MAN: Convert from Layer 3 to Dot1Q trunk?

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Mar 12th, 2009
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Hi all,

There two sites in our environment. One is a data-center (25 servers) and the other consists of about 300 users.

The link between these sites is a 100Mb OptiMAN fiber Ethernet connection. The link is connected via a 3750 switch (routed) interface on the data-center side and a 6513 switch (routed) interface on the user-site side.

There is a project underway that involves moving these servers from the data-center to the user-site. Both of these sites have different L2 VLANs and L3 VLANs.

In an effort to keep the servers from changing IP addresses(and subnets), I'm proposing that we remove the routed link and convert it to a Dot1Q trunk link. This would allow us to preserve the IP addresses of the servers when moving to the user-site.

Does anyone know of any drawbacks or problems with doing this? I'm going to contact the MAN provider and check with them as well.

Over the next few months, we plan on moving all of the servers from the data-center and eventually shutting down the site. So, this design is temporary.

As always, thanks for the help.


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Jon Marshall Thu, 03/12/2009 - 16:05
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Little confused. What is the current situation. Are you proposing to readdress the servers into one or more of the user site vlans then migrate them.

Surely at the moment the servers are on their own subnets. Why can you just not migrate these subnets to the user site or are you looking to migrate a part of the subnet at a time so you have a need to have the same subnet in 2 places ?


msrohman Thu, 03/12/2009 - 20:02
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Thanks, Jon.

The plan was originally to re-address the servers when they move to the user-site. But there may be a bunch of clients with hard-coded IP addresses in their applications. Thus, we want to keep the same IP address on the systems when moving them to the user-site. I figured that maybe I could use a layer-2 trunk, move the server and keep the same IP address om the systems. Of course, the default gateway IP address would still reside at the data-center site.

Yes we're looking at migrating a part of the subnet at a time so we would have the same subnet in 2 places.

Thanks again for your response.


glen.grant Thu, 03/12/2009 - 18:08
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Yes if its temp then you could that if the provider will allow trunking . That being said i think I would initially move the routing from the 3750 to the 6500 for the servers and then trunk across to the 3750 . This way when ready you just basically shutdown the link to the datacenter and not have to change anything.

Joseph W. Doherty Thu, 03/12/2009 - 18:47
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If you trunk across the WAN, where's the gateway router now going to be, on the 3750 or on the 6500? If the other non-gateway router still used as a router? If so, have you made for a possible unicast flooding issue?

msrohman Thu, 03/12/2009 - 20:15
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Thanks for the response, Joseph.

I didn't plan on changing the gateway router placement. Thus, if a server is moved to the user-site, it's gateway would still reside at the data-center.

Could you explain the possible Unicast flooding scenario?



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