IOS Gotcha : Frame Relay and PPP

frame-relay interface-dlci 304 ppp  

This is the first of a series of postings I intend to write covering some IOS ”gotchas”: pitfalls for the unwary in the CLI.

Suppose you have configured the Virtual-Template, configured the Frame Relay subinterface, but you have forgotten to specify the PPP, so you have something like this:

interface Serial0/0.34 point-to-point
 frame-relay interface-dlci 304
 end

You then realize you mistake and try and add on the PPP stuff:

R3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#interface Serial0/0.34 point-to-point
R3(config-subif)#frame-relay interface-dlci 304 ppp Virtual-Template1
R3(config-fr-dlci)#end
R3#

Has it taken it? No it hasn’t!

R3#show run int S0/0.34
Building configuration... Current configuration : 80 bytes
!
interface Serial0/0.34 point-to-point
 frame-relay interface-dlci 304
end

If you want to add the PPP stuff, you have to remove the interface DLCI, and then put it back again with the PPP link:

R3#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#interface Serial0/0.34 point-to-point
R3(config-subif)#no frame-relay interface-dlci 304
R3(config-subif)#frame-relay interface-dlci 304 ppp Virtual-Template1
R3(config-fr-dlci)#end
R3#

Now we have it:

R3#show run int S0/0.34
Building configuration... Current configuration : 99 bytes
!
interface Serial0/0.34 point-to-point
 frame-relay interface-dlci 304 ppp Virtual-Template1
end

The moral of the story is to make no assumptions, and always look at your running configuration after you have configured it.