This section covers policy-based routing (PBR), route summarization, route filtering, and route redistribution. You can use PBR to modify the next hop for packets from what is selected by the routing protocol. PBR is useful when path traffic engineering is required. Routes are summarized at network boundaries to reduce the size of routing tables. Redistribution between routing protocols is required to inject route information from one routing protocol to another. Route filtering is used to control network addresses that get redistributed or to control access to certain parts of the network. A CCNP enterprise designer must understand the issues with the redistribution of routes.

PBR

You can use policy-based routing (PBR) to modify the next-hop addresses of packets or to mark packets to receive differential service. Routing is based on destination addresses; routers look at the routing table to determine the next-hop IP address based on a destination lookup. PBR is commonly used to modify the next-hop IP address based on the source address. You can also use PBR to mark the IP precedence bits in outbound IP packets so that you can apply QoS policies. In Figure 4-18, Router A exchanges routing updates with routers in the WAN. The routing protocol might select Serial 0 as the preferred path for all traffic because of the higher bandwidth. The company might have business-critical systems that use the T1 but may not want systems on Ethernet 1 to affect WAN performance. The company could configure PBR on Router A to force traffic from Ethernet 1 out on Serial 1.

Figure 4-18 Policy-Based Routing

Route Summarization

Large networks can grow quickly, from 500 routes to 1000, to 2000, and higher. Network IP addresses should be allocated to allow for route summarization. Route summarization reduces the amount of route traffic on the network, unnecessary route computation, and the perceived complexity of the network. Route summarization also allows the network to scale as a company grows.

The recommendation for route summarization is to summarize at the distribution layer of the network topology. Figure 4-19 shows a hierarchical network. It has a network core, regional distribution routers, and access routes for sites. All routes in Brazil are summarized with a single 10.1.0.0/16 route. The North American and European routes are also summarized with 10.2.0.0/16 and 10.3.0.0/16, respectively. Routers in Europe need to know only the summarized route to get to Brazil and North America and vice versa. Again, a design best practice is to summarize at the distribution toward the core. The core needs to know only the summarized route of the regional areas.

Figure 4-19 Route Summarization to the Network Core

In this case, you can also use summarization to aggregate four contiguous Class C networks at the /22 bit level. For example, networks 200.1.100.0, 200.1.101.0, 200.1.102.0, and 200.1.103.0 share common bits, as shown in Table 4-9. The resulting network is 200.1.100.0/22, which you can use for a 1000-node network.

Table 4-9 Common Bits in Class C Networks

Binary Address	IP Address
11001000 00000001 01100100 00000000	200.1.100.0
11001000 00000001 01100101 00000000	200.1.101.0
11001000 00000001 01100110 00000000	200.1.102.0
11001000 00000001 01100111 00000000	200.1.103.0

It is important for an Internet network designer to assign IP networks in a manner that permits summarization. It is preferred that a neighboring router receive 1 summarized route rather than 8, 16, 32, or more routes, depending on the level of summarization. Summarization reduces the size of the routing tables in the network.

For route summarization to work, the multiple IP addresses must share the same leftmost bits, and routers must base their routing decisions on the IP address and prefix length. Figure 4-20 shows another example of route summarization. All the edge routers send network information to their upstream routers. Router E summarizes its two LAN networks by sending 192.168.16.0/23 to Router A. Router F summarizes its two LAN networks by sending 192.168.18.0/23. Router B summarizes the networks it receives from Routers C and D. Routers B, E, and F send their routes to Router A. Router A sends a single route (192.168.16.0/21) to its upstream router instead of sending eight routes. This process reduces the number of networks that upstream routers need to include in routing updates.

Figure 4-20 Route Summarization of Networks

Notice in Table 4-10 that all the Class C networks share a bit boundary with 21 common bits. The networks are different on the 22nd bit and thus cannot be summarized beyond the 21st bit. All these networks are summarized with 192.168.16.0/21.

Table 4-10 Summarization of Networks

Binary Address	IP Address
11000000 10101000 00010000 00000000	192.168.16.0
11000000 10101000 00010001 00000000	192.168.17.0
11000000 10101000 00010010 00000000	192.168.18.0
11000000 10101000 00010011 00000000	192.168.19.0
11000000 10101000 00010100 00000000	192.168.20.0
11000000 10101000 00010101 00000000	192.168.21.0
11000000 10101000 00010110 00000000	192.168.22.0
11000000 10101000 00010111 00000000	192.168.23.0

To summarize, the recommended practices regarding summarization include the following:

• Implement summarization at WAN connectivity and remote-access points toward the network core to reduce the size of the routing table.
• Summarize at the distribution layer for all network interfaces that point to the network core.
• Implement passive interfaces on access layer interfaces so that neighbor adjacencies are not established through the access layer. A more specific route might be created, which would be taken over by a summarized route.