OSPF and VLANs

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Dec 27th, 2006
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I've got 2 2620s connected via a point to point link and running OSPF. In one location I've got 2 VLANs, one for voice and one for DATA. I need to route or bridge those to the second location, as I am putting phones there as well.

Do I just use a BVI interface to do this or is there a better way?


Thanks,

Ben

Correct Answer by sundar.palaniappan about 10 years 7 months ago

As Rick stated earlier, VLAN split across WAN isn't a good design and should be avoided, if possible. It makes more sense in your case as you are dealing with voice traffic as it doesn't tolerate much latency. Bridging across WAN would mean all broadcast traffic extended across the p-t-p link and is less desirable.


If you can maintain separate subnets at both locations and route the traffic between them then that's probably the better route to take.



HTH


Sundar

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Richard Burts Wed, 12/27/2006 - 13:14
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Ben


If you want to bridge together two VLANs then you would probably want to configure BVI (and the rest of Integrated Routing and Bridging). If you do not need to bridge together 2 VLANs then you would probably just route (without BVI or IRB). Bridging together 2 VLANs sort of defeats the purpose of having separate VLANs. I am not sure that I understand your environment or your real requirements to know whether there is reason to bridge VLANs. Perhaps you can clarify what you are trying to accomplish? Then maybe we can give you an answer.


HTH


Rick

bregimand Wed, 12/27/2006 - 13:24
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Hey Rick-

Let me clarify. I don't want the VLANs to be able to talk to eachother - I just need to get them to exist on the other side of the T1.

I have a 2950 at site A with VLAN10 for voice and VLAN20 for data. I need these VLANs to exist at site B as well.


If I'm not explaining this well I can upload a diagram.


Thanks for your help,

Ben

Richard Burts Wed, 12/27/2006 - 14:03
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Your diagram might help. What you seem to be describing is a difficult thing to implement and usually not a good design. It sounds like you want a broadcast domain to exist in one location (actually one for data and one for vioce) and the same broadcast domain to exist in another location and for the broadcast domains to be linked over the T1. This implies that the same subnet exists on both ends of the T1 and that a device can issue an ARP request for a device in the other location and that the ARP will be transmitted over the T1 and delivered to the other device, and the response transmitted back to the first device. Bridging two VLANs over the T1 is difficult and is usually not advised. Is there a particular reason that you are trying to do it this way?


HTH


Rick

bregimand Wed, 12/27/2006 - 15:13
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Hmmm. I may have just found the way to do this, but I'm still curious to hear your take on it.


Thanks,

Ben

sundar.palaniappan Wed, 12/27/2006 - 16:17
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Ben,


I don't know what solution you had in your mind. Yes, you can bridge a range of VLANs across a p-t-p link using the feature, Bridge Control Protocol (BCP). VLAN tags would remain intact when the traffic traverses the p-t-p link.


You can find more info here:


http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801a6d95.html



HTH


Sundar

bregimand Wed, 12/27/2006 - 16:29
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Sundar-

Thanks for the reply. I think that this will not work as you have to disable IP routing between the 2 sites to do this, and these guys are running OSPF on that link.


I think that I will just enable DHCP on the site B router and put those phones on a different subnet than the phones in site A. Then I'll route all VoIP traffic back to site A while the data at site b stays at site b.

Thoughts?


Ben

Correct Answer
sundar.palaniappan Wed, 12/27/2006 - 16:55
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As Rick stated earlier, VLAN split across WAN isn't a good design and should be avoided, if possible. It makes more sense in your case as you are dealing with voice traffic as it doesn't tolerate much latency. Bridging across WAN would mean all broadcast traffic extended across the p-t-p link and is less desirable.


If you can maintain separate subnets at both locations and route the traffic between them then that's probably the better route to take.



HTH


Sundar

bregimand Wed, 12/27/2006 - 16:59
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Rick/Sundar

Thanks for all of your help. I am going to go ahead and implement the seperate subnet solution.


Much appreciated,

Ben

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