[cisco-voip] Qos for V3PN

[Lee Pedder]

Consider using IP DSCP values as they tend to make your QoS policy
easier to manage. Cisco phones will set some DSCP values by default.
E.g.

class-map match-all VOICE
  description IP Phones mark Voice to EF by default
 match ip dscp ef
class-map match-all CALL-SIGNALING
  description Call Signalling Marked Packets from Phones
 match ip dscp cs3

Also, if you are using GRE over IPsec be aware of the packetization
overhead this can add. For a G.729 call using 20ms packetization
intervals, this results in a 56kbps stream on the WAN when adding UDP,
IP, GRE, ESP and IP headers. You can reduce this slightly by changing
codec settings to 30ms but this increases call latency. I have not
tried this personally.

Also, ensure you set up the phones in a location with the appropriate
bandwidth setting in CCM. This should allow the exact number of calls
your QoS policy can handle, and present users with a "not enough
bandwidth" message on subsequent calls. Because of overhead, this will
need some tweaking as the bandwidth you tell callmanager will not take
this into account.