Cisco 2960s Switch Outdiscards Issue

ciscocisco-2960cisco-catalystnetwork-coreswitch

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?

Best Answer

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.

Related Solutions

Cisco – How to infer 2960S switch base ethernet MAC Address

Is there a reliable way to infer the switch "Base ethernet MAC Address" from receiving such a CDP interface MAC Address, so that I can use that "Base ethernet MAC Address" as a globally unique identifier for the switch device equipment on the network?

You can't determine the base ethernet mac from CDP without guessing or assumptions about how Cisco behaves today (as well as future behavior). On the other hand, if all you want is a unique identifier, use the Chassis serial number; however, that would require polling the CDP source for entPhysicalSerialNum... a small price to eliminate guesswork.

entPhysicalSerialNum has the added bonus of working across many of Cisco's current platforms.

Sample SNMP walk of entPhysicalSerialNum:

[mpenning@tsunami ~]$ snmpbulkwalk -OXsq -v 2c -c ciscoro -m ENTITY-MIB \
  172.16.1.200 entPhysicalDescr
entPhysicalDescr[1] 2650 chassis
entPhysicalDescr[2] 2600 Chassis Slot
entPhysicalDescr[3] C2600 Mainboard
entPhysicalDescr[4] 2600 DaughterCard Slot
entPhysicalDescr[5] 2600 DaughterCard Slot
entPhysicalDescr[6] AIM Container Slot 0
entPhysicalDescr[7] 2600 Chassis Slot
entPhysicalDescr[8] One port Fastethernet TX
entPhysicalDescr[9] AmdFE
entPhysicalDescr[10] AmdFE
[mpenning@tsunami ~]$ snmpbulkwalk -OXsq -v 2c -c ciscoro -m ENTITY-MIB \
  172.16.1.200 entPhysicalSerialNum
entPhysicalSerialNum[1] JAB05290Z8Q
entPhysicalSerialNum[2]
entPhysicalSerialNum[3] yyyyyyyyyyy
entPhysicalSerialNum[4]
entPhysicalSerialNum[5]
entPhysicalSerialNum[6]
entPhysicalSerialNum[7]
entPhysicalSerialNum[8] 25809045
entPhysicalSerialNum[9]
entPhysicalSerialNum[10]
[mpenning@tsunami ~]$

Interface capabilities: CISCO 2960S vs WS-X4648

The selection criteria between the two choices needs to be refined further in your question. If all you care about the throughput on the switch and ignore all other things, you can go with the device that provides more throughput. 4500 switch is a layer 3 switch used for access or distribution (even core for small networks) with huge capabilities and flexibility. It is a modular switch meaning that you add modules based on your needs. For sure it will be more expensive and might be an over kill for your needs if all you need is to add a few servers to your network.

Buying the 2960S stackable switch on the other hand will be cheaper as it is a layer 2 switch used only for access layer. You can always always buy more switches add stack them together if you need to add more servers.

Please keep in mind that your server performance is not only governed by the access layer switch you will buying. It will depend on the complete path your user data traverses before reaching your servers.

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