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How does Router Know, which interface should be forward multicast packet such as hello packet ospf

Dear All,

How does Router Know, which interface should be use to forward multicast packet such as hello packet ospf ?

thank you very much.

2 Accepted Solutions

Accepted Solutions

Jon Marshall
VIP Community Legend VIP Community Legend
VIP Community Legend

Regarding OSPF specifically, the router knows because you have told it to do that eg.. if you have an interface with an IP address 192.168.5.1 and you have this config

router ospf 10

network 192.168.5.1 0.0.0.0 area 0

then the router starts up OSPF on that interface and begins sending hello packets out of that interface.

The more general question of multicast packets is a much bigger one. With pim dense mode the router will simply flood multicast packets out of all interfaces except the interface it received it on. It is then up to routers further downstream to tell the originating router whether it wants to receive those packets (ie. it has members who want to receive the multicast stream) or whether it doesn't. With pim sparse mode it doesn't flood the packet, it informs the RP (Rendevous Point).  If another router has members attached to it that want to receive the packets then it has to tell the RP (Rendevous Point) that it wants to receive the packets and then it is sent them. The RP is a router that you designate for the multicast network.

Multicast routers using Cisco PIM use the existing IP routing table to forward multicast packets.

 

The above is a very brief outline and there is a lot more to multicast than that. Have a read of this link and if you have further questions please come back -

http://www.cisco.com/en/US/customer/docs/ios/solutions_docs/ip_multicast/White_papers/mcst_ovr.html

Edit - pay particular attention to the RPF section which describes how multicast uses the IP routing table.

Jon

View solution in original post

Hi,

224.0.0.5/224.0.0.6 are link-local multicast addresses with a TTL of 1 so the routers will never forward them even if it is doing multicast routing.

Regards

Alain.

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.

View solution in original post

6 Replies 6

Jon Marshall
VIP Community Legend VIP Community Legend
VIP Community Legend

Regarding OSPF specifically, the router knows because you have told it to do that eg.. if you have an interface with an IP address 192.168.5.1 and you have this config

router ospf 10

network 192.168.5.1 0.0.0.0 area 0

then the router starts up OSPF on that interface and begins sending hello packets out of that interface.

The more general question of multicast packets is a much bigger one. With pim dense mode the router will simply flood multicast packets out of all interfaces except the interface it received it on. It is then up to routers further downstream to tell the originating router whether it wants to receive those packets (ie. it has members who want to receive the multicast stream) or whether it doesn't. With pim sparse mode it doesn't flood the packet, it informs the RP (Rendevous Point).  If another router has members attached to it that want to receive the packets then it has to tell the RP (Rendevous Point) that it wants to receive the packets and then it is sent them. The RP is a router that you designate for the multicast network.

Multicast routers using Cisco PIM use the existing IP routing table to forward multicast packets.

 

The above is a very brief outline and there is a lot more to multicast than that. Have a read of this link and if you have further questions please come back -

http://www.cisco.com/en/US/customer/docs/ios/solutions_docs/ip_multicast/White_papers/mcst_ovr.html

Edit - pay particular attention to the RPF section which describes how multicast uses the IP routing table.

Jon

Dear Mr. Jon,

I make an ospf topology like this :

then I try to debug Ospf hello on Router 3, i think i should see hello packet from Router 1 too

since hello packet used multicast address But I Can't see hello packet From R1 on router R3.

What would Router OSPF do when they received hello packet from their neighbour ?

do they forward that hello packet to other neighbour, or just listen it

Hi,

224.0.0.5/224.0.0.6 are link-local multicast addresses with a TTL of 1 so the routers will never forward them even if it is doing multicast routing.

Regards

Alain.

Don't forget to rate helpful posts.

Don't forget to rate helpful posts.

Hi Alain,

Thanks for this information it really helped.

Hello,

Just like to add,

Ospf hello packets are used to confirm a bidirectional communication between opsf neighbors, so regards your topology hello packets will be seen between R1 - R2 & R3 & R2 not between R1 & R3 .

res

Paul

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Thanks.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
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Kind Regards
Paul

Just to add to Alain's reply. All 224.0.0.x addresses are link local. Routing protocols such as RIP/EIGRP and OSPF (as you have seen) use 224.0.0.x addressing as do other protocols. These are never routed.

In terms of OSPF they are only used to form neighborships as Paul said and you can only have a neighbor on the same subnet so there is no need for them to be passed to another OSPF router.

Jon

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