Monday, August 17, 2009

All Things Voice...Almost

This is going to be more config stuff than anything. But I read some voice/qos/cos stuff, so I need to offload at least a bit into this blog tonight. My notes f#$#!@ suck though, so this might be a bit more of a mess than some of my other posts. But here we go anyways.

Best effort deliver- No real quality of service (*QOS) here. The network devices just deliver packets or frames as they are recieved.

Integrated Services Model- This model essentially clears a path through the network for priority data to flow through using the resource reservation protocol (RSVP). The application requesting QOS via RSVP send the protocol its request. RSVP in turn goes out to the network devices along the path to see if they can support the request. If granted, the path is cleared for the priority traffic. Imagine a fire truck going through the city....the cops would have all the intersections cleared for it in advance of its arrival.

Differentiated Services Model (DiffServ)- QOS is handled on a per-hop basis rather than a per-flow basis. As the traffic approaches each network device, QOS decision will be made based on the information contained within the packet header. This frees up that end-to-end traffic tunnel that IntServ created.

TOS or type of service includes:
  • IP precendence value - 3 bits
  • TOS value - 4 bits
  • A Zero - 1 bit
Differentiated Services Code Point (DSCP)
  • Class Selector- 3 bits
Class 7 Network Control (STP, routing protocols, etc...)
Class 6 Internetwork Control
Class 5 Expedited forwarding (voice and other critical data)
Class 1 Assured forwarding (not left to best effort)
Class 0 Best effort delivery
  • Drop precedence- 3 bits
High - 3
Med - 2
Low - 1

Can be displayed as follows: AF (class)(drop prec); AF23 = Assured Forwarding class 2, high

Basic config:
(config)mls qos (enable globally on device)
(config-if)mls qos trust [cos] - trust cos value unconditionally on port
(config-if)mls qos trust device cisco-phone
(config-if)switchport priority extend trust (extends trust beyond cisco phone to pc)

show mls qos [int]

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