User`s guide

8-4 Preliminary November 23, 1998 MultiVoice Gateway for the MAX— User’s Guide
Configuring IP Routing
Introduction to IP routing and interfaces
IP routes
At system start-up, the MultiVoice Gateway builds an IP routing table that contains configured
routes. When the system is up, it can use routing protocols such as RIP or OSPF to learn
additional routes dynamically.
In each routing table entry, the Destination field specifies a destination network address that
may appear in IP packets, and the Gateway field specifies the address of the next-hop router to
reach that destination.
How the MultiVoice Gateway uses the routing table
The MultiVoice Gateway relies on the routing table to forward IP packets, as follows:
• If the MultiVoice Gateway finds a routing table entry whose Destination field matches the
destination address in a packet, it routes the packet to the specified next-hop router,
through its Ethernet interface.
• If the MultiVoice Gateway does not find a matching entry, it looks for the Default route,
which is identified in the routing table by a destination of 0.0.0.0. If that route has a
specified next-hop router, it forwards the packet to that router.
• If the MultiVoice Gateway does not find a matching entry and does not have a valid
Default route, it drops the packet.
Static and dynamic routes
A static route is a manually configured path from one network to another. It specifies the
destination network and the gateway (router) to use to get to that network. If a path to a
destination must be reliable, the administrator often configures more than one static route to
the destination. In that case, the MultiVoice Gateway chooses the route on the basis of assigned
metrics and availability. Each static route has its own Static Rtes profile.
The Ethernet > Mod Config profile specifies a static connected route, which states “to reach
system A, send packets out this interface to system A.”
A dynamic route is a path, to another network, that is learned from another IP router rather than
configured in one of the MultiVoice Gateway unit’s local profiles. Routers that use RIP
broadcast their entire routing table every 30 seconds, updating other routers about the usability
of particular routes. Hosts that run ICMP can also send ICMP Redirects to offer a better path to
a destination network. OSPF routers propagate link-state changes as they occur. Routing
protocols such as RIP and OSPF all use some mechanism to propagate routing information and
changes through the routing environment.
Route preferences and metrics
The MultiVoice Gateway supports route preferences, because different protocols have different
criteria for assigning route metrics. For example, RIP is a distance-vector protocol, which uses
a virtual hop count to select the shortest route to a destination network. OSPF is a link-state
protocol, which means that OSPF can take into account a variety of link conditions, such as the
reliability or speed of the link, when determining the best path to a destination network.
When choosing a route to put into the routing table, the router first compares preference
values, preferring the lowest number. If the preference values are equal, the router compares