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question about 3500yl and vlan qos tagged/untagged

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question about 3500yl and vlan qos tagged/untagged

I have a small network with a single 3500yl at the heart of it. I was wondering if I could get some clarification on the vlan/qos processing that the switch does. I have 2 vlans. One default and primarily untagged for data and one voice and primarily tagged for voip. I have assigned the voice vlan to a qos priority of 6. My question is: the port that the call manager (bcm50) exists on is dedicated to the bcm50. Can i have this port be an untagged member of the voice vlan (qos prioritized) and maintain qos priority on that port. How will the switch handle qos with packets that come from the bcm50 on the untagged voice vlan port and go to a phone on a tagged voice vlan port? How will the switch handle qos with packets that come from a phone on a tagged vlan port and go to the bcm50 on an untagged vlan port?
I know the simple solution is to just tag the bcm50 port and not worry about it but I would like to understand how the switch processes qos with regards to tagged/untagged vlan ports. Thank you for any help you can give.
Thank you very much for your help,
Jeff Carrell
Honored Contributor

Re: question about 3500yl and vlan qos tagged/untagged

qos settings can only be transported on a port that has been set for tagged (802.1q)...that is the only way to add the extra info of the qos settings, as the 'tagged' frame has an extra 4 byte field in it for vlan id, qos and a couple of other bits of data...

so, when the phone traffic enters a port that is tagged and has qos setting pri 6 on it, the switch processes that info faster in its hardware queues than data traffic (which is default at normal or 0 in the switch)...

so when the phone traffic has to get to the phone server, the switch processed that info faster, then sent it to the untagged phone srvr port...

phone traffic coming from the phone server will be processed at same priority as all other data traffic since that port is untagged...which is "generally" ok since more traffic is "generally" initiated by phones (i give this a very liberal definition here as this will depend on your system's use)...

in order to have untagged traffic processed at a higher priority, you have to set some layer 2 qos settings...you set the layer 2 DiffServ code points - 802.1p on the vlan...you also config a setting for the switch to map layer2 qos settings to layer3 qos settings - then all traffic (tagged or untagged) gets the higher priority processing in the switch...

see this section of the manual for details:


Re: question about 3500yl and vlan qos tagged/untagged

ty very much jeff for your help. i don't fully understand how to map layer 2 qos to layer 3 but i will read about it and find out. i thought that diffserv was layer 3 by default and was part of the packet information and 802.1p was layer 2 and part of the frame information. anyway, setting the port to tagged would work just as good right?
thank you again for all of your help. i truly appreciate it.
Preston :)
Jeff Carrell
Honored Contributor

Re: question about 3500yl and vlan qos tagged/untagged

opps, i did explain a bit somewhat incorrectly as you caught it...

a layer 2 switch can have a diffserv code point (DSCP) config in the vlan in order to map the traffic to a priority queue for the switch (untagged ports for inbound traffic like the phone server), and the switch will maintain that priority setting in the layer 2 packet component as it leaves the switch, even if its on a tagged port...this is an example of the config: switch1(vlan2) qos dscp 100100 - this maps to priority 6...

to map the DSCP to type of service (ToS), you would issue this command in the global config context: 'qos type-of-service diff-services', then the switch will map the vlan DSCP setting (as ref above) of an inbound packet (say untagged) to an 802.1p priority tag of the outbound packet...

yes, you could set the phone server port to tagged and if the phone server NIC supports the tag setting, then everything will have priority 6 as in your config...

when configuring qos in a network, you need to look at the actual switch queue capabilities (if more than 1) that all the traffic could pass thru, as many switch platforms/brands have different physical memory queues (some have 2,3,4,8)...then it could make a difference where traffic like some at pri 6 and some at pri 7 could actually get processed equally in the switches' queues...this is a little exaggerated, but it can occur...

btw the provision asic switches (3500/5400/6200/8212) all have 8 queues...5300/3400/2600/2800 have 4 queues...

again, the chapter on QoS, altho kinda hard to wade thru it all sometimes, does provide more details of what i a trying to say here...

happy to help, hopefully it does :-)