TMS zl Management and Configuration Guide ST.1.0.090213

9-10
Routing
Dynamic Routing
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 (on the TMS zl Module, TMS VLANs)
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 packets circulating through the network by
assigning different routers different roles. For example, only the desig-
nated 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.
■ When routers send and receive updates and hellos—To lower overhead
and conserve bandwidth, you can alter how often routers send certain
messages.
You can fine-tune the routing protocol to best fit your router’s role in your
network topology. Some protocols provide more flexibility in implementation
than others. In general, OSPF provides more options for customizing adver-
tisements for your particular network environment. However, the configura-
tion for that protocol can be more complex than the configuration for RIP.
Before you implement a routing protocol on your network, you should evalu-
ate the options each protocol provides and then determine which one will
work best for your network environment. Table 9-2 compares how RIP and
OSPF control basic options. You can learn about each protocol in more detail
in the overview for the configuration section on that protocol. See “RIP” on
page 9-13 and “OSPF” on page 9-25.