QOS

[ajay chauhan]

Hi,

what is the diffrence between DSCP or IP precedence and what is the Criteria to implement qos ...when we will use IP precedence and when DSCP, and what is the default DSCP or IP precedence for IP packet example :if any ip packet originating from PC and IP voice packet which is originating from IP Phone and why IP precedence to DSCP mapping is required.

[dgahm]

Both DSCP and IP precedence use bits in the TOS byte of the IP header. DSCP uses 6 bits, allowing more granularity than the 3 bits available with IPP. Here is a good doc that explains the relationship:

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml

The default values depend upon the device and version of code. Cisco phones use DSCP EF (TOS 46) for media packets and IPP 3 (TOS 24) for signalling. Older versions of Call Manager used AF31 (TOS 26) for signalling.

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[ajay chauhan]

suppose i have a network where applications like call manager ...www ftp is running ...how can we find out the default DSCP for ip packet belongs to diffrent application.and what is the need of IP DSCP to IP precedence mapping .If both are same just diffrence is this in dscp 3 bit we can use for drop probabilities.

[dgahm]

This document shows the marking recommendation for different types of traffic. In your case FTP as bulk data might be AF11, and WWW might be AF21 if it is considered to be important enough to be protected. It is up to you to customize this to fit your needs.

http://www.cisco.com/application/pdf/en/us/guest/tech/tk759/c1482/cdccont_0900aecd80295aab.pdf

It is usually not necessary to map DSCP to IP precedence as all current Cisco hardware can handle DSCP. A few years ago with older switches you did have to map IPP or DSCP (layer 3) to COS (layer 2) values on 802.1Q trunks.

The general approach is to mark traffic as close to the edge as possible, preferably on a switch rather than router. Service policies are configured at congestion points in the network to determine how the different classes of traffic are treated when congestion occurs.

This is a pretty complex subject to cover here, but this page has some good documents you may find helpful.

http://www.cisco.com/en/US/tech/tk543/tk759/tech_white_papers_list.html

This one would be a good starting point:

http://www.cisco.com/application/pdf/en/us/guest/tech/tk759/c1482/cdccont_0900aecd8019f3e0.pdf