MPLS QoS - Cisco recommendation

[kmpoon]

Hi Guys,

Was told that Cisco recommends not to configure a link to 100% priority/bandwidth traffic (ie. no best effort traffic on the link). I can't seems to find the relevant doc that explain this. Can someone help to explain if it's true?

Thanks.

[didyap]

Yeah, even I have read somewhere that the sum of all configured priority/bandwidth values must not exceed 75% of the link bandwidth.

[thisistim]

I recently read the same thing in the CCNP BCRAN book from Cisco Press. I think the book also went on to explain that the remaining 25% was used for IP routing and management etc...

[mpugliese]

You can find this information through the URLs bellow.

Just one comment. You may override the 75 percent limitation @ some platforms (CPEs), but you won't be allowed by the IOS to do it at the 75XX Series Routers.

Yours Truly.

Murilo Pugliese.

From : Class-Based Weighted Fair Queueing

(http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps1830/products_feature_guide09186a0080087a84.html#wp17641)

Configuring Class Policy in the Policy Map

(http://www.cisco.com/en/US/customer/products/sw/iosswrel/ps1830/products_feature_guide09186a0080087a84.html#wp25081)

You can configure class policies for as many classes as are defined on the router, up to the maximum of 64. However, the total amount of bandwidth allocated for all classes included in a policy map must not exceed 75 percent of the available bandwidth on the interface. The other 25 percent is used for control and routing traffic. (To override the 75 percent limitation, use the max-reserved bandwidth command.) If not all of the bandwidth is allocated, the remaining bandwidth is proportionally allocated among the classes, based on their configured bandwidth.

[richard.harvey]

Generally I would say it is good practice to override the 75% maximum-reserved-bandwidth for an interface as long as you design the MQOS service-policy with a class that reserves a minimum ammount of bandwidth for routing and control traffic.

I have seen similar configurations in use by ISP's on MPLS VPN's that assign around 1% to a reserve class. Matching on DSCP CS6 or similar will achieve this.

[brandon.bennett]

Actually i recently spoke to Tim Szigeti, the author of the new book End-to-End QoS Nework Design, and he recommends overridding this value.

The real reason why 75% is the default is to prevent starving out traffic and has nothing to do with routing protocols, overhead, etc.

It is a great book and I would recommend it to anyone. In addition Tim Szigeti is doing a QoS presentation at Networkers 2005 as well as helping rewrite the SRND (which should be out quite soon)