One stupid question

MACattack

Hi, Sorry for asking this but I have one stupid question.

In Vlan Trunk what is the MTU size for ISL and 802.1q, my answer is in
ISL it is 1500 + 18 + 26 and 4 = 1548 bytes
802.1q it is 1500 + 18 + 4 = 1522 bytes.

I am confuse when you use show vlan brief command, you will see mtu size showing 1500 in each VLAN and also tested in real switch is 1500.

Am I right or Wrong?

1500 normal payload without frame header
18 if with frame header

I am confuse too much book I have read some books it is 1500 some 1548 like chris said it is 1548 in trunk.

dtlokee

1500 is before the Data Link header and trailer are added. When you encapsulate it adds 4 bytes for 802.1q or 30 bytes for ISL

MACattack

I know but if someone ask you what is the frame size of an ISL data frame.. it should be 1548.

mikearama

Yep, 1548 is the max when dealing with ethernet for ISL.

Great link at

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094665.shtml

showing that the range of ISL frame sizes is from 94 to 1548 bytes.

sunil_vishnani

Bydefault the MTU size of VLAN is 1500 and the output of "Show VLAN " displays the default vlaue unless you change it .and it resembles only the payload size without the header appended.

Trunking headers are only appended when frame exits a Trunk Interface .

MACattack

Okay, I got it you mean when it ask what is the frame size of an ISL ethernet frame.
Answer is 1548 but when somebody ask you what is the frame size of an ethernet frame before sending to trunk interface. answer is 1500