A discard can be incremented for any of the following reasons: Layer 2 interfaces are considered to be switchport interfaces (all types), interfaces that function as switchports but have the Ethernet moniker (on the NetVanta 6355, 1224, and 7100), and Ethernet Subinterfaces (802.1q mode). However, discards for other than the following reasons should be very rare, and in this case would not warrant the reason to troubleshoot discards anyway. Note that this list only includes the most common reasons that a packet may be discarded - it does include every unique situation where it may occur. The following sections discuss when a discard may be incremented on the different types of interfaces for one of these reasons. When a packet/frame is received on an interface and when it is put in a queue to exit an interface, there are several checks that are run to make sure it is something that should be transmitted. The cause of discards should be investigated and troubleshot if they are incrementing at a high rate that can not be explained, or when they can be correlated to a specific network problem. Before continuing in this document, it is very important to remember that a healthy network will absolutely have discards and the presence of them does not necessarily indicate a network problem. In this case, a discard will be incremented indicating the interface is working as configured.ДЄdditional reasons that a unit can increment a discard legitimately will be explained further in this document.

If a packet is received with a VLAN tag of 3, it will be dropped. For security reasons, the administrator only allows VLANs 1 and 2 on the link with the switchport trunk allowed vlan 1,2 command. Consider configuring a switchport to trunk mode. In any healthy network, traffic needs to be discarded at certain points. What does this mean? That the discard stat is its own unique counter and should not be treated as an error. When a discard increments, the input and output errors stay the same. Whenever one of the other statistics like 'CRC errors' or 'overruns' increments, so does one of these general error counters.

In fact, examining the other Ethernet based stats (includes Switchports and VLANs) you will see there is a general set of errors called 'input errors' and 'output errors'. One of the biggest misconceptions concerning the discard statistic is that a discard is an error. For information on whether or not your unit has these interfaces, please see the AOS Feature Matrix - Product Feature Matrix. The discard stat appears on every Ethernet, Switchport, and VLAN interface in an AOS unit. Situations Where a Discard is Incremented This document explains the discard stat thoroughly as well as offering reasons for why a discard is incremented and what action you should take, if any, when you see this. Understanding the discard stat is important to evaluating your network health in correlation with them. On Ethernet and Switchport interfaces, the 'Discard' stat can be incremented for many different reasons some indicating healthy network operation and others indicating a network issue.

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