Tag Archive: Cisco

QoS overview

The most helpful overviews I’ve found to date on QoS for VoIP:


http://www.networkworld.com/community/node/37507 (You can browse through the preceding several posts until you arrive at the initial discussion on the QoS byte. Interesting reading.)

The key is to tag your VoIP traffic as DSCP class EF (Expedited Forwarding; the numeric value for EF is 46, binary is 101110(00), ToS 184).

The DSCP classes were created by Cisco and are the default markings applied to VoIP traffic. They are the evolution of the IETF’s ToS byte, which allowed the first bits to be used to specify a rough QoS (called “IP Precendence”). The other bits were never used, so Cisco used several of them to create a more granular set of QoS markings. EF 46 is one of them. EF 46’s first bits indicate IP Precedence 5, which is the class for voice traffic on the older IP Precedence system. So, if you mark traffic as DSCP EF, you’re also marking it as IP Precedence 5. Convenient!

Here’s a helpful tool to assess line quality:

http://myspeed.visualware.com/servers/namerica/iad.php?testtype=-1&codebase=mcsiad.visualware.com&location=USA: Dulles, Virginia&ver=9&cm=&map=namerica&lines=&pps=50&bpp=80&codec=G.726 (32 Kbps)&provtext=Visualware&provtextextra=&provlink=http://www.visualware.com

VLAN Management >

Create VLAN > Add > Set up the VLAN you want to be designated as your voice VLAN

Voice VLAN > Properties > Enable/​Pick your new VLAN

Voice VLAN > Telephone OUI > Make sure the leading characters from your phone’s MAC address are listed in there (and make sure your phone is configured to use LLDP, or none of this will work, obviously)

Port to VLAN > Filter > Select your new voice VLAN from the drop-down > Go, Click “Tagged” and for all ports and then “Apply” so that they can use the new VLAN as needed

Admin > Discovery LLDP >

Properties > Enable

LLDP MED Network Policy > Add > Type in the VLAN ID and save it

LLDP Port Settings > Don’t do anything with this

LLDP MED Port Settings > Pick a port and select Edit >

LLDP MED Status > Enable

Available Optional TLVs > Select the Network Policy TLV and click it over to the “Selected Optional TLVs” column

Available Network Policies > Select the network policy you created above and click it over to the “Selected Network Policies” pane