Summarization The Good the Bad and the Asymmetric

Summarization is a great tool for conserving network resources, from the amount of memory required to store the routing table to the amount of network bandwidth and router horsepower necessary to transmit and process routing information. Summarization also conserves network resources by "hiding" network instabilities.

For example, the network in Figure 2-4 has a flapping route—a route that, due to a bad physical connection or router interface, keeps transitioning down and up and down again.

Without summarization, every time subnet 192.168.1.176/28 goes up or down, the information must be conveyed to every router in the corporate internetwork. Each of those routers, in turn, must process the information and adjust its routing table accordingly. If router Nashville advertises all the upstream routes with the aggregate address 192.168.1.128/25, however, changes to any of the more-specific subnets are not advertised past that router. Nashville is the aggregation point; the aggregate continues to be stable even if some of its members are not.

The price to be paid for summarization is a reduction in routing precision. In Example 2-6, interface SI of the router in Figure 2-3 has failed, causing the routes learned from the neighbor on that interface to become invalid. Instead of dropping packets that would normally be forwarded out SI, however, such as a packet with a destination address of 192.168.1.75, the packet now matches the next-best route, 192.168.1.0/24, and is forwarded out interface E0. (Compare this to Example 2-2.)

Figure 2-4 A Flapping Route Can Destabilize the Entire Network

Unstable!

192.168.1.176/28

192.168.1.144/28

192.168.1.144/28

Example 2-6 A Failed Route Can Lead to Inaccurate Packet Forwarding

Cleveland#

%LINEPR0T0-5-UPD0WN: Line protocol on Interface Seriall, changed state to down %LINK-3-UPD0WN: Interface Seriall, changed state to down Cleveland#show ip route 192.168.1.75 Routing entry for 192.168.1.0 255.255.255.0

Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 10 Redistributing via ospf 1

Last update from 192.168.2.2 on Ethernet©, 00:00:20 ago

Routing Descriptor Blocks:

* 192.168.2.2, from 10.2.1.1, 00:00:20 ago, via Ethernet© Route metric is 20, traffic share count is 1

Cleveland#

This imprecision may or may not be a problem, depending on what the rest of the internetwork looks like. Continuing with the example, suppose the next-hop router 192.168.2.2 still has a.route entry to 192.168.1.64/26 via the router Cleveland, either because the internetwork has not yet converged or because the route was statically entered. In this case, a routing loop occurs. On the other hand, some router reachable via Cleveland's EO interface may have a "back door" route to subnet 192.168.1.64/26 that should be used only if the primary route, via Cleveland's S1, becomes invalid. In this second case, the route to 192.168.1.0/24 has been designed as a backup route, and the behavior shown in Example 2-6 is intentional.

Figure 2-5 shows an internetwork in which a loss of routing precision can cause a different sort of problem. Here, routing domain 1 is connected to routing domain 2 by routers in San Francisco and Atlanta. What defines these domains is unimportant for the example. What is important is that all the networks in domain 1 can be summarized with the address 172.16.192.0/18, and all the networks in domain 2 can be summarized with the address 172.16.128.0/18.

Figure 2-5 When Multiple Routers Are Advertising the Same Aggregate Addresses, Loss of Routing Precision Can Become a Problem

Figure 2-5 When Multiple Routers Are Advertising the Same Aggregate Addresses, Loss of Routing Precision Can Become a Problem

Rather than advertise individual subnets, Atlanta and San Francisco advertise the summary addresses into the two domains. If a host on Dallas' subnet 172.16.227.128/26 sends a packet to a host on Seattle's subnet 172.16.172.32/28, the packet most likely is routed to Atlanta, because that is the closest router advertising domain 2's summary route. Atlanta forwards the packet into domain 2, and it arrives at Seattle. When the host on subnet 172.16.172.32/28 sends a reply, Seattle forwards that packet to San Francisco—the closest router advertising the summary route 172.16.192.0/18.

The problem here is that the traffic between the two subnets has become asymmetric: Packets from 172.16.227.128/26 to 172.16.172.32/28 take one path, whereas packets from 172.16.172.32/28 to 172.16.227.128/26 take a different path. Asymmetry occurs because the Dallas and Seattle routers do not have complete routes to each other's subnets. They have only routes to the routers advertising the summaries and must forward packets based on those routes. In other words, the summarization at San Francisco and Atlanta has hidden the details of the internetworks behind those routers.

Asymmetric traffic can be undesirable for several reasons. First, internetwork traffic patterns become unpredictable, making baselining, capacity planning, and troubleshooting more problematic. Second, link usage can become unbalanced. The bandwidth of some links can become saturated, while other links are underutilized. Third, a distinct variation can occur in the delay times of outgoing traffic and incoming traffic. This delay variation can be detrimental to some delay-sensitive applications such as voice and live video.

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