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2406 Views 9 Replies Latest reply: May 14, 2011 11:04 AM by Julian RSS

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Frame relay- non broadcast

Feb 5, 2011 1:56 PM

Subramaniya Karthic 184 posts since
Aug 3, 2010

I have a really simple doubt

 

In FR networks, while using OSPF we configure static neighbors in OSPF. This is because the FR doesnot allow multicasting.

 

My doubt is when we use "frame-relay map ip <address> <dlci> broadcast" in a interface it means we are sending broadcast packets for the network

and when "broadcast" command is not used we are statically configuring neighbors in OSPF.

 

Does this means the capability of a FR nw to broadcast depends on the "frame-relay map" command? I dont think so?

If this is not the case why would a neighborship be formed in a network only when "broadcast" keyword is used ?

 

Can there be any configuration made in the frame relay switch that can stop broad casts?

 

Thank you all


R.S.Karthic

  • Currently Being Moderated
    1. Feb 5, 2011 2:12 PM (in response to Subramaniya Karthic)
    Re: Frame relay- non broadcast

    Technically, frame relay still doesn't support "broadcast" or "multicast" traffic, even with the broadcast command on the frame relay map, or when using Inverse ARP. 

     

    What happens, is that it sees that broadcast/multicast traffic needs to be sent to the far-end destination across a specific DLCI, it then wraps the broadcast/multicast in a unicast header, which allows it to be sent across the frame relay network just like any other traffic. 

  • Currently Being Moderated
    3. Feb 5, 2011 6:13 PM (in response to Subramaniya Karthic)
    Re: Frame relay- non broadcast

    There is a difference between OSPF network types, and frame-relay topologies.

     

    If your frame relay supports broadcasts, you can allow OSPF to communicate dynamically.

     

    If your frame relay does not support broadcasts, you need to tell OSPF this by setting the corresponding network type under the interface, and if necessary, configure static neighbors.

     

     

    Frame relay topologies are the key factor here, it sounds like you are still in the process of learning frame relay. I am no frame relay expert, but I will certainly help try to help until I reach the limit of my own personal comprehension of the topic!

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    5. Feb 6, 2011 5:44 AM (in response to Subramaniya Karthic)
    Re: Frame relay- non broadcast

    yes, the capability of FR to do broadcasts is dependent on the "broadcast" keyword (or point to point subinterfaces).  If you don't want broadcasts, don't use that keyword.  And it won't occur!

     

    Remember though, that routing protocols are just part of that.  CDP is similar.  IP Multicast traffic (if you run it) is similar.

     

    But by default, Frame Relay is an NBMA network (NON Broadcast Multiple Access).

     

    HTH,

     

    Scott

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    7. Feb 6, 2011 7:06 PM (in response to Subramaniya Karthic)
    Re: Frame relay- non broadcast

    The part of "though they are send as unicasts" you need to be careful with so that you don't confuse yourself.

     

    The format of the packet really doesn't change.  The IP header will still say 224.0.0.5 (OPSF) or 255.255.255.255 (if broadcast), or whatever particular group address you have...

     

    There's no MAC in a frame-relay header, so the "all-F's" is not really applicable.  It's just that the broadcast/multicast message will get COPIED to each of the DLCIs that has the "broadcast" parameter set.  That doesn't technically MAKE it a unicast message, it just makes it a replicated message.

     

    HTH,

     

    Scott

  • Scott, that is good to know.  I honestly haven't gotten to that kind of packet level detail on FR - nor have I seen it used outside of the lab in several years.

  • Julian 24 posts since
    Jan 2, 2011
    Currently Being Moderated
    Re: Frame relay- non broadcast

    Hi all,

     

    Only three short questions about this topic:

     

    1 - By the way, it is worthwhile to built a non-broadcast FR network, as it is by default?

     

    2 - I mean, what are the inconvenients of allowing broadcasts in such a network?

     

    3 - So, if we used for example RIP or OSPF in a FR network, we should allow broadcast to make it works well, shouldn't we?

     

    Regards!

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