On Thursday 21 September 2006 17:15, Rick Jones wrote: > I was reading David's blog entries on the netdev meeting in Japan, and > > have a question about this bit: > > Currently, things like Xen have to put the card into promiscuous > > mode, accepting all packets, which is quite inefficient. > > Is the inefficient bit meant for accepting all packets, or more broadly > that the promiscuous path is quite inefficient compared to the > non-promiscuous path? > > I ask because I would have thought that if the system were connected to > a switch (*), the number of packets received through a NIC in > promiscuous mode would be nearly the same as when it was not in > promiscuous mode - the delta being (perhaps) multicast frames. > > rick jones > > (*) "Today," it seems 99 times out of 10 systems are connected to > switches not hubs.
It depends on how good your switch is. Say you have a bank of 8 servers on a 8-port switch, each running 16 Xen instances with virtual NICs and different MAC addresses. If the switch does not have enough resources in its MAC table (likely for an 8-port switch) to cache 136 entries (8 * (16 + 1) mac addresses), it will broadcast any packet that is not in the cache to every port on the switch, effectively making the switch into a hub for certain usage patterns. Of course, this is an argument for getting a better switch, but the possibility of virtual MAC addresses might cause some surprising resource utilization problems for network administrators who are used to counting physical ports. - Brent - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
