OSPF Network Design Solutions: Redistribution

When routing information from one routing protocol, such as OSPF, is redistributed into IGRP, you must be mindful of possible routing loops.

OSPF Network Design Solutions


Chapter 6: Redistribution

This excerpt is reprinted with permission from Cisco Press. For more information or to order the book, visit the Cisco Press Web site.

By now, you have discovered that minimizing routing tables and choosing the next-hop destination path are critical for a well-tuned IP network. When routing information from one routing protocol, such as OSPF, is redistributed into IGRP, you must be mindful of possible routing loops.

A routing loop is a path to a remote network that alternates between two routers that assume the path is reachable via each other. Hence, the time to live that is present in every IP packet expires, and the packet or user data is dropped, resulting in loss of network connectivity.

Routing that uses a single routing algorithm is usually more desirable than running multiple IP and non-IP routing protocols, especially from a configuration and troubleshooting perspective.

However, in today's changing networks and with mergers, department politics, and acquisitions, more than one IP routing protocol is likely in use.

Before "diving in," you first need a definition of redistribution and summarization to clarify your understanding of these concepts. These are potentially confusing topics. Hopefully, they will be clearer to you after reading this chapter and Chapter 7, "Summarization."

  • Redistribution is when a router takes routing information it has discovered in one routing protocol and distributes it into a different routing protocol, thus allowing the redistribution of the first protocol 's networks into the second routing protocol. For example, a router running RIP and OSPF has a need for those in the OSPF section of the network to know the routes in the RIP network. Redistribution is used to accomplish this feat of routing magic! Sometimes, redistribution is referred to more specifically as route redistribution...both terms mean the same thing.

    Summarization is the taking of multiple route entries and representing them by a smaller number of routes. This smaller representation is known as summarizing your routes. For example, this technique is used to minimize the routing table size, which is useful when connecting to the Internet, conserving router resources, or simplifying the next hop. For example, routing table entries represent blocks of addresses and use the subnet mask to determine the size of the block. Through summarization, the router can tell the difference between 10.20.30.0 /24 and 10.20.30.0 /22.The former is a Class C–sized block of addresses, 256 addresses. The latter is four Class C–sized blocks, or 1024 addresses.

    This chapter is posted in full as a pdf file. To continue reading, click here.

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