Solved

diffserv questions

Posted on 2010-11-15
2
414 Views
Last Modified: 2012-05-10
I'm trying to understand diffserv a bit more

Is it true, that when marked traffic (say dscp ef) hits a switch inbound, the switch strips the marking if "trust dscp" is not enabled on the switchport?

Should you do your tagging on your edge devices (routers) and then trust dscp in your switch? Our routers have a service policy out on the outside interface and inbound on the inside interface

thanks
0
Comment
Question by:orus
2 Comments
 
LVL 21

Assisted Solution

by:from_exp
from_exp earned 100 total points
ID: 34142918
hi!
QoS is required by VoIP solutions to provide reasonable level of voice/video quality.
VoIP packets, leaving VoIP devices like ip-phones, PCs with softphones (btw, for windows you have to switch on QoS marking in registry) etc, are marked with appropriate diffserv values.
Modern QoS capable switches have a predefined set of diffserv settings, so when the switch trust dscp markings, it can handle traffic according to defined policies.
However not always VoIP device QoS markings correspond with switch markings, in this case you have to redefine qos policies on the switch instead of trusting incoming traffic.

As a general practice you can really trust dscp for ip-phones and voip pbxes, however you should be cautious with softphones, due malicious programs being able to set better priority for non voip traffic.
0
 
LVL 9

Accepted Solution

by:
Alex Bahar earned 400 total points
ID: 34145058
>>Is it true, that when marked traffic (say dscp ef) hits a switch inbound, the switch strips the marking if "trust dscp" is not enabled on the switchport?

"trust dscp" or "trust cos" refers to whether the switch should QUEUE the packets/frames by inspecting the IP DSCP value or the ETHERNET COS value. Please note that a layer 2 switch should normally trust COS, and a router should trust DSCP. Also note that ETHERNET COS value has to be mapped to a DSCP value when crossing the routing boundary, mainly because Ethernet is a layer 2 technology, and WAN links like frame relay, ATM, DSL, etc does not have COS bits in the protocol header. Please read Cisco documentation about COS-DSCP mappings.

>>Should you do your tagging on your edge devices (routers) and then trust dscp in your switch? Our routers have a service policy out on the outside interface and inbound on the inside interface

Tagging should be performed at the endpoint. In other words,  the ip phone, video endpoint, voice gateway, IPPBX, etc should mark the packets/frames properly. Network infrastructure should (ideally) just inspect and queue the traffic based on (trusted) markings as sent by the voip endpoints. General practice is to un-trust the PC VLAN, and trust the voip device markings. If your voip devices cannot provide QoS markings, or the markings are not complying with the rest of your network, then you may want to remark them at the switch interface. Or, if your service provider is expecting different markings, then you may need to remark your DSCP/TOS at your edge router to be compatible.
0

Featured Post

Save the day with this special offer from ATEN!

Save 30% on the CV211 using promo code EXPERTS30 now through April 30th. The ATEN CV211 connects a laptop directly to any server allowing you instant access to perform data maintenance and local operations, for quick troubleshooting, updating, service and repair.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
change extension to new user 5 68
BGP prefix and routing 3 87
Simple Router Management, Subnets and VLANs e.g. RV0xx 7 70
Cisco Edge Routers for BGP 6 92
There are two basic ways to configure a static route for Cisco IOS devices. I've written this article to highlight a case study comparing the configuration of a static route using the next-hop IP and the configuration of a static route using an outg…
How to set-up an On Demand, IPSec, Site to SIte, VPN from a Draytek Vigor Router to a Cyberoam UTM Appliance. A concise guide to the settings required on both devices
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
After creating this article (http://www.experts-exchange.com/articles/23699/Setup-Mikrotik-routers-with-OSPF.html), I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…

756 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question