mtu is 1504, or 1526, or 1500 in L2 protocol tunnel?

From: lee.stpeter@gmail.com
Date: Mon Aug 06 2007 - 05:02:17 ART

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according to http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3560/12237se/scg/swtunnel.htm
When configuring IEEE 802.1Q tunneling on an edge switch, you must use IEEE 802.1Q trunk ports for sending packets into the service-provider network. However, packets going through the core of the service-provider network can be carried through IEEE 802.1Q trunks, ISL trunks, or nontrunking links.

correct me is I am wrong.
1. if the core uses DOT1Q trunk, mtu is 1504 since 4 bytes tag is added, right?
2. if the core uses ISL trunk, mtu is 1526 since 26 bytes enc is added, right?
3. if the core does not use dot1q or isl, say access-vlan throughout the core, then mtu is 1500, since nothing is added, right?

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