IP Routing

OSPF Design Heuristics

The following sections provide a partial and ad hoc checklist to use when executing an OSPF design. As with any other discipline, the engineer will do best if he spends time understanding the details of OSPF and then designs his network as simply as possible.

OSPF Hierarchy

Building a large, unstructured OSPF network is courting disaster. The design of the OSPF network must be clearly defined: all changes in the OSPF environment must bear the imprint of the OSPF architecture. For example, when adding a new router, the network engineer must answer the following questions:

• Will the router be an area router, a stub router, or an ABR?

• If the router is an ABR or an ASBR, what routes should the router summarize?

• What impact would the failure of the router have on OSPF routing?

• Will this router be a DR/BDR?

• How will this router affect the performance of other OSPF routers?

IP Addressing

IP addresses must be allocated in blocks that allow route summarization at ABRs. The address blocks must take into account the number of users in the area, leaving room for growth. VLSM should be considered when planning IP address allocation.

Router ID

Use loopback addresses to assign router IDs. Choose the router IDs carefully -- the router ID will impact DR/BDR election on all attached multi-access networks. Keep handy a list of router IDs and router names. This will make it easier to troubleshoot the network.

DR/BDR

Routers with low processor/memory/bandwidth resources should be made DR-ineligible. ...