Query Regarding EIGRP

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Jul 29th, 2010
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Hi All,


One of my friend asked a simple question on EIGRP which I was unable to answer.. Please help me on this...


Q : Is it necessary to turn ON Multicast routing for EIGRP to send updates (via 224.0.0.10),,, If not then hw those updates are delivered....????


Regards,

Bhargav

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Jayakrishna Mada Thu, 07/29/2010 - 22:19
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Hi Bhargav,


The answer is no. As you can see EIGRP multicast ip 224.0.0.10 converts into 0100.5e00.000A. On cisco switches any multicast packet with destination mac-address 0100.5e00.00xx will be forwarded to the CPU. Thats how the switches form neighbors. Same goes for OSPF.


Hope that answers your question.


JayaKrishna

Mahesh Gohil Thu, 07/29/2010 - 22:44
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Hi bhargava,


you can use neighbor statement in place of network command incase the intermediate hops are not supporting

multicast. please find below description taken from cisco which might help you to understand the concept


"The neighbor command is used in EIGRP in order to define a neighboring router with which to exchange routing information. Due to the current behavior of this command, EIGRP exchanges routing information with the neighbors in the form of unicast packets whenever the neighbor command is configured for an interface. EIGRP stops processing all multicast packets that come inbound on that interface. Also, EIGRP stops sending multicast packets on that interface.

The ideal behavior of this command is for EIGRP to start sending EIGRP packets as unicast packets to the specified neighbor, but not stop sending and receiving multicast packets on that interface. Since the command does not behave as intended, the neighbor command should be used carefully, understanding the impact of the command on the network."


Hope this will be helpful to you


Thanks & Regards

Mahesh

Hitesh Vinzoda Fri, 07/30/2010 - 03:44
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Hi,


You use multicast routing only when you want to route the multicast packets. while the multicast destination address 224.0.0.10 is a link local multicast address which never croses a layer 2 boundary. so a router can send multicast packets on link local multicast address like


eigrp 224.0.0.10

ospf 224.0.0.5 / 224.0.0.6

ripv2 224.0.0.9


when eigrp uses 224.0.0.10 for forming neighborship it creates it on the directly connected neighbors on the interface from where the packet was sent. have you ever seen eigrp neighborship few hops away from the router... of course not, because the hellos are meant for link local and they never cross the layer 2 boundary. interestingly we have 224.0.0.1 for all host on the link and 224.0.0.2 for all routers, even hosts can send multicast packets on these addresses and we dont enable multicast routing on them ... do we..??


on the other hand Multicast routing is used when you want to route a multicast packet arrived on one interface to route out other....same like the unicast routing.


HTH


Hitesh Vinzoda


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