TMS zl Management and Configuration Guide ST.1.1.100430

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Routing
Dynamic Routing
How Routing Protocols Work
The module constructs its routing table using the information it receives from
other modules and routers. The module changes its routing table in response
to routing updates that provide additional information or notification that
conditions in the network have changed (for example, a link has failed). This
responsiveness explains why using a routing protocol is often called dynamic
routing.
A routing protocol governs how routers exchange routes and other network
information with each other. The protocol must dictate parameters such as
the following:
■ How routers compute a route’s metric and select the best route for their
routing table—Routing protocols can have a relatively complicated sys-
tem for calculating a route’s metric. Usually, you do not need to under-
stand exactly how this calculation is performed. However, you should
understand the criteria that the routing protocol uses to calculate a route,
so that you can select the best routing protocol (or protocols) for your
network environment. If necessary, you can change which routes are
chosen by altering the default metrics that a protocol assigns certain
routes.
■ What information routers include in routing updates—With some routing
protocols, routers exchange their entire routing tables. With other routing
protocols, routers exchange only portions of the routing table. Routers
that are running a link-state protocol, such as OSPF, do not exchange
actual routes. Instead, these routers exchange information about their
links. Each router then uses this information to generate a network
topology and calculates its own best routes according to this topology. In
addition, some routing protocols allow routers to generate route
summarizations or summary routes, which advertise an entire range of
networks in a single entry in the routing update.
■ Which routers and router interfaces send and receive updates—Most
protocols specify that when routers receive an update on an interface,
they do not send the same update from that interface. This common sense
rule minimizes overhead. Some routing protocols also cut down on pack-
ets circulating through the network by assigning different routers differ-
ent roles. For example, only the designated router (DR) in an OSPF subnet
floods link-state advertisements (LSAs) to other routers in the subnet, and
only area border routers (ABRs) store information about the entire AS. If
you understand the role of each router in your WAN, you can configure
routers to minimize congestion.