Misunderstanding with Frame Relay

[Vuqar Abdullayev]

Hello to everyone

I will be glad if you resolve the following problem.

I  designed frame relay in packet tracer and configured subinterfaces in each router(point-to-point) and enabled Rip so router can exchange information with each other. Everything works well but I do not understand one thing ,since Frame relay does not broadcast by default so how Rip updates propagated through network ?

I uploaded the picture so you can have a look.

[Peter Paluch]

Hi,

Everything works well but I do not understand one thing ,since Frame relay does not broadcast by default so how Rip updates propagated through network ?

This is because instead of the Frame Relay network replicating broadcasts/multicasts, the senders themselves replicated them.

Take any of your routers and enter the show frame-relay map command. This command shows you the mappings of remote IP addresses to virtual circuits (DLCIs) that lead to them. In this output, notice the broadcast keyword at each dynamic mapping. This flag tells the router to create a copy of every broadcast and multicast packet when it exits the Frame Relay interface, and send a copy of this packet to each neighbor whose mapping is flagged with the broadcast flag. Frame Relay itself did not do anything with the broadcasts/multicasts, it just carried them just like any other packets. It was the router itself that performed the replication.

Does this make sense? Feel welcome to ask further!

Best regards,
Peter

[Vuqar Abdullayev]

Hi Peter

Thank you for reply.

Yes ,I noticed that in sh fram map. Broadcast word is shown but I do not understand one issue, since frame relay is non broadcast by default  and should not allow propagating RIP updates so why it shows broadcast in sh frame map now? Which command has turned it on? Maybe subinterface point-to-point  command has made it broadcast ,right?

Thank you

[Peter Paluch · Accepted solution]

Hi,

It is true that Frame Relay is what we call a Non Broadcast Multi Access technology. However, that does not mean that Frame Relay does not support broadcasts at all. What it truly means is that Frame Relay does not care. It does not distinguish between a unicast, multicast, and a broadcast. Frame Relay's only task is to take a frame that has been put onto a specific virtual circuit, and deliver it to the other end of that virtual circuit. Virtual circuits have only two ends (they are point-to-point in nature) and they are entirely independent, which means that a frame from one virtual circuit cannot leak or become replicated into another virtual circuit. That is why multicasts and broadcasts cannot be replicated and multiplied inside a Frame Relay network. However, if a router attached to a Frame Relay sends a broadcast packet by intentionally placing it on one or more virtual circuits, Frame Relay will happily deliver this packet just like any other unicast packet to the other end of each virtual circuit where the packet was originally placed by the router. This is what NBMA truly means.

So in NBMA, you can transmit multicast and broadcast frames. The difference to BMA networks such as Ethernet is that Frame Relay will not replicate the multicasts and broadcasts for you. It is you who must send multiple copies of a broadcast packet, one through each virtual circuit, so that the broadcast truly gets to all intended recipients. In other words, NBMA networks shift the task of replicating multicasts and broadcasts to the end devices attached to NBMA networks.

Broadcast word is shown but I do not understand one issue, since frame relay is non broadcast by default  

Frame Relay is and always stays a NBMA technology. The broadcast flag does not change the nature of Frame Relay, it only changes the way how your router uses the Frame Relay. The broadcast flag was added automatically to the mapping by the InverseARP protocol that did the task of mapping remote IPs to local DLCIs. All dynamic mappings created by InverseARP will have the broadcast flag added automatically - this behavior is hardwired in Cisco IOS. However, this flag does not mean that the Frame Relay has somehow become a broadcast-compliant network. This flag, as I already explained, tells your router to replicate any multicast/broadcast over this particular virtual circuit, so that the broadcast and multicast applications have a feeling that the multicasting and broadcasting works, while in fact, it never worked in Frame Relay, and it is just emulated by your router by doing the replication and multiplication of all multicast/broadcast packets over all DLCIs marked with the broadcast keyword.

Remember: the broadcast keyword does not say what is the type/nature of the network - rather, it tells your router how to use a particular DLCI.

Does this make more sense?

Best regards,
Peter

[Vuqar Abdullayev]

Dear Peter

Thank you so much for a clear explanation. I appreciate your work.So as far as I understand the issud was due the Inverse Arp and broadcast word just meant the behavior of DLCI.Now I understood the behavior of NBMA Frame Relay.

Thank you so much.

[Peter Paluch]

Hello,

You are welcome! :)

Best regards,
Peter