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OSPF confused just a quick clarification will help?
Llukman1
Member Posts: 21 ■□□□□□□□□□
in CCNA & CCENT
I am confused on the address 224.0.0.5 multicast address.
-Used to send OSPF messages to all OSPF routers on the same network. The AllSPFRouters address is used for Hello packets. The DR and BDR use this address to send Link State Update and Link State Acknowledgment packets.
I want to know when is it a hello packet and when is it used for LSA's..I know hello packets are sent in order to establish an adjacency with another router and to maintain a relationship with that router by sending hello packets back and forth...but I just don't know when is it meant to for hello packets and when is it meant for LSA updates....
sorry if I am not explaining it that well....just confused.
-Used to send OSPF messages to all OSPF routers on the same network. The AllSPFRouters address is used for Hello packets. The DR and BDR use this address to send Link State Update and Link State Acknowledgment packets.
I want to know when is it a hello packet and when is it used for LSA's..I know hello packets are sent in order to establish an adjacency with another router and to maintain a relationship with that router by sending hello packets back and forth...but I just don't know when is it meant to for hello packets and when is it meant for LSA updates....
sorry if I am not explaining it that well....just confused.
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OptionsCertifiedMonkey
Member Posts: 172 ■■□□□□□□□□
You explained the question pretty well IMO. Let's hope that I can do the same with my answer.
To help answer the question, I am going to compare the behaviors of the 224.0.0.5 and 224.0.0.6 addresses on point-to-point networks vs broadcast multiaccess networks like Ethernet.
On point-to-point networks:
Routers use 224.0.0.5 as the address to send ALL types of OSPF packets, Hello, DBD, LSR, LSU, and LSAck. 224.0.0.6 isn't used
On broadcast Multiaccess networks:
On this type of network we need a DR and BDR to reduce the flooding of LSAs. However, we can't use 224.0.0.5 because all routers are already listening to that address. So what do we do? We use a new address for both the DR and BDR (224.0.0.6). All non-DR/BDR routers send their LSAs to 224.0.0.6. This way, only the DR/BDR receive LSAs from non-DR/BDR routers (DROTHERs). It's then the DRs job to send the LSA out to 224.0.0.5 (the all SPF routers address). Again, this is done to minimize flooding. As far as the Hello packet goes, it is always sent to 224.0.0.5. Remember, the DR/BDR is elected at the 2-WAY state. In order to get there, the routers need to exchange Hellos. Therefore, no router should be using the 224.0.0.6 address until they have reached the 2-WAY state and the DR/BDR have been elected.
To summarize. We use 224.0.0.5 for ALL OSPF packets on point-to-point networks. On broadcast multiaccess networks we use a special address (224.0.0.6) to send our LSAs to the DR/BDR to minimize flooding. However, 224.0.0.5 is still used to send all OSPF messages to all non-DR/BDR routers.
Might be wrong... Hopefully someone will correct me if I am.