Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Routing Information (Rev. 03)
Controlling and Modifying Routing Information
© 2008 Foundry Networks, Inc.
Page 20 of 48
Controlling and Modifying Routing Information
Foundry Networks provides flexible filtering features to control the routing information
redistribution between routing devices. Route filtering is used for manipulating the exchange of
routing information between routing devices. Once a routing filter is defined, it can be used in the
routing protocols (such as OSPF).
A routing filter enables the device to manipulate the routing information of inbound packets, based
on standard protocol fields. Routing filters are applied to incoming and outgoing routing update
traffic. Routing filter criteria indicate the actions to be performed with the control packets.
The exchange of routing information occurs within logical boundaries referred to as Autonomous
Systems (AS). Routing information within the application software is controlled and modified by
means of routing filters. The access lists, IP prefixes and route maps represent the routing filters in
Foundry Networks. Routing filters are used to exchange and redistribute routing information as
well as for policy routing. Filtering features are included in the routing protocol implementations.
Overview
Autonomous Systems
The Autonomous System (AS) is a collection of routers (gateways) and hosts. Local Area
Networks (LANs) and Wide Area Networks (WANs) are interconnected by IP routers and form a
group of networks. An AS generally is a network controlled by a single administrative authority.
For administrative purposes, the Internet is divided into autonomous systems. An autonomous
system consists of a group of routers and networks sharing a common routing protocol.
Routing Protocols and Updates
The purpose of dynamic routing protocols is to exchange information on the networks. The routing
protocols provide the routers with real-time information about the location of the networks, the
preferred path for accessing a particular network, which parallel paths can be used to balance the
load among multiple channels, etc. If routers have this information, they can maintain adequate
routing tables.
Open Shortest Path First (OSPF) Protocol
- The OSPF protocol is an interior gateway protocol
(IGP) intended for use in large networks. OSPF exchanges routing information between routers in
an autonomous system using a link state algorithm. Routers synchronize their topological
databases. Once the routers are synchronized and the routing tables are built, the routers will return
topology information in response to a topological change. For OSPF, the best path to a destination
is the path that offers the least cost metric delay. In OSPF, cost metrics are configurable, allowing
the user to specify preferred paths.
Exterior Gateway Protocols (EGP)
Routers in separate autonomous systems use an exterior gateway protocol to communicate with
each other..
EGP protocols connect separate ASs, providing a transit path between and through these ASs to
facilitate traffic forwarding across the Internet. The Exterior Gateway Protocol is used to exchange
network reachability information between routers in different autonomous systems. An IGP, such
as OSPF, is used within an AS to facilitate the communication of routing information within the
autonomous system.