Skip navigation
Cisco Learning Home > CCNP R&S Study Group > Discussions
This Question is Answered
3415 Views 14 Replies Latest reply: Sep 8, 2011 8:10 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

Currently Being Moderated

OSPF Multicast Question ?

Sep 6, 2011 8:24 AM

Rajesh Agrawal 71 posts since
Nov 2, 2009

Hi All,

 

 

I have a questioned arosen from mind regrading OSPF multicast packet. As we all know OSPF Multicast works on 224.0.0.5 and 224.0.0.6.

 

Yesterday i was going through Anthony Sequeira and Keith Video on youtube for multicast and understood good. However i have doubt about that how ospf client or any OSPF running router joins the multicast group (224.0.0.5 or 6) and if all the router by default enable multicast routing for OSPF or any other routing protocol multicast ?

 

As I understood by the Video that IP Sparse mode or  Dense or IP sparse dense mode we need to enable ip multicast routing on router. As well as we have to use "ip igmp join-group" command to join any group

 

 

Please if you can tell me the process about OSPF or other routing protocol which use multicast.. How those protocol works?

  • miftaul haque 4 posts since
    Mar 9, 2011
    Currently Being Moderated
    1. Sep 6, 2011 8:42 AM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    224.0.0.0/24 are multicast addresses range reserved for local networks.

     

    In OSPF, 224.0.0.5 (all routers) and 224.0.0.6 (Designated Routers) are used, but 224.0.0.6 is only found on MultiAcces networks where DR/BDR elections occurs.

     

    The moment a router is configured with OSPF, it joins itself to the 224.0.0.5 multicast group automatically.
    When the DR/BDR election occurs, then the DR/BDR router joins to 224.0.0.6 multicast address automatically.

     

    Thanks

    Miftaul

  • Martin 13,076 posts since
    Jan 16, 2009
    Currently Being Moderated
    2. Sep 6, 2011 1:10 PM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    OSPF Multicast and Sparse mode or Dense or sparse-dense mode have nothing to do with each other;

     

    You run OSPF on the router, it listens to 224.0.0.5 and/or 224.0.0.6 multicast traffic;

     

    RiP v2 and EIGRP use multicast as well on 224.0.0.9 and 224.0.0.10.

     

    ip multicast routing is for different type of M-cast traffic

  • LeeBrownUSA 102 posts since
    Apr 1, 2009
    Currently Being Moderated
    4. Sep 6, 2011 8:58 AM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    Certain multicast functions related to dynamic routing protocols, OSFP, EIGRP, etc are allowed by default.  Like having multicast enabled all the time with a ACL to restrict it to just what is needed by the dynamic routing protocols.

     

    Cheers,

    Lee

  • LeeBrownUSA 102 posts since
    Apr 1, 2009
    Currently Being Moderated
    6. Sep 6, 2011 9:05 AM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    yes, for the dynamic routing protocols, multicast is allowed by default.

  • Currently Being Moderated
    8. Sep 6, 2011 12:49 PM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    Hi Rajesh

     

    Multicast routing doesn't need to be enabled with routing protocols that multicast updates because the TTL is set to one.  This means that these multicast packets can not be routed beyond a single hop.  When a receiving router receives the update that was multicasted to the 224.0.0.5 or the 224.0.0.6, the TTL will be decremented to 0 and the packet wil then be discarded.  So it's not possible to route these packets throughout the network.

     

    HTH

     

    Erick

     

    Message was edited by: ErickB

  • cadetalain 2,642 posts since
    Sep 18, 2008
    Currently Being Moderated
    12. Sep 7, 2011 4:06 AM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    Hi,

     

    they won't get past the router anyway because this is link-local scope multicast address and furthermore we haven't got any multicast routing protocol enabled.

     

    Alain.

  • Currently Being Moderated
    13. Sep 7, 2011 4:30 AM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    Hi Rajesh

     

    Look at what RFC 5771 has to say about the multicast addresses in the range of 224.0.0.0 - 224.0.0.255, which includes the EIGRP example you mentioned.

     

    4.  Local Network Control Block (224.0.0/24)



       Addresses in the Local Network Control Block are used for protocol

       control traffic that is not forwarded off link.  Examples of this

       type of use include OSPFIGP All Routers (224.0.0.5) [RFC2328].

     

    So even though Cisco decided not to play by the rules officially when it comes to EIGRP, that control plane traffic is not going to be forwarded past the local link, meaning one hop.

     

    Erick

  • Scott Morris - CCDE/4xCCIE/2xJNCIE 8,396 posts since
    Oct 7, 2008
    Currently Being Moderated
    14. Sep 8, 2011 8:10 AM (in response to Rajesh Agrawal)
    Re: OSPF Multicast Question ?

    Link local multicast messages (224.0.0.x/24) will not get routed.

     

    HTH,

     

    Scott

Actions

More Like This

  • Retrieving data ...

Bookmarked By (0)