14 Replies Latest reply: Sep 8, 2011 8:10 AM by Scott Morris - CCDE/4xCCIE/2xJNCIE RSS

    OSPF Multicast Question ?

    Rajesh Agrawal

      Hi All,



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


      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 ( 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?

        • 1. Re: OSPF Multicast Question ?
          miftaul haque

 are multicast addresses range reserved for local networks.


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


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




          • 2. 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 and/or multicast traffic;


            RiP v2 and EIGRP use multicast as well on and


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

            • 3. Re: OSPF Multicast Question ?
              Rajesh Agrawal

              My last Question Reg the same as i asked  "if by default a newly router has multicast enable" as OSPF also uses multicast.


              As we do "IP multicast routing" command on router to enable the multicast routing. What about in case of OSPF how does it work in new router without "IP multicat routing" enable

              • 4. 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.




                • 5. Re: OSPF Multicast Question ?
                  Rajesh Agrawal

                  It means in all the router for all kind of muticast routing protocol by default multicast routing is enable?

                  • 6. Re: OSPF Multicast Question ?

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

                    • 7. Re: OSPF Multicast Question ?
                      Rajesh Agrawal

                      Thanks every one to help me clear my doubt..

                      • 8. 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 or the, 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.






                        Message was edited by: ErickB

                        • 9. Re: OSPF Multicast Question ?
                          Rajesh Agrawal

                          Thanks Eric,


                          This is new and good information i found in entire chain, Actually i was clearing my concept so that i can make sure what goes where and could clear the thing which is very basic.

                          • 10. Re: OSPF Multicast Question ?
                            Rajesh Agrawal

                            I have checked in just 2 router R1 and R2 and captured their entire EIGRP traffic and found that TTL is set to 2 when R1 send first Hello packet to . In this case what happen to TTL - 2 does it die or forward further to another router.


                            I know its EIGRP question under OSPF umbrela however the mail point about the multicast routing protocol behaviour

                            • 11. Re: OSPF Multicast Question ?
                              Rajesh Agrawal

                              Frame 11: 74 bytes on wire (592 bits), 74 bytes captured (592 bits)

                                  Arrival Time: Sep  7, 2011 13:33:12.551575000 India Standard Time

                                  Epoch Time: 1315382592.551575000 seconds

                                  [Time delta from previous captured frame: 1.965000000 seconds]

                                  [Time delta from previous displayed frame: 0.000000000 seconds]

                                  [Time since reference or first frame: 32.573000000 seconds]

                                  Frame Number: 11

                                  Frame Length: 74 bytes (592 bits)

                                  Capture Length: 74 bytes (592 bits)

                                  [Frame is marked: False]

                                  [Frame is ignored: False]

                                  [Protocols in frame: eth:ip:eigrp]

                                  [Coloring Rule Name: TTL low or unexpected]

                                  [Coloring Rule String: ( ! ip.dst == && ip.ttl < 5 && !pim) || (ip.dst == && ip.ttl != 1)]

                              Ethernet II, Src: c0:00:1d:54:00:00 (c0:00:1d:54:00:00), Dst: IPv4mcast_00:00:0a (01:00:5e:00:00:0a)

                                  Destination: IPv4mcast_00:00:0a (01:00:5e:00:00:0a)

                                      Address: IPv4mcast_00:00:0a (01:00:5e:00:00:0a)

                                      .... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)

                                      .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)

                                  Source: c0:00:1d:54:00:00 (c0:00:1d:54:00:00)

                                      Address: c0:00:1d:54:00:00 (c0:00:1d:54:00:00)

                                      .... ...0 .... .... .... .... = IG bit: Individual address (unicast)

                                      .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)

                                  Type: IP (0x0800)

                              Internet Protocol Version 4, Src: (, Dst: (

                                  Version: 4

                                  Header length: 20 bytes

                                  Differentiated Services Field: 0xc0 (DSCP 0x30: Class Selector 6; ECN: 0x00: Not-ECT (Not ECN-Capable Transport))

                                      1100 00.. = Differentiated Services Codepoint: Class Selector 6 (0x30)

                                      .... ..00 = Explicit Congestion Notification: Not-ECT (Not ECN-Capable Transport) (0x00)

                                  Total Length: 60

                                  Identification: 0x0000 (0)

                                  Flags: 0x00

                                      0... .... = Reserved bit: Not set

                                      .0.. .... = Don't fragment: Not set

                                      ..0. .... = More fragments: Not set

                                  Fragment offset: 0

                                  Time to live: 2

                                      [Expert Info (Note/Sequence): "Time To Live" != 1 for a packet sent to the Local Network Control Block (see RFC 3171)]

                                  Protocol: EIGRP (88)

                                  Header checksum: 0xc395 [correct]

                                  Source: (

                                  Destination: (

                              Cisco EIGRP

                              No.     Time        Source                Destination           Protocol Length Info

                                   12 37.503000              EIGRP    74     Hello

                              • 12. Re: OSPF Multicast Question ?



                                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.



                                • 13. Re: OSPF Multicast Question ?

                                  Hi Rajesh


                                  Look at what RFC 5771 has to say about the multicast addresses in the range of -, 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 ( [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.



                                  • 14. Re: OSPF Multicast Question ?
                                    Scott Morris - CCDE/4xCCIE/2xJNCIE

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