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Need help with configuring frame-relay switching

I have read the cisco's docos and it seems frame-relay switching only works at interface level not subinterface. I have a Cisco 3725 with two serial ports. One port is connected to the frame-relay cloud and the other port is connected back-to-back with another router. There are multiple PVCs coming into the router and I need to switch only one of the PVCs to the other router. This is my config. I like to know if this would work? Or would it effect my other PVCs because the interface is now dce and not dte.

frame-relay switching

interface Serial0/0

description ** 2 MBs Frame-relay **

bandwidth 2000

no ip address

encapsulation frame-relay IETF

clockrate 2000000

no fair-queue

frame-relay lmi-type q933a

frame-relay intf-type dce

frame-relay route 16 interface serial 0/1 16

hold-queue 2048 out


Re: Need help with configuring frame-relay switching

In your configuration the following statement seems to be incorrect

frame-relay route 16 interface serial 0/1 16

Actual syntax of the command

frame-relay route in-dlci interface out-interface-type out-interface-number out-dlci

both the in-dlci and out-dlci seems to be the same, which is incorrect.

The following link has a good exmaple for the FR-Switch, Please go through the America routers config and the Spicy router which acts a hub. If you want only one PVC to be switched the other router, It will have only one frame-relay route command in the above configuration in which you have to correct the in-dlci and out-dlci numbers respectively, And the above config will work.


Re: Need help with configuring frame-relay switching


The fact that your FR interface-type is now DCE only impacts the type of LMI messages it sends to the router connected to it. It should not impact those DLCIs that you are not switching.

Sub-interfaces are only useful for terminating frame-relay connections in order to get to the IP (or other Layer 3) header. When you are doing frame-relay switching, the router just deals with the frame-relay header (does not un-encapsulate it to get to the layer 3 header). Therefore, you need to configure the frame-relay route statement on the main interface, and not a sub-interface.

Even if your frame-interface is configured as DCE, you can create sub-interfaces under it in order to terminate frame-relay connections.

Ok, now to the frame-relay route statement. There is absolutely nothing wrong with having the 'in-dlci' equal to the 'out-dlci'. Remember, that the DLCI has only local significance (i.e. significant only to a single interface). Your in-dlci is 16, which is the DLCI for your Serial0/0 interface. Your out-dlci is also 16, but this time it refers to the Serial 0/1 interface.



Cisco Employee

Re: Need help with configuring frame-relay switching

Hi, you are correct in that frame relay switching works at the interface and not subinterface level. Subinterfaces are a logical way of identifying and segmenting dlci's at layer 3 (for routing purposes) or layer 2 (if your bridging, but that is still above the frame relay layer here).

The config you have is close. ;^) Frame switching is a tricky one, but you need 2 frame relay routes for every dlci. One for incoming in one direction, and another for the return path. For your config, you will also need to go to interface serial 0/1 and add a frame relay route 16 int ser 0/0 16 to route that dlci back out this interface as well.

Switched pvc's dont get terminated, so they "pass through" the main interface. Terminated pvc's need to be mapped to interfaces or subinterfaces for appropriate termination and definition of the traffic.

The dte and dce configs are ONLY relevant across the cable - so as long as whatever you have plugged into serial 0/0 is a dte you should be ok. However, i bet that is your wan cloud, and if that is the case you'll need this to be a dte.

wan (always dce)---- serial 0/0 (dte)-serial0/1(dce)---------"other router"(dte)

hope this helps

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