This chapter covers the following key topics:

•  Symmetry, Redundancy, and Load Balancing
Configuration examples for dynamically and statically learned defaults, multihoming to single and multiple providers, load balancing, and customers sharing a backup link.

•  Following Defaults Inside an AS
Configuration examples for setting defaults in a variety of architectures. This section examines a particular routing problem assuming a variety of different IGPs.

•  Policy Routing
Configuration example for routing based on source rather than destination.

•  Route Reflectors
A practical example of using route reflectors in conjunction with peer groups.

•  Confederations
A practical example of how to configure confederations.

•  Controlling Route and Cache Invalidation
Syntax and practical example of BGP’s soft reconfiguration feature.

•  Route Dampening
Syntax and practical example of BGP’s route dampening feature.

Chapter 11
Configuring Effective Internet Routing Policies

The preceding chapter covered configuration examples for all the basic functions and attributes of BGP. In this chapter, we will consider examples that address the challenging, potentially conflicting design goals of routing architecture. We will also look at examples designed to help manage growing, complex ASs.

Perhaps the most challenging part of configuring your network's routing architecture is determining what your routing policies should be. This step, of course, must come before the actual configuration process, and there is no simple, lockstep method for determining your policies. A careful analysis of network needs, behavior, and potential growth will reveal a unique set of problems and optimal solutions for every network.

Redundancy, Symmetry, and Load Balancing

The following examples will illustrate the implementation of different route redundancy, symmetry, and load balancing scenarios. Please remember that these scenarios are not cast in stone. Many variations of these techniques can be used to fit your situation. The examples presented here should guide you to a better understanding of how policies are set. We will first go over a brief implementation of default routes.

Dynamically Learned Defaults

It is important to control defaults in BGP because, if they are originated randomly, they could cause everybody serious problems. Problems occur when a BGP speaker that intends to originate a default to a specific peer ends up flooding the default over all its neighbors. Cisco provides a way to target the default toward a specific neighbor.

In figure 11-1, RTA is originating a default route 0.0.0.0 0.0.0.0 toward RTC only. IBGP neighbors, such as RTF, will not get the default. RTA's configuration follows.

Figure 11-1  Dynamically learned defaults.

RTA configuration:

    router bgp 3
    no synchronization
    network 172.16.1.0 mask 255.255.255.0
    neighbor 172.16.20.1 remote-as 1
    neighbor 172.16.20.1 default-originate
    no auto-summary

The default-originate option of the neighbor router subcommand will cause 0/0 to be sent toward RTC. This is shown in the BGP and IP routing tables of RTC:

    RTC#show ip bgp
    BGP table version is 14, local router ID is 192.68.11.1
    Status codes: s suppressed, d damped, h history, * valid, > best,
    i - internal Origin codes: i - IGP, e - EGP, ? - incomplete

    Network         Next Hop           Metric LocPrf Weight Path
    *> 0.0.0.0        172.16.20.2             0 3 i
    *> 172.16.1.0/24  172.16.20.2      0           0 3 i
    *> 192.68.11.0   0.0.0.0       0           32768 i

    RTC#show ip route
    Codes: C - connected, S - static, I - IGRP, R - RIP,
      M - mobile, B - BGP
      D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
      E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
      i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2,
      * - candidate default U - per-user static route

    Gateway of last resort is 172.16.20.2 to network 0.0.0.0

    C  192.68.11.0/24 is directly connected, Ethernet0/0
    C    172.16.20.0/24 is directly connected, Serial2/1
    B*  0.0.0.0/0 [20/0] via 172.16.20.2, 00:04:40

The routing table of RTC indicates that RTC has dynamically learned the 0/0 default from RTA and has set its gateway of last resort to 172.16.20.2, which is RTA.

Defaults can also be originated over all BGP peers by using the "network 0.0.0.0" router command, as long as the router advertising this default already has its own default. The following configuration can be used assuming that RTA has a default route itself (default could be created via a static route).

RTA configuration:

    router bgp 3
    no synchronization
    network 0.0.0.0
    network 172.16.1.0 mask 255.255.255.0
    neighbor 172.16.20.1 remote-as 1
    no auto-summary