In any IP-based deployment, whether using standard IP routing protocols or running IP across an MPLS network, route summarization is an important part of the network structure. Route summarization provides the mechanism necessary to reduce the size of the Layer 3 routing table by bundling a number of prefixes into a less specif ic summary route, which helps reduce the amount of memory required by devices in the network but also helps reduce the overhead when computing paths through the network topology.
In an MPLS deployment, this summarization can help reduce the number of labels because only one label is necessary for the summary route. You can see an example of this summarization, and relevant label distribution, in Figure 5-11.
You can see in Figure 5-11 that the Washington LSR receives two /24 prefixes, 126.96.36.199/24 and 188.8.131.52/24, from the Paris LSR via its internal routing protocol. The Washington LSR is configured to send a summary route, 174.24.0/16, which covers both of the more specific routes that it learned from the Paris LSR.
Using this configuration, the Washington LSR becomes the aggregation point for LSPs that use the summary route. This means that each LSP that uses the summary route needs to terminate on the Washington LSR. The result is that the Washington LSR needs to examine the second-level label of each packet and, depending on what it finds, depends on the action that is taken. If a label exists, the LSR switches the packet based on this label. If a label does not exist, the LSR needs to examine the Layer 3 header information so it can reclassify the packet.
Because of the necessity to reclassify packets at the aggregation point, it is imperative that the device that provides the aggregation not be an ATM switch. This is because an ATM switch has no hardware to process Layer 3 information with which to reclassify any packets and just uses the incoming VPI/VCI as reference to determine the outgoing port, and the outgoing VPI/VCI that should be used for the incoming cell.
Summarization also has major implications when used in an MPLS/VPN environment. Chapter 13 discusses this in more detail.