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Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring Routing Information (Rev. 03)
Controlling and Modifying Routing Information
© 2008 Foundry Networks, Inc.
Page 21 of 48
Route Updates
All routing protocols exchange information about networks and subnets through route updates.
Route updates transmit the reachability information that routers need for building and maintaining
their route tables. Some routing protocols transmit these updates as broadcasts to everyone on the
subnet while others send them out as multicasts.
Upon initialization, all routers, regardless of routing protocol, send out their entire route tables to
neighbor routers. Thereafter updates (depending on the routing protocol) are sent when triggered
either by periodic timers or by events, such as a change in the network, that cause updates to be
sent. Routing protocols that send periodic updates send them even if no changes have occurred in
the network, thus wasting bandwidth.
How to Control and Modify Routing Information
Redistribution and Policy Routing
More than one routing protocol (i.e. OSPF) can be enabled simultaneously on the device. To
redistribute (or inject) means to allow routing information obtained by another protocol to be
distributed in the update messages. Route redistribution allows the device to exchange routes,
including static routes between the two routing protocols. Such a router is called an Autonomous
System Border Router (ASBR).
Static routes are user-defined routes and are entered manually into the routing table. Static routes
determine a specified path for packets that move from a source to a destination. Redistributing a
static route is the most stable way to advertise a supernet, because route confusion is prevented.
Networks can be advertised by redistributing dynamic routes. A dynamic route is a route that is
learned from another protocol or represented by the IP address of an IP interface that comes up and
was not known when the router was deviceed on.
Routes learned via any routing protocol can be redistributed into any other routing protocol. At the
interdomain level, OSPF can import routes learned via EGP. OSPF routes can be exported into
EGP.
To direct packets to take different paths, to restrict bandwidths or to apply selective rules, the user
can use the approach of policy-based routing. Policy routing provides a mechanism for routing the
information through routers, in addition to the existing mechanism provided by routing protocols.
Routing Metrics and Costs
Metrics are cost values used by routers to determine the best path to a destination network.
Dynamic routing protocols use metrics and algorithms to evaluate the preferred or shortest path to
a particular destination.
For some routing protocols, these metrics are static and may not be changed. For other routing
protocols, the user may assign these values. The most common metric values are hop, bandwidth,
delay, reliability, load, and cost. The metric(s) used depends on the routing protocol implemented.
Table 10
lists the metrics and costs used in the application software.
Table 10: Routing Metrics and Costs
Metric/Cost
Description
Hop
A hop is the unit used by distance vector protocols such as RIPv1 and RIPv2
to measure distance in terms of network nodes traversed by a packet. Each
time a router forwards the packet onto a segment, one hop is added to the
packet metric value. When multiple paths to a destination are available,
routing protocols that use hops as their primary metric value consider the
path with the least network hops to be the best or most preferred path.