This chapter covered the elements of advanced routing design, and touched on the merits of a well-planned IP addressing scheme. The IP addressing scheme is the foundation for greater efficiency in operating and maintaining a network. Without proper planning in advance, networks might not be able to benefit from route summarization features inherent to many routing protocols.
Cisco favors a transition strategy from IPv4 to IPv6 that begins from the edges of the network and moves in toward the core. This strategy allows you to control the deployment cost and focus on the needs of the applications, rather than complete a full network upgrade to a native IPv6 network at this stage. Cisco IPv6 router products offer the features for a such an integration strategy. The various deployment strategies permit the first stages of the transition to IPv6 to happen now, whether as a trial of IPv6 capabilities or as the early controlled stages of major IPv6 network implementations. IPv6 can be deployed as dual stack, hybrid, and service block.
The general advanced routing design discussion can be encapsulated in the following key points:
- Route summarization and default routing are important in scaling routing designs.
- Route filtering can be used to manage traffic flows in the network, avoiding inappropriate transit traffic and as a defense against inappropriate routing updates.
- Redistribution can be useful for manipulating and managing routing updates but needs to be designed properly to prevent routing loops or other problems.
EIGRP converges quickly as long as it has a feasible successor. With no feasible successor, EIGRP sends queries out to its neighbors. To limit the scope of these queries, use route summarization and filtering. By limiting EIGRP query scope, you can speed up EIGRP convergence and increase stability. In addition, large numbers of neighbors should be avoided for any one router. Multiple autonomous systems may be used with EIGRP providing that you understand that they do not directly limit EIGRP query scope. You would use them to support migration strategies, different administrative groups, or very large network design.
OSPF scaling depends on summarization and controlling how much LSA flooding is needed. Simple, stub, summarized designs scale most effectively. Several techniques speed up convergence for OSPF, including fast hellos, and BFD.
Finally, IBGP requires a full mesh of all IBGP routers, but full-mesh peering does not scale gracefully. Route reflectors pass along routing information to and from their clients. The route reflector clients are relieved of the burden of most IBGP peering. Confederations allow an autonomous system to be divided into sub-autonomous systems, where the sub-autonomous system border routers peer with each other and then pass along routes on behalf of the other sub-autonomous system routers. Confederation sequences are used to prevent information loops. Sub-autonomous systems can have different BGP polices from each other.
The key points to remember include the following:
- IP address design allows for route summarization that supports network scaling, stability, and fast convergence.
- Route summarization, route filtering, and appropriate redistribution help minimize routing information in the network.
- EIGRP converges quickly as long as it has a feasible successor. Multiple autonomous systems with EIGRP may be used, with care, to support special situations, including migration strategies and very large network design.
- Simple, stub, summarized OSPF designs scale most effectively. Several techniques speed up convergence for OSPF, including fast hellos and BFD.
- IBGP designs can be scaled using route reflectors to pass routing information to and from their clients and confederations to allow an autonomous system to be divided into sub-autonomous systems.