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MTU on port with tagged VLAN
The default for the MTU configuration on all ports is 1518, while a VLAN tagged ethernet frame can be as large as 1522 with a L3 payload MTU of 1500. Should I change all MTU values to 1522 for ports ...
Hi firstsn，
Yes， all right.
In the 'Monitoring / Port Detailed Statistics' menu, 'Packets Transmitted > 1518 Octets it include 4 bytes VLAN-tag.
Regards
Carl
Hi fritsn
Welcome to Netgear Community,
In general，it no need to change all MTU values to 1522，since most of switch have function of fragment.
Hope it helps
Reagards
Carl
Netgear Employee
Hi Carl,
I do not want fragments!!!
That will slow down my traffic.
BTW., only routers can fragment frames, not L2 switches. I do not use the GS748T as a router, but purely as a L2 switch.
I also doubt that the L2+ GS748Tv5 (=static L3 switch) can fragment. Can you show me in the manual where it describes that?
The switch manual only states:
- Maximum Frame Size - The maximum ethernet frame size the interface supports or is configured, including ethernet header, CRC, and payload. (1518 to 9216). The default maximum frame size is 1518 .
It does not mention VLAN tag, which is between ethernet header and payload. With a normal maximal payload of 1500, the frame size becomes 1522 for a VLAN tagged ethernet frame.
Why is this not mentioned in the manual?
See this reference from Cisco:
http://www.cisco.com/c/en/us/support/docs/switches/catalyst-4000-series-switches/29805-175.html
This document discusses the support of varying Maximum Transmit Units (MTUs) on Catalyst 4000/4500 series switches
"Standard Ethernet frame MTU is 1500 bytes. This does not include the Ethernet header and Cyclic Redundancy Check (CRC) trailer, which is 18 bytes in length, to make the total Ethernet frame size of 1518. In this document, MTU size or packet size refers only to Ethernet payload. Ethernet frame size refers to the whole Ethernet frame, including the header and the trailer. Baby giant frames refer to Ethernet frame size up to 1600 bytes, and jumbo frame refers to Ethernet frame size up to 9216 bytes."
So there is a standard MTU size of 1500 of the L3 payload of an ethernet frame. That makes the Ethernet Frame Size 1518 for non-VLAN frames and 1522 for VLAN-tagged frames.
Does Netgear mean MTU=Ethernet Frame Size?
What happens if the switch receives or sends a frame that is larger than the configured MTU for a port, drop it?
Should the MTU be set to 1522 for ports that carry VLAN tagged frames, in order to prevent dropping frames?
Regards,
Frits.
Hi fritsn
Sorry，I wrongly assumed your refering to a L3 issue. You are right only routers can fragment frames, not L2 switches.
For GS748T , the MTU not include vlan tag.
For example,
Set port mtu at 1518
untag package size 1518------pass
untag package size 1519（or larger）-----drop
tag package size 1522------pass
tag package size 1523（or larger）-----drop
Hope it helps
Regards
Carl
Hi Carl,
When I said MTU earlier, I really meant the 'Maximum Frame Size' configuration in the 'Switching / Ports' menu.
So you say this:
The VLAN tag is not counted against the Maximum Frame Size, so there is no need to put the number to 1522 for VLAN tagged ports if a normal MTU of 1500 is used for the payload.
True?
In the 'Monitoring / Port Detailed Statistics' menu, I see for a VLAN tagged port: 'Packets Transmitted > 1518 Octets'.
Does the packet size in this menu include the 4 bytes VLAN-tag?
Regards,
Frits
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