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QOS over VLAN

Posted on 2004-10-11
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Last Modified: 2006-11-17
I have 3 3750 SMI and 1 3550 EMI switches connected in a straight line (daisy chained) across 4 sites.  Sites are connected with Gigabit SFPs using fiber.  The core routing switch is located at Site 1.  I have 1 VLAN at each site with the 3750 SMI doing inter-vlan routing at Site 1 for all the rest of the VLANs.

I want to add a second VLAN at Site 4 using QOS over that VLAN from Site 1, through Site 2 and Site 3 to Site 4.  The new VLAN will be used for Video Conferencing.

Can you tell me how to configure this for each site so that traffic destined for Video Conferencing is prioritized over all other traffic.
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Question by:cisdoz2
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Expert Comment

by:Dr-IP
ID: 12283257
The easiest way to do what you want to do is to turn on QOS, if not already on, and set the switchport priority on the ports used for video conferencing equipment to 7. With that done anything coming from the video conferencing gear should get higher priority over every thing else.

One thing I should note, is with all the routing being done only on the end switch, it creates a lot of extra traffic on its link, as every packet that needs to be routed, not local to the end switch, has to go all the way to the end switch, and then back to its destination. Besides being inefficient, if something goes wrong with the routing on that end switch, connectivity between all sites goes down.

What I would do is set each switch to do the routing for its local VLAN, and route any non local traffic directly to the correct switch, instead of out to the end, and back again. Also if the end switch were to die, at least the 3 other sites would still be able to communicate with each other.


interface FastEthernet0/1
switchport access vlan 5
switchport priority default 7
 
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Author Comment

by:cisdoz2
ID: 12286050
That is a good point and I had already given that some thought.  The problem I had with that is that the video equipment will not work with NAT and must be assigned a public ip address.  Is there a way to route across several routing switches to a single ip address that belongs to the same subnet as the first switch without doing the VLAN setup I created? I have provided a brief diagram:

                                                                 Routed Switches
                                      |---------------------------------------------------------|
Internet ----> Router ----> Switch1 ----> Switch2 ----> Switch3 ----> Switch4 ----> Video Conferencing Device
                 X.X.X.225                                                                                                   X.X.X.226
                  -----------                                                                                                   -----------
            255.255.255.224                                                                                          255.255.255.224

I realize I could split my assigned public addresses into smaller subnets, but with so few, that is really not an option.
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Dr-IP earned 350 total points
ID: 12287200
Maybe you could create some kind of GRE tunnel, but that would be a real klutzily solution. The best way to do what you want to do is use trunks between the switches, and create a separate VLAN for the video traffic to go through.

If you plan your steps carefully, the impact on the network should be minimal. One hint, if you don’t have trunking between the switches yet, start enabling on the furthest switches link port first, and work your way back, and before issuing the trunk command for the port, always issue a reload in 2 command before on the far end switch first. That way if something goes wrong and you can't reestablish connectivity on the far switch after setting the links on booth switches to trunking, the far end switch will reboot reloading the old config allowing you to reestablish a connection to it.

This is a handy trick by the way, as it has allowed me to safely do some very extensive network reconfiguring on remote equipment thousands of miles away from where I work. That I would have never dared to do unless I have a technician standing by at the remote site.                  
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Author Comment

by:cisdoz2
ID: 12287263
Looks like a plan, Doc

Thanks
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