On Thu, 2009-03-19 at 17:39 -0400, Josh Luthman wrote: > In my experince this is caused by half duplex equipment (which was > already checked and replaced with switches I have used often). The > issue still remiands, however.
It is typical that a duplex mismatch will cause this type of issue. It is not always the case, however. If there is a wireless link in the path, then there is a half duplex link, but maybe no duplex mismatch. > It was suggested to me to change the MTU for qos reasons. Is this a > likely solution? If so how will it help if I do this at the > customer's router? How would I put this into the firewall? Well, when a wireless link is involved, the benefit to changing MTU is somewhat cloudy and often not the right approach. Changing the MTU on a wireless link (I'm assuming it is wireless) will have a couple of significant impacts that you need to be aware of before you attempt this. If you have traffic coming into the router on (for example) ether1 and leaving on wlan1, you must recognize that the MTU for both of those networks will be different. This is not a huge issue, but for every packet that comes into the router on ether1 that is larger than the MTU on wlan1, it will have to be segmented to be delivered. Again not a big issue, but it will impact CPU on both ends of the wireless link. This effect will cause a higher packet rate on the wireless network, which may or may not be desirable. Additionally, if the AP on this wireless link is a PtMP network, then all devices connected to the AP must change their MTU as well (because the AP will need to be changed, too). The reason MTU changes MAY help out depends on the wireless link. If the link, or more specifically the wireless network, is seeing a significant number of retransmissions (anything over about 3-5%), then changing the MTU has a chance of helping. Keep in mind the higher packet rate mentioned earlier, because that will have a potentially severe negative impact on the network behavior. Either way, the theory is that if I am retransmitting every 1 of 10 packets, if I make the MTU smaller, then I am retransmitting a much smaller volume of data. In other words, if MTU=1500 and I retransmit 1/10 packets, then I am retransmitting 1500/150k bytes. If the MTU is 1000 with the same 1/10, I am retransmitting 1000/100k bytes. Same ratio, but fewer bytes being retransmitted. That's the theory anyway. My experience is that MTU changes usually have either no impact at all or a negative impact. To change the MTU, you would do something like: /interface wireless set wlan1 mtu=1000 mru=1000 Note that MRU is a similar parameter, but will tell the interface what size packet it can receive. -- ******************************************************************** * Butch Evans * Professional Network Consultation* * http://www.butchevans.com/ * Network Engineering * * http://www.wispa.org/ * WISPA Board Member * * http://blog.butchevans.com/ * Wired or Wireless Networks * ******************************************************************** _______________________________________________ Mikrotik mailing list Mikrotik@mail.butchevans.com http://www.butchevans.com/mailman/listinfo/mikrotik Visit http://blog.butchevans.com/ for tutorials related to Mikrotik RouterOS
