OSPF, Part 12: Stub Areas

Last time, we had just finished configuring Area 1 as an OSPF stub area.

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 and EIGRP clouds via defaults from R3 and R4 (cost = 1)

Now let’s suppose that R5 wants to get a packet to the RIP cloud. Since R5 is receiving the default route from both R3 and R4, if the metrics are the same (which they would be unless we change them), R5 will load-share traffic for the RIP cloud between R3 and R4. In other words, half of the packets bound for the RIP cloud will take the sub-optimal path via R4. Likewise, since R5 will load-share packets for the EIGRP cloud, half of those packets would take the sub-optimal path via R3. The bottom line is that as a result of hiding information from R5, routing is no longer optimal. The possibility of sub-optimal routing exists whenever summarization is used and there are multiple paths (and OSPF stub areas are a type of summarization).

Let’s look at another example. Suppose that we now configure Area 1 as an OSPF “totally-stubby” area. This is done by using the OSPF “area stub” command on the internal routers within that area (R5), and the OSPF “area stub no-summary” command on the ABRs (R3 and R4). If we do, the Type-3 LSAs for subnets A, B, C and D will not be advertised into Area 1, and the view from R5 will be:

  • Everything outside of Area 1via defaults from R3 and R4 (cost = 1)

Obviously this saves RAM, bandwidth and CPU within Area 1, but these savings come at a cost. Since R5 no longer knows the metrics to the subnets within Area 0, it will now load-share traffic headed for those prefixes between R3 and R4. As with traffic headed from Area 1 to the RIP or EIGRP clouds, traffic from Area 1 for the prefixes within Area 0 could take a sub-optimal route.

The bottom line is that the goal of summarization is to conserve RAM, bandwidth and CPU by hiding information from some of the routers, but as a result of hiding information, routing may no longer be optimal. Note that this is not just a characteristic of OSPF, it applies to any routing protocol when there are multiple paths available and summarization is being used. That being said, summarization techniques are still tremendously useful, as long as they’re not applied blindly.

Author: Al Friebe

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