OSPF, Part 10

Having discussed how OSPF works without summarization options, let’s take a look at the various summarization options and their effects.

If there are 100 prefixes in the RIP cloud, and another 100 in the EIGRP cloud, each being injected into the OSPF domain by the ASBRs, each OSPF router will be tracking over 200 individual prefixes, and using RAM to store each in its LSDB and routing table. Also, a change to any external prefix will require advertisement of the change to all OSPF routers, which consumes bandwidth. Finally, any change to a router’s LSDB triggers Dijkstra’s SPF algorithm, which consumes CPU. For this reason, running a large OSPF topology “wide open” (without summarization) is not scalable.

Let’s start by using the OSPF “summary-address” command on R3, an ASBR. If we’re lucky (in other words, if the addresses in the RIP cloud have been allocated correctly), we can summarize the prefixes in the RIP cloud into one summary block. Assuming that’s the case, R3 will now be injecting only one Type-5 LSA into the OSPF cloud.

One might wonder, “What OSPF cost will be assigned to the Type-5 LSA for the summary block?” There are two options. One is to set the cost of the block equal to that of the lowest-cost prefix within the block, in accordance with RFC 1583. This is the default method. The second option is to set the cost of the block equal to that of the highest-cost prefix within the block, in accordance with RFC 2328. This is configured with the OSPF command “no compatible rfc1583”. In general, it probably doesn’t matter which method is used, but to prevent sub-optimal routing, it should be consistent within the OSPF autonomous system. We’ll just go with the default (“compatible rfc1583”).

If the EIGRP prefixes fall into a nice block, we can use the OSPF “summary-address” command on R4 to advertise that block into OSPF. At this point, the best paths from R5’s perspective are:

  • Subnet A via R3 (cost = 3)
  • Subnet B via R3 (cost = 3)
  • Subnet C via R4 (cost = 3)
  • Subnet D via R4 (cost = 3)
  • RIP cloud via R3 (1 summary route at cost = 21)
  • EIGRP cloud via R4 (1 summary route at cost = 21)

Note that we’ve saved RAM, bandwidth and CPU within the OSPF cloud due to the reduction in Type-5 LSAs being generated, advertised and stored. We’ve also saved additional RAM due to the reduction in sizes of the routing tables. So far, so good.

Next time, we’ll discuss additional summarization options.

Author: Al Friebe

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