Question about David's blog entry for NetCONF 2006, Day 1
Subsystems (2)
I was reading David's blog entries on the netdev meeting in Japan, and have a question about this bit:
Quoted 2 lines
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.
Quoted 15 lines · Rick Jones
I was reading David's blog entries on the netdev meeting in Japan, and
have a question about this bit:
Quoted 2 lines
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.
Quoted 4 lines
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