3Com SuperStack 4, Configuration Manual

Page: 103 / 432

Static Routes 115
Import Routes of Other Protocols” on page 125 , “Configuring OSPF to Import the
Default Route” on page 146 and “Importing Routing Information Discovered by
Other Routing Protocols” on page 158 .
Static Routes
A static route is a route that is manually configured by the network administrator.
You can set up an interconnected network using static routes. However, if a fault
occurs in the network, the static route cannot change automatically to steer
packets away from the fault without the help of the administrator.
In a relatively simple network, you only need to configure static routes to make the
router work normally. Proper configuration and usage of the static route can
improve network performance and ensure bandwidth for important applications.
The following routes are static routes:
■
Reachable route — The IP packet is sent to the next hop towards the
destination. This is a common type of static route.
■
Unreachable route — When a static route to a destination has the reject
attribute, all the IP packets to this destination are discarded, and the originating
host is informed that the destination is unreachable.
■
Blackhole route — If a static route to a destination has the blackhole attribute,
all the IP packets to this destination are discarded, and the originating host is
not informed.
The attributes reject and blackhole are usually used to control the range of
reachable destinations for the router, and to help troubleshoot the network.
Default Route
The default route is also a static route. The default route is used only when no
suitable routing table entry is found. In a routing table, the default route is in the
form of the route to the network 0.0.0.0 (with the mask 0.0.0.0). You can
determine whether a default route has been set by viewing the output of the
display ip routing-table
command. If the destination address of a packet fails
to match any entry of the routing table, the router selects the default route to
forward this packet. If there is no default route and the destination address of the
packet fails to match any entry in the routing table, the packet is discarded, and an
Internet Control Message Protocol (ICMP) packet is sent to the originating host to
indicate that the destination host or network is unreachable.
In a typical network that consists of hundreds of routers, if you used multiple
dynamic routing protocols without configuring a default route then significant
bandwidth is consumed. Using the default route can provide appropriate
bandwidth for communications between large numbers of users.
Static Routes configuration is described in the following sections:
■
Configuring Static Routes
■
Troubleshooting Static Routes