- 54 -
Configuring IP Routing
IP routing protocols are divided into two classes: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols
(EGPs).
Note:
The word
gateway
is often a part of a routing protocol’s name, since many routing protocol specifications refer
to routers as gateways. However, a protocol translation gateway is usually defined by the Open System
Interconnection (OSI) reference model as a Layer 7 device, whereas a router is a Layer 3 device, and routing
protocol activities occur at the Layer 3 level.
Interior gateway protocols are used to exchange routing information among routers in an autonomous network, such
as a company’s LAN. A routing protocol determines how routers in a network share and update information and
report changes, enabling a network to be dynamic instead of static. All IP interior gateway protocols must be specified
with a list of associated networks before routing activities can begin on the switch. The IC35516 supports the Open
Shortest Path First (OSPF) and Routing Information Protocol (RIP) as interior gateway protocols.
Exterior protocols are used to exchange routing information between networks that do not share a common
administration. The supported exterior gateway protocol is Border Gateway Protocol (BGP).
With any of the IP routing protocols, you must create the routing process, associate networks with the routing
process, and customize the routing protocol for a particular network.
5.3 Configuring Static Routes
Static routes are user-defined routes that cause packets that are moving between a source and a destination to take
a specified path. Static routes can be important if the switch cannot build a route to a particular destination.
To configure static routes, perform the following task in global configuration mode:
Command Purpose
ip route
{
prefix mask
|
prefix-length
}
address
|
interface
} [<
1-255
>]
Establish a static route.
Note
: The numeric value is the static administrative distance. Enter a number between 1 and 255. See Table 5-3 for
a list of default administrative distances for common routing protocols.
The software retains the configured static routes until they are removed, using the
no ip route
global configuration
command. However, you can override the static routes with dynamic routing information through the assignment of
administrative distance values. Each dynamic routing protocol has a default administrative distance, as listed in Table
5-3. If you would like a static route to be overridden by information from a dynamic routing protocol, you will need to
ensure that the administrative distance of the static route is
higher
than that of the dynamic protocol, since the lower
value will be used. For example, if a route is known both by OSPF and RIP, the OSPF route will be used, since its
default administrative distance is lower than RIP.
Note
: Static routes that point to an interface will not be advertised via RIP, nor by other dynamic routing protocols,
unless a
redistribute static
command is specified for these protocols.
When an interface goes down, all static routes through that interface are removed from the IP routing table. Also,
when the software can no longer find a valid next hop for the specified forwarding router's address in a static route,
the static route is removed from the IP routing table.
Route Source
Default Distance
