Engineering rules and guidelines

Overview

To use IS-IS Level 2 Routing, the user must plan and engineer the provisioning of Level 2 routers and area addresses in the network.

The goal is to create a network with all nodes having OSI LAN or DCC connectivity with each other so that access to all the nodes can be gained from any node in the network.

Rules must be followed to assure OSI LAN and DCC message routing. Guidelines are recommendations that add survivability in the event of a node, OSI LAN or DCC failure. Rules must be followed. Guidelines should be followed. There may be customer applications that cannot adhere to some or all of the guidelines. In most cases, the potential consequence is that a single failure would at least partially disrupt operations communications.

Rule: OSI LAN or DCC connectivity within an area

All nodes in an area must have OSI LAN or DCC connectivity within the area to all other nodes in the same area. Otherwise, the area would be segmented and message routing could not be expected to function properly. The following figure illustrates a correct and an incorrect example of assigning nodes to areas.

Figure 6-9: Assigning areas

Rule: single Level 2 router sub-domain

All Level 2 routers must have OSI LAN or DCC connectivity, either directly or indirectly via other Level 2 routers, to all other Level 2 routers in the network to form a single Level 2 router sub-domain that provides connectivity to all areas. If more than one Level 2 router sub-domain exists, the network becomes segmented and routing will not function properly.

The following figure illustrates a correct and two incorrect examples of assigning a Level 2 router sub-domains.

Figure 6-10: Assigning sub-domains

Guideline: redundant routes within the Level 2 sub-domain

When practical, redundant routes in the Level 2 subdomain should be available between all Level 2 routers to ensure that proper routing will occur in the event of a single failure. To satisfy this guideline, every Level 2 router has at least two other Level 2 routers as direct neighbors.

The following figure illustrates a recommended example and an example that is not recommended for redundant routes within the Level 2 subdomain:

Figure 6-11: Redundant routes with the Level 2 sub-domain

Guideline: Level 2 router assignment in a ring

The recommended number of Level 2 routers in a ring may be one of the following:

None
One
All nodes in the ring
Two; for example, when dual-homing or dual ring interworking (DRI) is used.

The following figure illustrates examples of recommended Level 2 router assignments in various ring configurations.

Figure 6-12: Recommended Level 2 router assignments

Guideline: Level 2 router area assignments in a ring

For a ring of Level 2 routers, the recommended area assignments may be one of the following:

All Level 2 routers are part of the same area.
Each Level 2 router is part of a different area.
A pair of adjacent Level 2 routers are in the same area (for example, with dual-homing or DRI), with either the other Level 2 routers similarly paired off or individually in different areas.

The following figure illustrates examples of recommended Level 2 router area assignments in a ring.

Figure 6-13: Recommended area assignments

Guideline: adjacent Level 2 routers within an area

It is recommended that dual Level 2 routers in an area be adjacent to each other and have a redundant route within the area. Failure to meet this guideline would result in some messages not being routed properly in the event of a single failure that divides an area into two separate islands.

The following figure illustrates a recommended example and two examples that are not recommended of dual Level 2 routers in an area:

Figure 6-14: Recommended placement of Level 2 routers

Rule: Level 2 routers on an OSI LAN

If an OSI LAN (or WAN) is used to connect between areas, then at least one node in each area on the OSI LAN must be provisioned to be a Level 2 router. Otherwise, the areas won't route to each other via the OSI LAN.

The following figure illustrates a correct and an incorrect example of Level 2 router assignments on an OSI LAN:

Figure 6-15: Level 2 router assignments on an OSI LAN

Guideline: OSI LAN redundancy

If an OSI LAN (or WAN) is the only method of connecting between areas, then at least two Level 2 routers directly connected to the OSI LAN for each area are recommended for redundancy purposes. A second, separate OSI LAN hub would provide additional redundancy.

The following figure illustrates a recommended example for providing redundancy between areas connected solely by OSI LANs: