bridging across WAN links

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Apr 9th, 2007
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Is the idea behind bridging across WAN links to allow a remote site to be on the same layer 2 link and subnet as the Main site?


For example in a DR scenario, you could configure the DR site to be on the same subnet as the Main site as far as the remote branches are concerned?

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paolo bevilacqua Mon, 04/09/2007 - 13:27
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What is DR?

yes, with bridging everything will be in the same L3 subnet. That is usually a bad idea especially for WAN links because:


- wasted bandwidth due to broadcast/multicast flooding

- overhead due to L2 addresses in WAN packets

- cannot to compress traffic

- cannot crypt traffic

- poor control of traffic flow due to inability of L3 separation

- harder to diagnose and troubleshoot.

- much harder to design for a backup.

- no performance advantage over routing, actually the opposite.


If you are new to networking, don't think that bridging is a shortcut to success, it is not.

richmorrow624 Mon, 04/09/2007 - 15:17
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Thanks for the reply,


DR is the Disaster Recovery site.


This was set up by someone else, they have a bridge connection from the Main Site to the Disaster Recovery site.


All the Server subnets are in the same VLAN(I think why they did it this way).



paolo bevilacqua Tue, 04/10/2007 - 00:51
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There is no reason for a disaster recovery site to use bridging instead of routing. The real problem is: "If I bring down the main site, how and by which addresses the branch sites and other offices will connect ?".

Bridging in itself does't offer a solution to this problem.


Unfortunately, there is no easy answer. each application, vendor, integrator can propose a different solution and I have yet to see one that is totally satisfying. Then it depends by the customer. Some are happy just having an almost ready copy of all-data, other pretend small switching times.


Designing something that works for real (not on paper only) can be difficult, expensive and time-consuming.

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