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. Figure 6-26, Assigning areas illustrates a correct and an incorrect example of assigning nodes to areas.
Figure 6-26: 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.
Figure 6-27, Assigning sub-domains illustrates a correct and two incorrect examples of assigning a Level 2 router sub-domains.
Figure 6-27: Assigning sub-domains
Guideline: redundant routes within the Level 2 sub-domain
When practical, redundant routes in the Level 2 sub-domain 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.
Figure 6-28, Redundant routes with the Level 2 sub-domain illustrates a recommended example and an example that is not recommended for redundant routes within the Level 2 sub-domain:
Figure 6-28: 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:
Figure 6-29, Recommended Level 2 router assignments illustrates examples of recommended Level 2 router assignments in various ring configurations.
Figure 6-29: 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:
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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.
Figure 6-30, Recommended area assignments illustrates examples of recommended Level 2 router area assignments in a ring.
Figure 6-30: 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.
Figure 6-31, Recommended placement of Level 2 routers illustrates a recommended example and two examples that are not recommended of dual Level 2 routers in an area:
Figure 6-31: Recommended placement of Level 2 routers
Guideline: OMS own area address and Level 2 router
If OMS (or any other OS) access is via OSI LAN (or WAN), OMS should be assigned to a different area and rely on a generic Level 2 router to communicate with a large network. (OMS is not expected to be a Level 2 router itself.) This is recommended to avoid routing all messages to/from OMS through a single NE serving as a Level 2 router.
Although not shown in the examples below, OMS can support a second, redundant OSI LAN port. Each OMS OSI LAN port has its own unique NSAP but both must be assigned the same area address.
Figure 6-32, Recommended OMS access via OSI LAN/WAN illustrates a recommended example, an example that is not recommended, and an incorrect example of OMS access via OSI LAN/WAN.
Figure 6-32: Recommended OMS access via OSI LAN/WAN
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.
Figure 6-33, Level 2 router assignments on an OSI LAN illustrates a correct and an incorrect example of Level 2 router assignments on an OSI LAN:
Figure 6-33: 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.
Figure 6-34, OSI LAN redundancy illustrates a recommended example for providing redundancy between areas connected solely by OSI LANs:
Figure 6-34: OSI LAN redundancy
