L2TPv3 MTU

i'm trying to understand L2TPv3 header overhead from CCO -

http://www.cisco.com/en/US/products/ps6587/products_white_paper09186a00800a8444.shtml
it says the following -

The MTU in the IP backbone must be x bytes larger than the MTU that
operates on the pseudo-wire. The value for x is:

802.1Q = 50
Ethernet = 46
POS = 36
Frame Relay = 34
CHDLC = 36

My understanding is for e.g. in Frame-relay , if i have default IP MTU
 = 1500, and i generate an IP packet of 1500bytes,
1. 1504-byte frame will be received on a Frame Relay attachment
circuit including the 4 byte Frame-Relay header
2. 20 bytes L2TP IP packet header
3. 12 bytes L2TPv3 session header
4. 4 bytes - default Layer 2 specific sublayer (considering sequencing
is enabled)
Total = 1540 bytes.

Can somebody explain Cisco's breakup? i cant understand how Cisco came
with th figure of "34 bytes" extra overhead over IP MTU.

Similarly for Ethernet its listed as 46 bytes which is not clear.

This comes from a real OEQ in the SP lab, hence i'm being lil extra meticulous

Thanks.

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Received on Wed Mar 24 2010 - 01:58:41 ART