Re: Default Queue size in cisco routers

I guess I'd ask the question why this is an important issue unless you work in the bowels of the router in a network engineering context. What problem are we trying to solve? Hardware buffer tuning is somewhat esoteric, and one can get themselves into some trouble by manually adjusting the nerd-knobs without careful consideration. :-)

I would *especially* take exception to this if this request was coming from an application development context. The network service end-user should not know, care, and/or be able to "manipulate" transport device operation at the hardware/interface level. At best, this opens the door to suboptimal operation, and at worst, a potential security/service vulnerability scenario.

Now, as for hardware buffer management, Cisco utilizes several techniques. As an example, one of the techniques allows limited control of packet buffer processing (particle-based systems), but some of those pools can't be tuned at all. Of course Cisco offers some guidelines for tuning, but in general, one won't see a major impact to network traffic. Tuning these buffer pools is really meant to address large bursts in traffic.

One item that is worth mentioning is interface ring buffer tuning. This particular tuning mechanism can address nasty issues such as head-of-line blocking. However, depending on the type of network, ingress/egress interface mix (high and low speed) and traffic pattern, tuning the ring buffers is generally not needed.

I guess my point here is that if we're attempting to compensate for layer 4 (and above) performance deficiencies, tweaking router hardware buffers is not really the place to do it except in some fairly specialized packet transport situations. In that case, now we're looking at tweaking the QoS nerd-knobs in IOS. We can impact things more granularly and not impact hundreds (or perhaps thousands) of global flows because we dorked the physical interface...which led to the development of random packet and tail drop techniques in the first place. ;-) I'd argue IOS is the appropriate place to classify and manipulate production traffic, not at the interface/hardware level.


Just my $.02 + Indiana sales tax.


- A Cisco guy in Fort Wayne, Indiana



Scott Perry wrote:
Ok, first... take whoever has used this phrase and "random tail drop" them!!! (that's a bad network joke)

The command to show traffic queue buffers is simply "show buffers" from exec or privileged mode. These buffers are adjustable and can increase in size if needed. It is not common and the Cisco documentation I read recommends that the administrator has a grasp over what they are doing before proceeding with buffer adjustments.
There are also buffers for traffic queing when prioritizing network traffic which should be taken into consideration.

.

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