3Com S7902E, Configuration Manual

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The network administrator can configure a default route with both destination and mask being
0.0.0.0. The router forwards any packet whose destination address fails to match any entry in the
routing table to the next hop of the default static route.
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Some dynamic routing protocols, such as OSPF, RIP and IS-IS, can also generate a default route.
For example, an upstream router running OSPF can generate a default route and advertise it to
other routers, which install the default route with the next hop being the upstream router.
Application Environment of Static Routing
Before configuring a static route, you need to know the following concepts:
1) Destination address and mask
In the ip route-static command, an IPv4 address is in dotted decimal format and a mask can be either
in dotted decimal format or in the form of mask length (the digits of consecutive 1s in the mask).
2) Output interface and next hop address
While configuring a static route, you can specify either the output interface or the next hop address
depending on the specific occasion. The next hop address can not be a local interface IP address;
otherwise, the route configuration will not take effect.
In fact, all the route entries must have a next hop address. When forwarding a packet, a router first
searches the routing table for the route to the destination address of the packet. The system can find the
corresponding link layer address and forward the packet only after the next hop address is specified.
When specifying the output interface, note that:
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If the output interface is a Null 0 interface, there is no need to configure the next hop address.
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If the output interface is a point-to-point interface, there is no need to configure the next hop
address. You need not change the configuration even if the peer’s address changes. For example,
a PPP interface obtains the peer’s IP address through PPP negotiation, so you need only specify
the output interface.
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If the output interface is an NBMA or P2MP interface, which support point-to-multipoint network,
the IP address to link layer address mapping must be established. Therefore, it is recommended to
configure both the next hop IP address and the output interface.
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You are not recommended to specify a broadcast interface (such as an Ethernet interface, virtual
template, or VLAN interface) as the output interface, because a broadcast interface may have
multiple next hops. If you have to do so, you must specify the corresponding next hop for the output
interface.
3) Other attributes
You can configure different preferences for different static routes so that route management policies can
be applied more flexibly. For example, specifying the same preference for different routes to the same
destination enables load sharing, while specifying different preferences for these routes enables route
backup.
Configuring a Static Route
Configuration Prerequisites
Before configuring a static route, you need to finish the following tasks:
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Configure the physical parameters for related interfaces
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Configure the link-layer attributes for related interfaces