Frame relay (point to point and multipoint)

[iamtheone12345]

Hi Guys,

I couldnt help but notice that the #show frame-relay command doesn't show the layer 3 addresses that the DLCI is used to reach for

pont-to-point interfaces.

Like for example your :

R4#sh frame-relay map

Serial0/0.401 (up): point-to-point dlci, dlci 401(0x191,0x6410), broadcast

          status defined, active

Clearly instead of listing the ip address (layer 3 address) it lists point-to-point.

Can you please tell me the reason why,Because it does show for the multipoint interfaces.

[Peter Paluch]

Hello Arjun,

On point-to-point subinterfaces in Frame Relay, the logic is very simple: all IP addresses are on the other side of the PVC. If your routing table tells you to send a packet out some point-to-point subinterface, you do not search for any IP-to-DLCI mapping because the point-to-point logic mandates: it is at the other side, so just send it. There is nothing to choose from, anyway: there is only a single DLCI terminated under the point-to-point subinterface so it does not make sense to look for particular mappings.

That is why your router does not display any IP mapping for point-to-point subinterfaces. In fact, even your own IP address is considered to be located at the other end: if your ping your own subinterface, the ICMP packet is sent to the other side, and the other side routes it back to you This is, by the way, how all point-to-point interfaces work, including PPP and HDLC framing.

Best regards,

Peter

[iamtheone12345]

Hi Peter(that was fast!!!),

Thanks for your reply.I understood the first part of your answer.But i dint understand what you meant by "even your own ip

address is considered to be located at the other end" ??

Could you throw some light on it ??

[Peter Paluch · Accepted solution]

Hello Arjun,

even your own ip address is considered to be located at the other end

Oh, sure. Usually, if you ping your own IP address, your IP driver discovers that you are pinging yourself, and does not really transmit any real packet over an interface. Rather, the packet is generated and delivered inside your operating system only, but it is not truly sent over any interface. It remains stored only in the memory of your operating system.

This logic does not apply to point-to-point interfaces, however. If you ping your own IP address on a point-to-point (sub)interface, your operating system generates the packet, applies the logic of all IP addresses are on the other side of the PVC and following it, it truly sends the IP packet out the point-to-point interface. This packet will be carried through the PVC, received by the device on the other end, and there, the device will simply route it back to you.

Does this make it a little more clear? Please feel welcome to ask further!

Best regards,

Peter

[iamtheone12345]

Hi Peter,

I couldnt have asked for a better explanation.Thank you very much.i am curious though as i see a VIP icon at the side of your name.On a lighter note did i get an answer from a guest CCIE ???

[Peter Paluch]

Hello Arjun,

i am curious though as i see a VIP icon at the side of your name.On a lighter note did i get an answer from a guest CCIE ???

I am not sure if I understand your question correctly. Regarding the CCIE, I am CCIE certified in the RS track. I do not quite understand the question about the VIP icon, sorry

Best regards,

Peter