Monday, 28 November 2011

What is the Maximum Ethernet Frame Size supported by Non -VLAN switch

I can across this question and it left me thinking. I searched in Net and could not find any satisfactory answer. What will be the maximum Ethernet frame size allowed to be passed from any Non- Vlan switch? Before answering this question Lets refresh some basics.

We Know that the MTU of the Ethernet is 1500. So anything above 1500 should not be allowed. This does not include header so lets see how much with header .

We have 14 bytes of Ethernet header = 6 byte source MAC address + 6 byte destination Mac address+ 2 Byte protocol type

So total byte excluding CRC will be 1500 + 14 byte ==1514 bytes

Lets say we take example of ICMP packet.

For any ICMP packet this 1500 byte includes === IP header 20 byte + ICMP header 8 bytes + 1472 data

This you can verify by using ping command on your PC

ping 192.168.3.1 -t -l 1472

If your Ethernet interface MTU is 1500 if you give 1473 the packets will be fragmented by the PC . This can be seen in wireshark.

So now total is ==== 14 ( Ethernet header) + IP header 20 byte + ICMP header 8 bytes + 1472 data + 4 byte CRC ===1518 Byte including header


OK this was without VLAN , now we have VLAN 4 bytes so :

Total allowed Ethernet frame with VLAN should be 1518+4 ==1522 bytes

So to accommodate this some changes has been done in standard which is known as 802.1ac-1998. This states that

Section : 4.2.4.2.1 Framing

a) Maximum Frame Size. The receiving CSMA/CD sublayer is not required to enforce the frame size
limit, but it is allowed to truncate frames longer than maxUntaggedFrameSize octets and report this
event as an (implementation-dependent) error. A receiving CSMA/CD sublayer that supports tagged
MAC frames (see 3.5) may similarly truncate frames longer than (maxUntaggedFrameSize + qTag-
PrefixSize) octets in length, and report this event as an (implementation-dependent) error.


This states that if an interface receives a frame more than the size limit it can drop the frame.

So if a switch does not support the vlan it can drop the received frame if the size is above the maxUntaggedFrameSize i.e. 1518 bytes in our case.

And if it supports the VLAN frame it can drop the frame above the (maxUntaggedFrameSize + qTag-PrefixSize) i.e. 1522 bytes.

Also a Non Manageable switch does not mean that it cannot forward or support VLAN . I have seen non manageable switch which can support Jumbo frames also. So it all depends on Vendors how they implement and how much maximum allowed Ethernet size they support.

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