To be sure, 25,000 addresses is still a huge number, more than enough for the vast majority of data centers. Then again, some heavy users of virtualization already are pushing above this figure. Further, we think data-sheet numbers should give users meaningful guidance on the limits of switch performance, not theoretical best-case estimates.
Link aggregation fairness
The S4810 allows up to eight ports to be combined into a link aggregation group (LAG) and uses the link aggregation control protocol (LACP) to dynamically add and remove LAG members. We took one LAG member offline, as might occur in the event of a link or transceiver failure, to see how the switch would distribute that port's traffic across remaining members of the LAG.
Traffic distribution was not uniform in this failover test. After we disabled a port, the switch redistributed all of its traffic to the first two ports in the LAG. On a lightly loaded network this wouldn't be a problem, but it could result in oversubscription and frame loss on a heavily loaded LAG. Still, this is an improvement over LAG behavior we saw on some switches in last year's test, where all traffic from a failed LAG port was redistributed to just one other LAG member.
As a final test of unicast performance, we checked the S4810 for "forward pressure," a mechanism some switches use to avoid congestion by forwarding frames illegally fast. The S4810 doesn't have that problem. Its clock is set to run at 40 parts per million (ppm) faster than Ethernet's theoretical line rate, but that's well within the 100-ppm tolerance allowed in the Ethernet specification.
We measured the S4810's multicast performance with tests of IGMP group capacity; group join and leave times; and throughput and latency. The first two of these stress the switch's control plane via the switch's software and CPU, while throughput stresses the data plane via the ASIC.
Using IGMP snooping, the switch learned 3,000 multicast groups in our capacity test. That's higher than all but one top-of-rack switch tested last year, and a useful figure for trading and videoconferencing applications that require large number of multicast groups.
The switch's join/leave times were another story. With all receivers subscribed to 989 multicast groups, the S4810 took an average of 21.7 seconds to join each group and 18.3 seconds to leave. That's much higher than most switches in last year's test, which also handled 989 groups. The S4810's maximum join and leave times were higher still, at 49.8 and 53.7 seconds respectively. These high IGMP processing times suggest an overload of the switch's CPU.
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