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I have an issue with a pair of 2960s switches. I can see a lot of outdiscards on some of the interfaces and the end users are noticing network issues. I also have some access points that can't download the configuration from the controller because of this.

The interfaces are trunks which are connected to other switches in the other end via multimode fiber 100Mbit. They are configured like this:

interface GigabitEthernet2/0/7
 switchport mode trunk
 speed 100
 duplex full

The command show interfaces counters errors:

Port        Align-Err     FCS-Err    Xmit-Err     Rcv-Err  UnderSize  OutDiscards 
Gi2/0/17            0         899           0         903          0     42443970

I have searched around via different forums and noticed that some says that this issue is because of the incoming traffic is higher than 100Mbit and forces the switch to discards some packets because it can't send them out on the slow interface. Incoming interface is with Gigabit speed.

Is it possible to increase the buffer on the switch for the outgoing traffic or is the only solution to install gigabit traffic on these as well?

Is it the 2960s that is slow or is it not possible to have different speeds in a LAN environment?

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What is probably happening is that your access bandwidth is too big for your uplink bandwidth. The only real solution is to increase the uplink bandwidth. For instance, with 48 100 Mb access ports and only a 100 Mb uplink, one host could monopolize the uplink, forcing the situation you describe.

Cisco has a recommendation for access to distribution bandwidth of 20:1. That is for every 20 access ports of 100 Mb, you need 100 Mb in the uplink. This is for normal everyday use, but if you are streaming something at 1 Gb, you may need something less than 20:1.

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  • Hi Ron, thank you for your comment. I believe you are right. The only thing to do is to change the uplink bandwidth.
    – Muffe
    Dec 9 '15 at 12:26

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