Which Packets Can Be Counted on a VLANIF Interface?

Only the packets sent to the CPU can be counted on a VLANIF interface.
Multicast, unicast, and broadcast packets forwarded by using the chip are not counted on a
VLANIF interface.

NOTE 

V100R005 and later versions support packet statistics on VLANIF interfaces. By default, the packet statistics function in a VLAN is disabled. To enable the packet statistics function for the VLAN and VLANIF interface, run the statistic enable command in the corresponding VLAN view.

What Is the Function of MTU on a VLANIF Interface? Will an Improper MTU Cause Protocol Negotiation Failure?

On the switch, MTUs are configured on VLANIF interfaces. The default MTU is 1500 bytes. If the size of an IP packet exceeds the MTU on the outbound interface, the switch fragments the packet.
The MTU takes effect only for packets that are sent to the CPU for software forwarding, and it does not affect packets that are forwarded by hardware. 

Two interconnected interfaces must have the same MTU value. If they use different MTU values, errors may occur during protocol negotiation.

good sharing.

The default MTU is 1500 bytes. why is 1500 bytes?

or 12000 bits )

btw, from wiki:

In computer networking, the maximum transmission unit (MTU) of a communications protocol of a layer is the size (in bytes) of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). Standards (Ethernet, for example) can fix the size of an MTU; or systems (such as point-to-point serial links) may decide MTU at connect time.

A larger MTU brings greater efficiency because each packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed; the resulting higher efficiency means an improvement in bulk protocol throughput. A larger MTU also means processing of fewer packets for the same amount of data. In some systems, per-packet-processing can be a critical performance limitation.

However, this gain is not without a downside. Large packets occupy a slow link for more time than a smaller packet, causing greater delays to subsequent packets, and increasing lag and minimum latency. For example, a 1500-byte packet, the largest allowed by Ethernet at the network layer (and hence over most of theInternet), ties up a 14.4k modem for about one second.

Large packets are also problematic in the presence of communications errors. Corruption of a single bit in a packet requires that the entire packet be retransmitted. At a given bit error rate, larger packets are more likely to be corrupt. Their greater payload makes retransmissions of larger packets take longer. Despite the negative effects on retransmission duration, large packets can still have a net positive effect on end-to-end TCP performance

Good info.

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