Cisco IOS Access Lists

Chapter 6. Route Maps

For more advanced network policies, Cisco routers have a policy structure called route maps. Route maps combine policy set definition with policy application, using the access list formats discussed in previous chapters. In this chapter I talk about using route maps for intranet routing policies and for routing policies in the Internet. Along the way, I cover some new access list types, key BGP (Border Gateway Protocol, the routing protocol used on the Internet) concepts, and how to implement commonly used Internet routing policies, focusing on the needs of multihomed organizations, not ISPs.

Other access list types

For most configurations, standard and extended access lists are enough to specify which sets of IP addresses or networks you want to influence with a policy. When you want to set policies with BGP or manipulate sets of networks based on their prefix length (the number of bits in the network mask), these types of access lists fall short. BGP is an Exterior Gateway Protocol (EGP) routing protocol. EGPs are designed for sending routing updates between large administrative domains. As a result, BGP routes carry a lot of information. Unlike Interior Gateway Protocols (IGP) such as OSPF or EIGRP, BGP routing updates carry complete path information. The routing updates can also carry flags added by network administrators called community attributes. Both paths and communities are often the basis of Internet routing policies. In this section, I ...