Route Summarization
As the result of corporate expansion and mergers, the number of subnets and network addresses in routing tables is increasing rapidly. This growth taxes CPU resources, memory, and bandwidth used to maintain the routing table. Route summarization and CIDR techniques can manage this corporate growth much like Internet growth has been managed. With a thorough understanding of route summarization and CIDR, you can implement a scalable network. This section describes summarization; CIDR is covered in the later section "Classless Interdomain Routing." The relationship between summarization and VLSM is also examined. With VLSM, you break a block of addresses into smaller subnets; in route summarization, a group of subnets is rolled up into a summarized routing table entry.
Route Summarization Overview
In large internetworks, hundreds, or even thousands, of network addresses can exist. It is often problematic for routers to maintain this volume of routes in their routing tables. Route summarization (also called route aggregation or supernetting) can reduce the number of routes that a router must maintain, because it is a method of representing a series of network numbers in a single summary address.
For example, in Figure 1-16, router D can either send four routing update entries or summarize the four addresses into a single network number. If router D summarizes the information into a single network number entry, the following things happen:
Bandwidth is saved on the link between routers D and E.
Router E needs to maintain only one route and therefore saves memory.
Router E also saves CPU resources, because it evaluates packets against fewer entries in its routing table.
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Figure
1-16 Routers Can Summarize to Reduce the Number of Routes
Key Point: Summary Routes
A summary route is announced by the summarizing router as long as at least one specific route in its routing table matches the summary route.
NOTE
Router D in Figure 1-16 is advertising that it can route to network 172.16.12.0/22, including all subnets of that network. However, if there were other subnets of 172.16.12.0/22 elsewhere in the network (for example, if 172.16.12.0 were discontiguous), summarizing in this way might not be valid.
Another advantage of using route summarization in a large, complex network is that it can isolate topology changes from other routers. For example, in Figure 1-16, if a specific link (such as 172.16.13.0/24) is flapping (going up and down rapidly), the summary route (172.16.12.0/22) does not change. Therefore, router E does not need to continually modify its routing table as a result of this flapping activity.
NOTE
Flapping is a common term used to describe intermittent interface or link failures.
Route summarization is possible only when a proper addressing plan is in place. Route summarization is most effective within a subnetted environment when the network addresses are in contiguous blocks in powers of 2. For example, 4, 16, or 512 addresses can be represented by a single routing entry because summary masks are binary masks—just like subnet masks—so summarization must take place on binary boundaries (powers of 2). If the number of network addresses is not contiguous or not a power of 2, you can divide the addresses into groups and try to summarize the groups separately.
Routing protocols summarize or aggregate routes based on shared network numbers within the network. Classless routing protocols (such as RIPv2, OSPF, IS-IS, and EIGRP) support route summarization based on subnet addresses, including VLSM addressing. Classful routing protocols (RIPv1 and IGRP) automatically summarize routes on the classful network boundary and do not support summarization on any other bit boundaries. Classless routing protocols support summarization on any bit boundary.
NOTE
Summarization is described in RFC 1518, An Architecture for IP Address Allocation with CIDR, available at http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc1518.html.
As an example of the power of summarization, imagine a company that operates a series of pizza shops, with 200 stores in each of the 50 states in the U.S. Each store has a router with an Ethernet and a Frame Relay link connected to headquarters. Without route summarization, the routing table on any of those routers would have 200 * 50 = 10,000 networks.
Instead, if each state has a central site to connect it with all the other states, and each of these routes is summarized before being announced to other states, every router sees its 200 state subnets and 49 summarized entries representing the other states. This results in less CPU, memory, and bandwidth usage.
Route Summarization Calculation Example
Router D in Figure 1-16 has the following networks in its routing table:
172.16.12.0/24
172.16.13.0/24
172.16.14.0/24
172.16.15.0/24
To determine the summary route on router D, determine the number of highest-order (leftmost) bits that match in all the addresses. To calculate the summary route, follow these steps:
|
Step 1 |
Convert the addresses to binary format and align them in a list. |
|
Step 2 |
Locate the bit where the common pattern of digits ends. (It might be helpful to draw a vertical line marking the last matching bit in the common pattern.) |
|
Step 3 |
Count the number of common bits. The summary route number is represented by the first IP address in the block, followed by a slash, followed by the number of common bits. As Figure 1-17 illustrates, the first 22 bits of the IP addresses from 172.16.12.0 through 172.16.15.255 are the same. Therefore, the best summary route is 172.16.12.0/22. |
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Figure
1-17 Summarizing Within an Octet, for Router D in Figure 1-16
NOTE
In this network, the four subnets are contiguous, and the summary route covers all the addresses in the four subnets and only those addresses. Consider, for example, what would happen if 172.16.13.0/24 were not behind router D, but instead were used elsewhere in the network, and only the other three subnets were behind router D. The summary route 172.16.12.0/22 should no longer be used on router D, because it includes 172.16.13.0/24 and might result in confusing routing tables. (However, this depends on how other routers in the network summarize. If the 172.16.13.0/24 route is propagated to all routers, they choose the route with the most bits that match the destination address and should route properly. This is further described in the section "Route Summarization Operation in Cisco Routers.")
In Figure 1-17, the subnets before and after the subnets to be summarized are also shown. Observe that they do not have the same first 22 bits in common and therefore are not covered by the 172.16.12.0/22 summary route.
Summarizing Addresses in a VLSM-Designed Network
A VLSM design allows for maximum use of IP addresses as well as more-efficient routing update communication when using hierarchical IP addressing. In Figure 1-18, route summarization occurs at the following two levels:
Router C summarizes two routing updates from networks 10.1.32.64/26 and 10.1.32.128/26 into a single update: 10.1.32.0/24.
Router A receives three different routing updates. However, router A summarizes them into a single routing update, 10.1.0.0/16, before propagating it to the corporate network.
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Figure
1-18 VLSM Addresses Can Be Summarized
Route Summarization Implementation
Route summarization reduces memory use on routers and routing protocol network traffic, because it results in fewer entries in the routing table (on the routers that receive the summarized routes). For summarization to work correctly, the following requirements must be met:
Multiple IP addresses must share the same highest-order bits.
Routing protocols must base their routing decisions on a 32-bit IP address and a prefix length that can be up to 32 bits.
Routing updates must carry the prefix length (the subnet mask) along with the 32-bit IP address.
Route Summarization Operation in Cisco Routers
This section discusses generalities of how Cisco routers handle route summarization. Details about how route summarization operates with a specific protocol are discussed in the corresponding protocol chapter of this book.
Cisco routers manage route summarization in two ways:
Sending route summaries—Routing information advertised out an interface is automatically summarized at major (classful) network address boundaries by RIP, IGRP, and EIGRP. Specifically, this automatic summarization occurs for routes whose classful network addresses differs from the major network address of the interface to which the advertisement is being sent. For OSPF and IS-IS, you must configure summarization.
Selecting routes from route summaries—If more than one entry in the routing table matches a particular destination, the longest prefix match in the routing table is used. Several routes might match one destination, but the longest matching prefix is used.
Route summarization is not always a solution. You would not want to use route summarization if you needed to advertise all networks across a boundary, such as when you have discontiguous networks. When using EIGRP and RIPv2, you can disable this automatic summarization.
For example, if a routing table has the paths shown in Figure 1-19, packets addressed to destination 172.16.5.99 are routed through the 172.16.5.0/24 path, because that address has the longest match with the destination address.
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Figure
1-19 Routers Use the Longest Match When Selecting a Route
NOTE
When running classful protocols (RIPv1 and IGRP), you must enable ip classless if you want the router to select a default route when it must route to an unknown subnet of a network for which it knows some subnets. Refer to the section "The ip classless Command" in Chapter 2 for more details.
Note that by default (and for historical reasons) the routing table on Cisco routers acts in a classful manner, as described in the sidebar "The Routing Table Acts Classfully" in Chapter 2.
Route Summarization in IP Routing Protocols
Table 1-2 summarizes the route summarization support available in the various IP routing protocols.
Table 1-2 Routing Protocol Route Summarization Support
|
Protocol |
Automatic Summarization at Classful Network Boundary? |
Capability to Turn Off Automatic Summarization? |
Capability to Summarize at Other Than a Classful Network Boundary? |
|
RIPv1 |
Yes |
No |
No |
|
RIPv2 |
Yes |
Yes |
No |
|
IGRP |
Yes |
No |
No |
|
EIGRP |
Yes |
Yes |
Yes |
|
OSPF |
No |
— |
Yes |
|
IS-IS |
No |
— |
Yes |
NOTE
Cisco IOS 12.0 introduced RIPv2's manual summarization feature with the ip summary-address rip command. This command provides limited summarization support; RIPv2 advertises a summarized local IP address pool on the specified interface to dialup clients.
More information on this feature is available in IP Summary Address for RIPv2 at http://www.cisco.com /en/US/products/sw/iosswrel/ps1830/products_feature_guide09186a0080087ad1.html.