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Basic Asynchronous Configuration 79
Example 4-4 shows the distinction between the physical and logical parameters and where they
are defined in the router configuration file.
Three types of router interfaces can be configured for serial communication:
·
Asynchronous interfaces
·
Synchronous/asynchronous interfaces (A/S)
·
Synchronous interfaces
Router interfaces that are synchronous only cannot be used for modem or asynchronous
communication. On the router models with A/S ports, the serial ports default to synchronous,
and the interface must be declared for asynchronous usage using the physical-layer async
command.
The configuration in Example 4-4 is for the first (port 0) synchronous/asynchronous interface
on a four-port A/S card in the third slot of a 3600. The physical-layer async is needed because
this device has A/S ports. Hence, the physical-layer async command is entered at the
router(config-if)# prompt for Serial 3/0. On the other hand, in the case of those routers that
have ports designated as asynchronous, only the physical-layer async command is not used.
Logical Considerations on the Router
Logical considerations are configured on the interface of the router. These include the network-
layer addressing, the encapsulation method, the authentication, and so on. The configuration in
Example 4-5 is for a serial interface that is used to receive an inbound call.
In Example 4-5, the physical-layer async command places the serial 2 interface in
asynchronous mode. Once this command is issued, the router treats the interface as an
asynchronous port. This can be done on ONLY those interfaces that are defined as A/S.
The ip unnumbered Ethernet0 command declares that the interface assume the address of the
E0 interface. This enables the saving of IP addresses but makes the interface non-SNMP
manageable. This command could be replaced with the desired IP address of the interface (refer
Example 4-5
Router Configuration for Serial Interface Receiving Inbound Calls
interface Serial2
physical-layer async
ip unnumbered Ethernet0
ip tcp header-compression passive
encapsulation ppp
bandwidth 38
async mode interactive
peer default ip address pool remaddpool
no cdp enable
ppp authentication chap