[R1]---[FRSW]---[R2] - is the topology. Everything is L2, there are no PVC's, all physical interfaces. At startup everything is running with HDLC. Enable CDP - we get CDP neighbors across the board.

Next steps - enabled FR switching on FRSW, made its interfaces FR encapsulation, switched interface type to DCE, the branch routers simply changed encapsulation to FR (so they're FR DTE's). Then manually enabled CDP on each FR interface. No CDP neighbors.

Why? CDP is a L2 technology. I show up/up for all interfaces. Thanks in advance !

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Commented Aug 9, 2017 at 18:52

1 Answer 1


Via (one of many) very old discussions on the Cisco Support Forums:

CDP will transit through the Frame Relay PVC carried by the FR switch. By default it's disabled on FR interfaces, but you can enable it. Then the end point routers will see one another, but you won't see the intermediate frame switch(es). [link]

Yes, CDP is layer-2. However, the frame switch is a frame switch. CDP isn't frame-relay, so it will ignore it. Once a PVC is created, CDP will flow through it, but not before. As there's no VCs, R1 and R2 aren't connected to each other.

  • So, I'm glad you brought this up, and it's one I had read. It says the FRSW will just 'ignore' the CDP packets, but while capturing on a non-FRSW like R1, the interface actually stopped issuing CDP packets altogether - even after I manually renabled CDP on the capture interface, and bounced the interface etc. I'm also not 100% on why you highlighting 'frame switch' - can you elaborate? I think maybe you mean it won't process anything but FR frame, but don't want to assume. Thanks for your response !
    – A L
    Commented Jan 16, 2015 at 4:40

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