Suppose I have one ospf network where two multicast group exists. And some routers is ospf which belongs to one group or multiple group (Like: routers E, H) and router is layer3 router (Like: routers A, C, K), not ospf router. Like given below:

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I divided the whole network into two parts Multicastgroup 1 and Multicastgroup 2.Like given below: enter image description here enter image description here

I have some confusions which are given below:

Confusion 1:: We know that all routers which belongs to same group one multicast address. So my question is routers which belongs to different subnets, but they are also using same multicast address? For example the adjacent link between routers H and I belongs to one subnet and the adjacent link between routers I and J belongs to another subnet , but they are using one multicast address i. e. belongs to same group. Am I correct?

Confusion 2:: The adjacent link between two routers should belongs to same group? I mean the adjacent link between two routers, is there possible one interface of adjacent link belongs to one group and another interface belongs to another group or is there possible one interface belongs to any group and another is layer3 interface?

Confusion 3:: We see that in routers A, C, K don't running routing protocol ospf. Suppose one router belongs to group 1,say J floods the routing protocol information when that information comes into interfaces of layer3 routers A, C, K they just route routing protocol information and don't exchange the routing protocol information because layer3 routers A, C, K running upto layer3 but routing protocol information is application database? Am I correct?

Confusion 4:: When one router interface belongs to one multicastgroup, when that interface get another multicastgroup routing protocol information, both exchange the routing protocol information? I mean group1 router interface could exchange the routing protocol information with group2 router interface?

Confusion 5:: Router one interface could run more than one than one routing protocol? I mean router one interface could belongs to both multicast groups?

1 Answer 1


OSPF uses a link-local multicast group that does not cross a router. Each OSPF multicast is confined to a single link.

There is nothing about your diagrams that would have an OSPF multicast on more than two routers. In fact, it would be a best practice to use unicast for your diagram.

Every interface on a router would use the OSPF multicast groups for all routers ( for IPv4, FF02::5 for IPv6) and DR/BDR ( for IPv4, FF02::6 for IPv6). Those multicasts are confined to the link on which they are originate, and are not propagated across a router to other links.

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    Routing protocols are applications that run on a router. They use transport protocols (RIP/UDP, EIGRP/RTP, OSPF/OSPF, BGP/TCP). OSPF is hierarchical in that multi-area OSPF implementations require that all areas connect to Area 0 (backbone). OSPF areas (other than Area 0) will only exchange information with Area 0. You cannot have Area0<->Area1<->Area2 (unless you use a virtual link that connects Area 2 to Area 0) because Area 1 and Area 2 cannot exchange routing information with each other except through Area 0.
    – Ron Maupin
    Apr 5, 2022 at 20:45
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    OSPF is an application that has its own transport protocol that it uses to exchange information. The transport protocol does not have a separate name, only protocol number 89. EIGRP has its own transport protocol (number 88), but Cisco named it RTP.
    – Ron Maupin
    Apr 5, 2022 at 20:57
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    It is an application that requires a hierarchy. Really just OSPF and IS-IS require a hierarchy (basically the same way, although the terminology and details are different).
    – Ron Maupin
    Apr 5, 2022 at 21:07
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    "Then now Area2 could exchange data with Area1 directly through virtual link without Area0 temporarily" No. A virtual link must be created between Area 0 and an area not directly connected to Area 0. Other areas cannot directly communicate. Also, virtual links do not automatically get created, they must be configured.
    – Ron Maupin
    Apr 5, 2022 at 21:24
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    @AlokMaity, it may be confusing, but routers are not in areas. Router interfaces are placed in areas. That is how you have an ABR: one interface is in Area 0, and other interfaces are in a different area.
    – Ron Maupin
    Apr 5, 2022 at 21:26

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