To: "Norman W. Finn" Cc: P8021 From: Peter Wang/HQ/3Com Date: 11 Apr 96 19:56:48 EDT Subject: Re: ["Robin Tasker": Priority in 802.1p] > 1 * (the number of endstations) packets propagated throughout the switches > and backbone is an absurd amount of traffic for (I would claim) most any > installation. Look at the effort that went into 802.1D to ensure that > there is only one BPDU per LAN segment per hello time. That's not to say > that the notion would be entirely useless; a few switches on a Gbit > backbone using 2-level tagging comes to mind. But the potential for > drowning a bridged network in end-station registrations seems very real. > How much control traffic? We are seeing installations with 10,000 > endstations distributed among 100 VLANs. (10,000 endstations) * (8 > priority levels) / LeaveAllPeriod is a heck of a lot of packets to be > distributed to all switches. Reduce 8 to 2 and it's still way too many. Yes, one can imaging a lot of things! If you think through how GARP is specified (at least the intention of the current design), you should see that it is not a whole lot different than the combination of IGMP and DVMRP/PIM/MOSPF. From our discussions with Prof. Cheriton, it's apparently that if one implements it correctly, the GARP overhead should actually be slightly less (since it allows for multiple addresses to be registered within a single packet as opposed to the 1 address per message for IGMP). Hence, if everyone thought IP multicast control traffic was acceptable for routers (which are slower in general than switches), I don't see why there should be any objection to doing similar things at the MAC level. > We seem to be standardizing one mechanism (802.1p) that cannot scale > to large installations, and another (802.1q) able to increase dramatically > the practical size of a bridged installation. The VLAN management and database distribution schemes are yet to be worked out. GARP may actually work out to be as effective a mechanism for these as well. > Compare the 802.1p plan to priorities kept separate from MAC addresses, > (e.g. in the VLAN tag or the I/G bit) which requires no control overhead. If priority flag can be kept separate from MAC addresses without incurring any control overhead and is still backwards compatible, I don't think there will be any objections. -Peter