Install guide

Filtering IP Routes 2-3
Software Version 2.7.5
C613-10454-00 REV A
Introduction
This chapter describes the router or switch’s functions for filtering IP routes. IP
route filtering enables you to control your routing tables, for example, to meet
the terms of business relationships you have with the networks you are
connected to.
If you are a network provider, you can filter the routing information that your
routers or switches receive from the networks they connect to, and that they
advertise to those networks. This gives you control over the path of any traffic
originating from or traversing your network. Usually, one or more of your
routers or switches form peer relationships with routers or switches at other
ISPs with which you have entered into data transporting agreements. The
process of filtering is, in effect, the process of specifying the routes that your
routers or switches send or receive from each of their peers.
The router or switch provides several different mechanisms for filtering routes.
Some of the functionality of these mechanisms overlaps, so sometimes you can
achieve a given filtering effect in several ways. This chapter discusses all the
different mechanisms and places them in context within the overall picture of
how you can filter routes.
In very general terms, configuring any filter involves three steps. This chapter
is divided into sections that describe each of the steps:
1. Select the required filter type, as described in Types of Filters.
2. Create the filter, as described in Creating Filters.
3. Apply it, as described in Applying Filters.
When to use filters You can use route filtering to select which routes:
■ the router or switch copies from a routing protocol into its Routing
Information Base (RIB). This determines which routes the router or switch
uses to send traffic (Applying Filters When Writing to the RIB).
■ the router or switch copies from its RIB into a routing protocol. This
determines which routes the protocol has available for advertising to
neighbouring devices (Applying Filters When Redistributing from
the RIB).
■ routing protocols actually advertise to neighbouring devices (Applying
Filters Before Advertising Routes).
The RIB is another term for the router or switch’s main IP route table, which is
described in The Routing Table in the Internet Protocol (IP) chapter of the
Software Reference.
Types of routes you
can filter
As explained above, this chapter first divides the information about filtering
into sections about each type of filter, rather than each type of routing protocol.
Then it summarises the available filters for each routing protocol, in the
following sections:
■ Border Gateway Protocol (BGP-4)
■ Open Shortest Path First (OSPF)
■ Routing Information Protocol (RIP)
■ Interface Routes
■ Statically-Configured Routes.