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Implementing Basic DDR 147
To function, the router must understand what phone number to dial as well as when to dial it.
How does it come to know this information? The same way it knows everything else it knows--
the administrator tells it.
The administrator uses dialer maps to tell the router how and when to dial a particular
destination. Dialer maps serve the same basic function as does ARP in a LAN: the mapping of
network layer addresses to data link layer addresses. In this case, the data link layer address is
the phone number. To get to a specific destination, the router must associate the proper
destination phone number with the next logical hop protocol address.
Step 5: Configuring Protocol Addressing
Once the encapsulation has been decided upon, you must apply a protocol addressing scheme.
You can configure DDR with any routable protocol. Each protocol that must pass across the link
must have a configured address.
For IP implementations, you must supply an IP address and subnet mask to the interface. The
protocol addressing scheme should be decided upon well in advance of any deployment of any
networking technology.
In IPX implementations, you must apply an IPX network number to the BRI interface. The host
portion of the address is hard-coded in the global configuration or is taken from the Burned In
Address (BIA) of the lowest numbered LAN interface (that is, Ethernet 0). When IPX routing
is enabled and IPX network numbers are configured on interfaces, the IPX RIP and the SAP
protocols are automatically enabled for those interfaces.
IPX RIP and SAP are broadcast-based updates for routing table information and Novell
NetWare service propagation, respectively. These broadcasts are on independent 60-second
timers. You might or might not wish for this traffic to go across your ISDN link. To avoid this
traffic, you can simply not include RIP and SAP in your interesting traffic definitions. This is
accomplished by implementing IPX access lists to filter out RIP and SAP. The access lists are
then associated with the dialer list defining interesting traffic. At this point, RIP and SAP go
across the link only as long as the link is up because of the transfer traffic that fits the interesting
parameters.
You can also define IPX static routes and static SAP entries. Example 6-8 shows the
encapsulation as well as the application of an IPX network number and an IP address to the BRI
interface.
Example 6-8
Protocol Addressing
RouterA(config)#interface BRI0
RouterA(config-if)#ip address 10.12.1.1 255.255.255.0
RouterA(config-if)#ipx network number 80fa
RouterA(config-if)#encapsulation ppp