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Webcast-Catalyst9k
New Member

Query regarding ospf behavour for Non broadcast multi access network

Hi All,

Actually i am confused with OSPF behaviour for Non-broadcast multiaccess network..

First take the case of RIP.

If we configured the RIP in Non broadcast multi access network it is working.. But RIP send the Route updates to neighbor routers via broadcast, so that broadcast will pass therough NONn braodcast network.. I have attached the figure in Attachment.

Now in case of OSPF. IF we configure OSPF in non-broadcast multiaccess network then routers would not be able to communicate bcoz hello packets will not pass through network...

but we give broadcast command on router int s0 on both sides, ospf starts communication.

So after giving broadcst command will it start sending hell opackets to any other ip address rather than 224.0.0.5 or something else will be there.

I am very confused with this concept.

I have attched the snap shot..

If we configure frame realy.. and using routing protocol RIP or IGRP then how they will send the route update.. because network in non-braodcast and both the protocols broadcast thire update only.. how the update packet will pass the cloud.

If we use ospf and eigrp.. they use multicast..so will multicast will pass through cloud.. bcoz OSPF uses 224.0.0.5 for hello messages.

Thanks for ur help in advance..

Kindly help me out..

Regards

Mahi

3 REPLIES
Purple

Re: Query regarding ospf behavour for Non broadcast multi access

Hi Mahi,

Because NBMA networks do not have any inherent broadcast/multicast capabilities, some method has to be used in order to emulate such behavior - this is often called pseudo-broadcasting. On a Cisco router, you enable this using the broadcast keyword when defining DLCIs etc. When a router has to send a multicast packet destined to 224.0.0.5/224.0.0.6 out of an interface, it will check each DLCI for the broadcast command. On each DLCI where the command exists, the packet will be forwarded out. Note that the destination address of the packet will NOT change - it will remain whatever multicast/broadcast address it was.

Hope that helps - pls rate the post if it does.

Paresh

New Member

Re: Query regarding ospf behavour for Non broadcast multi access

Hi Paresh,

Thanks for ur response.

from where i can get the study material on pseudo-broadcasting.

What abour RIP....

How RIP will work on NBMA network...

I have installed rip Version 1 on nbma network and without any furthur configuration it was working.but RIP send updates to other routers as a broadcast. SO how this broadcast is passing through NBMA network...

Waiting for ur response.

Mahi

Purple

Re: Query regarding ospf behavour for Non broadcast multi access

Hi Mahi,

There is a brief description of the 'broadcast' keyword at the following URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hwan_r/wan_f2h.htm#wp1080980

Apart from that, I don't think there is any document that specifically addresses this point.

As for your RIP version 1 case, it also needs broadcast capability for it to work over NBMA networks. If you are using frame-relay inverse-arp to learn your IP to DLCI mappings, then you should be aware that broadcast is enabled automatically.

Pls do remember to rate posts.

Paresh

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