DR/BDR OSPF

[MajidShirzadeh]

Hello guys, as far as I know all SPF routers talk to each other with 224.0.0.5 in broadcast media, and they use unicast in NBMA, what's the 224.0.0.6, is that for DR and BDR router talk to each other or what, and can someone please explain DROTHER for me.

Thanks

[Edwin Summers · Accepted solution]

224.0.0.6 is the all DR/BDR router address.  Remember on networks where a DR (and BDR) are elected, routers only form full adjacencies with the DR/BDR.  The DR would then handle the task of rebroadcasting any updates to the other routers on the net.

DROTHER is a router on such a network that has not been elected DR or BDR.  It could be because it "lost" the election, or was explicitly configured to not become DR/BDR (such as setting priority of 0).

[MajidShirzadeh]

ok that's mean all DR/BDR talk to each other with 224.0.0.6, but DROTHER can not talk to 224.0.0.6, it just only talk to 224.0.0.5, is that correct ?

[smehrnia · Accepted solution]

Hi Majid,

the 224.0.0.5 is ALL OSPF Router multicast address, OSPF routers use this address to send OSPF Hello packets to eachother in the network, but when it comes to networks with the presence of DR and BDR, OSPF routers which want to talk to DR/BDR use 224.0.0.6 to send their LSA updates.

then, all "official" network updates are sent to 224.0.0.5 to ALLRouters address.

HTH,

plz Rate helpful posts.

Soroush.

[MajidShirzadeh]

Hi Soroushm!

make sense

Thank you

[Edwin Summers · Accepted solution]

Sorry if I caused you confusion earlier, Majid.

On the broadcast network, all routers that are NOT a DR or BDR are called DROTHER.  DROTHER will send their link state updates and LSAck to the AllDRouter address, 224.0.0.6.  While DROTHER will continually send their HELLO packets to the AllSPFRouter address (224.0.0.5), they send LSUs and LSAcks to 224.0.0.6.

The DR will "re-flood" the received update to 224.0.0.5 to maintain database sync with all SPF routers on the broadcast net.  DROTHER will LSAck to 224.0.0.6.

So, yes.  DROTHER do talk directly to the DR on 224.0.0.6.  Theoretically, only the DR and BDR listen on 224.0.0.6, while all OSPF routers listen on 224.0.0.5.

Dry reading, but the RFC may help as well:

http://www.rfc-editor.org/rfc/std/std54.txt

Best regards!

[asif.naveed]

Every time a DR-OTHER router sends an update, it sends it to the DR and BDR on the multicast address 224.0.0.6. The DR will then send the update out to all DR-OTHER routers in the area, to the multicast address 224.0.0.5.
Point-to-Point networks use the address (224.0.0.5) for all OSPF messages
All DR/BDR Address: 224.0.0.6
All Shortest Path First (SPF) Routers Address: 224.0.0.5