Tolly's column: The prioritisation paradox

"There is only one way, yet there are many ways." -- AnonymousAlthough that maxim might easily pass for philosophical guidance from some Far Eastern mystic, it is not. Rather, it is the essence of an apparently paradoxical statement emanating these days from the mouths of LAN switch vendors when they profess their bandwidth prioritisation beliefs. Allow me to explain.

"There is only one way . . .

When it comes to the question of how Ethernet, Fast Ethernet and Gigabit Ethernet frames should be tagged to indicate a desired level of priority, all vendors agree there is only one way. That mechanism is the IEEE 802.1p standard, which uses a field defined by the IEEE 802.1Q standard to tag Ethernet frames. I don't know any vendor that is opposed to the ratification and acceptance of this standard. Clearly, using the bits defined in 802.1p is the accepted, multi-topology, multivendor method of explicitly indicating priority in a frame.

. . . yet there are many ways."

Appropriately, this sounds somewhat mysterious. To put a diabolical spin on it, these "many ways" are proprietary, nonstandard approaches to an alternative method of prioritising traffic without using or setting any priority bits. To be fair, I should note that there are no standards available to implement. We might call this alternate approach "implicit" prioritisation. The paradox referred to in the quote is that many vendors actually promote the use of 802.1p for priority tagging, while simultaneously promoting so-called policy-based approaches that don't use 802.1p at all.

The situation becomes less confusing when considered on another plane. In the explicit prioritisation approach, which is based on 802.1p, intelligence in the endstation tags each and every outbound packet that requires special treatment by the network. The result is "network-aware applications."

When implicit prioritisation is implemented through proprietary policy-based schemes, the endstations take no action. Rather, intelligence located in the switching core automatically enforces prioritisation policy by analysing traffic streams in real time and queuing it accordingly. The result, vendors say, is "application-aware networks."

What we have may no longer be a paradox, but it qualifies as a quandary - neither approach is without problems.

Implicit prioritisation counts on the fact that it can see inside packets to decide what to do with the data. However, as users implement secure features, such as IP Security, on campus LANs, implicit prioritisation will hit a brick wall because it can't see inside the packets.

Because explicit prioritisation relies on a new standard, it's not backward compatible with existing endstations, network interface cards (NIC) and switches. It may require significant expenditures of time and money to bring existing gear up to date.

Tolly is president of The Tolly Group, a strategic consulting and independent testing firm in Manasquan, New Jersey. He can be reached at or

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