subinterface question

RUN-DMC

Could someone explain or give a quick scenerio on how and when a subinterface should or could be implemented?

thanks1

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Comments

markzab

Ok, so basically if I understand it correctly...

Without a frame switch in frame relay no LMI's are sent. The keepalive "entity" (for all intensive purposes) looks out for these LMIs and since they arent being sent it shuts the interface down/down, thinking that the link is down. In order to keep this from happening, you tell the "entity" to take a hike (no keepalives) and that brings the interface up/up.

Now to go a bit further. I did try searching for the answer before I posted the above and read in one thread that you could actually have LMIs sent on a back to back connection by making the DTE side act as a frame switch? Something about a "frame-relay dte" command? Or something like that?

EdTheLad

Frame relay interfaces are categorized as UNI or NNI, when you setup the FR switch you specify the frame-relay int type as DCE, by doing this you are allowing the switch to respond to Full status enquiry messages and also send the status message(i.e. link integrity message).
A UNI interface can send a status message i.e. link integrity,keepalive whatever you want to call it,different vendors have different names, the interface is also configured to send a full status enquiry message every certain number of keepalives.Only the NNI interface can respond to a full status enquiry and it responds with a full status messages which contains info on all the active dlcis.
If a keepalive is not received by the UNI or NNI, LMI senses a problem and drops, the physical link can be up, but lmi down "show frame-relay lmi" is useful.When you set the "no keepalive" lmi is not waiting for the received status message and is permanently up.No lmi messages are being exchanged but the dlci stays logically up so you can blindly send frames over the interface dlci. This is just a nice workaround for a lab environment.

markzab

It was your post I saw Ed, in regards to the DTE frame-relay command.

So in the end, I CAN set one of my routers on a back-to-back connection to be the switch and proivide LMI messages and therefor keep keepalives on? What was the command to do that? Frame-relay DTE dlciXX??? Something or other.

EdTheLad

In the b2b setup you dont have a FR switch hence you are sending data over a dlci blindly.
The FR interface on a router will work only as a DTE regarding LMI, this is completely seperate to the clocking.The FR switch is configured as a DCE LMI device using the "frame-relay intf-type dce" but in the b2b scenario you dont need to worry about it.

markzab

EdTheLad wrote:

In the b2b setup you dont have a FR switch hence you are sending data over a dlci blindly.

The FR interface on a router will work only as a DTE regarding LMI, this is completely seperate to the clocking.The FR switch is configured as a DCE LMI device using the "frame-relay intf-type dce" but in the b2b scenario you dont need to worry about it.

I don't even need to know this for the test I'm going for now, but I like to know things anyway. Got back into this stuff not especially for my career, but I was bored of knowing everything there is to know about mortgages. In this field there is more than enough information to feed my brain the knowledge it misses.

So in a back to back scenario like mine I could turn one of the machines on my b2b connection into a theoretical frame switch by using the "frame-relay intf-type dce" command. And in the end, by doing that I wouldn't have to turn off keepalives on my s-interfaces. Right?