Configuring SDLC for Multidrop Connections

2017-11-03 09:05:05

Problem

You want to configure a serial port for an SDLC multidrop line supporting several devices.

Solution

SDLC supports multidrop connections. These are serial links that connect to several downstream devices in series. Each device has its own SDLC address, which must be configured in the router. The global DLSw configuration for this example is omitted here because it is identical to the previous example:

dlsw-branch#configure terminal Enter configuration commands, one per line. End with CNTL/Z. dlsw-branch(config)#interface Serial1 dlsw-branch(config-if)#description Connection to three remote SDLC devices dlsw-branch(config-if)#encapsulation sdlc dlsw-branch(config-if)#no keepalive dlsw-branch(config-if)#nrzi-encoding dlsw-branch(config-if)#clock rate 4800 dlsw-branch(config-if)#sdlc role primary dlsw-branch(config-if)#sdlc vmac 4000.CCCC.0000 dlsw-branch(config-if)#sdlc poll-pause-timer 200 dlsw-branch(config-if)#sdlc address 20 dlsw-branch(config-if)#sdlc xid 20 017A0006 dlsw-branch(config-if)#sdlc partner 4000.3745.AAAA 20 dlsw-branch(config-if)#sdlc address 21 dlsw-branch(config-if)#sdlc xid 21 017A0007 dlsw-branch(config-if)#sdlc partner 4000.3745.AAAA 21 dlsw-branch(config-if)#sdlc address 22 dlsw-branch(config-if)#sdlc xid 22 017A0008 dlsw-branch(config-if)#sdlc partner 4000.3745.AAAB 22 dlsw-branch(config-if)#sdlc slow-poll 30 dlsw-branch(config-if)#sdlc dlsw 20 21 22 dlsw-branch(config-if)#end dlsw-branch#

Discussion

The basic router configuration is the same here as it is for the single device shown in Recipe 15.5, with a few differences. The first difference you should notice is that all three of the SDLC addresses are configured, where the previous recipe had only one. Each SDLC address appears in four places, and you must ensure that all four are configured for all SDLC devices.

There must be an sdlc address command for each address. You must define the XID for each device with an sdlc xid command. And you must associate each SDLC address with a FEP Token Ring MAC address with the sdlc partner command. In this example, the sdlc partner command for the third SDLC address, 22, is associated with a different FEP MAC address than the other two, just to show how this can be done. In most cases, you would probably want to use the same FEP for all of the devices on a port, though.

Finally, you must associate each of these SDLC addresses with a DLSw bridge as follows:

dlsw-branch(config-if)#sdlc dlsw 20 21 22

This tells the router to share these three specific SDLC addresses with DLSw. The router will not accept this command unless there is a matching sdlc address command defining each of these addresses.

Besides defining the additional SDLC addresses for the multidrop operation, this example also includes the command sdlc slow-poll with an argument of 30. This tells the router only to poll each device to ask for its data every 30 seconds rather than the default 10 seconds. This is useful in multidrop configurations because it is often difficult to service all of the devices within the required time interval.

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