameter value.

In a Phase IV networks consisting of more than one area, at least one of the nodes in each area is also a member of the level 2 network. This node (called a level 2 routing or area routing node) must maintain two routing databases; one for the local level 1 net-work and one for the netnet-workwide level 2 netnet-work. Each node in the level 2 netnet-work is a member of the networkwide level 2 network and one of the level 1 networks.

Communication between nodes in the same area occurs exactly as it occurred in Phase III. When the source and destination nodes are in the same area, the level 1 routing nodes forward the request using their local routing databases.

When access to a node outside a local area is desired, the source node sends the request to the nearest level 2 router. The level 2 routing node will route the packet to a level 2 router at the correct destination area. The packet is then routed from the level 2 router to the destination node in that area.

In designing a network that supports area routing, several guidelines must be followed to ensure proper network operation.

• Every level 1 routing node must be in one area only. It is illegal for a level 1 router to have a circuit connected to a node outside its area.

• Each area (level 1 network) must be physically intact. All nodes within an area must be connected in some way to all other nodes within the same area. The nodes need not be physically adjacent; however, all nodes in the area must be connected by a physical path that lies totally within the area. This path can involve level 2 rout-ing nodes in the same area.

• The level 2 network must be physically intact. All level 2 routing nodes must be connected in some way to all other level 2 nodes. This connection must involve level 2 nodes only. Level 1 nodes or Phase III nodes cannot form a part of the path between level 2 routing nodes.

• All Phase III routing nodes are treated as level 1 routing nodes, with some important restrictions:

• All Phase III nodes (both routing nodes and end nodes) can communicate only with nodes within the same area as themselves.

• All Phase III routing nodes can support communication only with nodes for which they can maintain routing information. Phase IV DECnet networks can have more nodes than can Phase III nodes (Phase Ill's limit was 255). For this

reason, any Phase IV node with an address above 255 is "invisible" to all Phase III nodes in the area. This is a good reason to configure a network into areas, each of which does not consist of more than 255 nodes.

• A Phase III node cannot be in the path of two high address Phase IV nodes.

Because of the Phase III address limitation, a Phase III node must not be in the path of two Phase IV nodes with addresses higher than the Phase III node can handle (255).

• A Phase III node must not be in the path of any interarea communication.

Because Phase III nodes do not know about areas, they cannot handle interarea packets correctly.

• Areas should reflect expected traffic flow. Observing this guideline will greatly improve overall network performance. Routing within an area is less costly than is routing between areas. Try, therefore, to include within the same area nodes that are logically or functionally related. Area bounds should reflect a combination of geographical and logical node location.

• Areas should not be used to enforce protection. Although at first glance the con-cept of areas seems to offer penetration protection across areas, the DNA architec-ture does not specify protection guarantees for areas. No assumption about such guarantees should be made when setting up areas. Areas were not designed to be used as an accounting or security tool.

Additional information about areas and area parameters can be found in the DECnet-RSX Network Management Concepts and Procedures manual.