3Com 5500G-EI Command Reference Guide - Page 158

158 CHAPTER 6: USING ROUTING PROTOCOL COMMANDS mask-length Enter the number of consecutive 1s in the mask. Because 1s in the 32-bit mask must be consecutive, the mask in dotted decimal format can be replaced by mask-length. interface_name Specify the transmission interface name of the route.Packets that are sent to a NULL interface, are discarded immediately which decreases the system load. gateway_address Specify the next hop IP address of the route. preference_value Enter the preference level of the route in the range 1 to 255. The default is 60. reject Enter to indicate an unreachable route. blackhole Enter to indicate a blackhole route. Description Use the ip route-static command to configure a static route. Use the undo ip route-static command to delete the configured static route. By default, the system can access the subnet route directly connected to the router. If you do not use the parameters preference, reject or blackhole, the route will be reachable by default with a preference level of 60. A static route is a special route. You can set up an interconnecting network with a static route configuration. The problem for such configuration is when a fault occurs to the network, the static route cannot change automatically to steer away from the node causing the fault without the help of an administrator. In a relatively simple network, a system adminstrator may chose to implement static routes rather than a dynamic routing protocol. The proper configuration and usage of static routes can improve the network performance and ensure bandwidth for important applications. All the following routes are static routes: ■ Reachable route - A normal route. That is, the IP packet is sent to the next hop via the route marked by the destination. It is the most 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 will be discarded, and the originating host will be informed destination unreachable. ■ Blackhole route - If a static route to a destination has the "blackhole" attribute, the outgoing interface of this route is the Null 0 interface regardless of the next hop address, and any IP packets addressed to this destination are dropped without notifying the source host. The attributes reject and blackhole are usually used to control the range of reachable destinations of this router and to help troubleshoot the network.