Skip to main content
Question

Yosemite Networking Issues

  • October 6, 2015
  • 2 replies
  • 10 views
M
Forum|alt.badge.img+1

We are having network issues with our Yosemite 10.10.5 clients. We have around 150 iMacs which are losing network connectivity every hour or so at random intervals.

This seems to have been since flow control was turned on, on our switches. Clearing the ARP cache or unplugging and plugging the network cable in fixes this immediately.

I have upgraded my iMac to El Capitan and I am no longer seeing the issue but do not want to roll this out yet.

When the iMacs go off they can only see their VLAN so lose internet connection and the network shares.

Does anyone have any ideas how to fix this? We have a support case open with our switch manufacturer but they seem to think the Macs are cause.

I have already tried the following script with no luck: https://github.com/MacMiniVault/Mac-Scripts/blob/master/unicastarp/unicastarp-README.md

Any other ideas?

    2 replies

    I
    Forum|alt.badge.img
    • ivaylomihaylov
    • New Contributor
    • October 6, 2015

    Hi Mark,

    I have found that on the internet and I hope it helps.

    "The primary application for flow control is in high performance applications with protocols that are highly loss sensitive.
    Both sides of a link have to be configured and if you are going to use flow control, it is generally recommended to be enabled on every link along the path.
    Using Flow Control is typically not a good recommendation and is rarely used. If you are planning on using QoS, then you MUST NOT use flow control.
    This is because flow control can cause head of line blocking across ports and queues.
    For example, priority 0 buffer is full, so the switch sends a pause frame to the transmitter on the side of the link. This will cause the TX switch to block all higher priority traffic until the receiving switch says its ok to resume.
    Likewise, a single downlink port on a switch that is congested can cause all uplink traffic being sent to that switch to be blocked.
    Draft IEEE 802.1Qau specification for congestion notification is adding support for per-priority pause to mitigate the head-of-line blocking for QoS across priority queues.
    Bottom line: unless you really know what you are doing...don't use flow control."

    my link text

    Best to look at the switch logs to try and see what is going on. Maybe some debugging will be helpful as well like:
    debug ip tcp congestion
    debug ip tcp ecn

    Install Wireshark on one Mac and capture some traffic if a packet capture on the switches in no possible.

    J
    Forum|alt.badge.img+8
    • jrwilcox
    • Contributor
    • October 6, 2015

    I disagree with the above. I know Macintoshes have defaulted to flow control enabled since before the switch to Intel processors. This was done because it showed much improvement for most of the metrics used to rate ethernet performance and server performance. To enable flow control successfully you do need to have better than average switches and routers. They have to be able to withstand periods when they must cache data. If the switches are working correctly they will not cause issues with flow control on. It really sounds like the switches are not configured correctly.

    Terms & ConditionsCookie settingsAccessibility statement