OSPF multicast

[Yair Sofee]

---

Hey all,

I have a question regarding the OSPF multicast group of 224.0.0.5

We know each transit network has a different Designated Router and there isn't one Designated Router for the whole area. But I read that this multicast group contains all routers in an area. So if that's the case why do we need a different DR per transit network and nor one per area who sends all updates therein. Not only that but it makes all the DRs in OSPF redundant since every DR sends all updates on a multicast group that contains all routers in an area. I can't really wrap my head around this. Would appreciate if anyone can explain. Thanks!

[Sergey Lisitsin]

Yair Sofee,

That is not entirely correct. Not every transit network contains a DR. Point-to-Point network type for example doesn't require a DR.

DR is only necessary in the multiaccess network segment, where you have more than 2 OSPF routers. The DR is an efficiency mechanism that removes a necessity to build full neighbour relationships between all the routers on the segment. You don't need a DR for the entire area, because not all the routers in the area are necessary neighbours of each other.

[rais]

" But I read that this multicast group contains all routers in an area "

 224.0.0.5 = ospf-all.mcast.net 

A packet sent to this address is only going to be received by router(s) on the same broadcast segment/link not the whole area as 224.0.0.0/24 is a link-local address range.

HTH.

[Joseph W. Doherty]

I recall (?) hop limit might also be set to one.

[Sergey Lisitsin]

That's correct. 224.0.0.0/24 is a link-local scope of addresses. Anything within that range should be sent with a TTL set to 1.

[Giuseppe Larosa]

Hello @Yair Sofee ,

OSPF is a link state protocol that for each OSPF area keeps a separate database.

The objective of the per Area database is to:

a ) building a complete and updated topology map describing all links in the area either they are stub networks ( 0 OSPF neighbor on it ) point to point  ( 2 routers )  broadcast LAN segments ( potential for 2 or more neighbors) or other forms of multiaccess.

b) to describe network topology each router with at least one active interface in the area generates its own Router LSA that describes the node OSPF RID, the active links in the area.

For broadcast LAN segments each router creates an entry pointing to the elected DR LAN address on the segment. The DR creates and mantains a network LSA listing the OSPF RIDs of all routers connected to the segment. The network LSA link id = DR address.

This allows each router in the area to know the topology of a remote LAN segment .

However, in short why we have two multicast link local addresses ?

224.0.0.6  All OSPF DR on segment

224.0.0.5 All OSPF routers on segment

because  DR /BDR have been introduced for efficiency reasons in the number of OSPF adjacencies required in a LAN. Each DRother router builds only two FULL adjacencies with DR and BDR.

When an update has to be sent by a DRother it happens in two steps:

it sends to 224.0.0.6 DR and BDR receive it

The DR re-sends the update to 224.0.0.5 so that all other DRother can process it

This is the reason why OSPF uses two link local multicast addresses

Hope yo help

Giuseppe