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Beginner

What is the ethertype for STP?

Hi,

I don't see the ethertype for STP in the file:

standards.ieee.org/regauth/ethertype/eth.txt

Would it be similar to the protocol identifier in 802.1D, that is 00-00?

Thanks for your opinion,

best regards,

Michelot

1 ACCEPTED SOLUTION

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Hall of Fame Cisco Employee

Re: What is the ethertype for STP?

Hello Ian,

Good work!

Just a brief additional comment: neither IEEE STP nor Cisco PVST+/RPVST+ are encapsulated into Ethernet_II frames, that is why there is no information about the assigned EtherType. An Ethernet frame carrying these protocols carries the length of the frame in the respective field, and the actual payload type is determined by additional headers.

The IEEE STP/RSTP/MSTP uses LLC encapsulation with the DSAP=SSAP=0x42. The Cisco's PVST+ uses SNAP encapsulation where, because of SNAP, DSAP=SSAP=0xAA and the Protocol=0x010B.

Best regards,

Peter

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Enthusiast

Re: What is the ethertype for STP?

After digging around I found this:

1. LSAP 0x4242 -> IEEE STP and PVST (ISL trunks, access ports)

2. ether 0x010B -> PVST+  (802.1q trunks)

3. LSAP 0xAAAA -> All SNAP encaps, including PVST+, CDP, VTP, etc, which can all be matched with ethertype (even though is protocol ID).

HTH,

Ian

Highlighted
Hall of Fame Cisco Employee

Re: What is the ethertype for STP?

Hello Ian,

Good work!

Just a brief additional comment: neither IEEE STP nor Cisco PVST+/RPVST+ are encapsulated into Ethernet_II frames, that is why there is no information about the assigned EtherType. An Ethernet frame carrying these protocols carries the length of the frame in the respective field, and the actual payload type is determined by additional headers.

The IEEE STP/RSTP/MSTP uses LLC encapsulation with the DSAP=SSAP=0x42. The Cisco's PVST+ uses SNAP encapsulation where, because of SNAP, DSAP=SSAP=0xAA and the Protocol=0x010B.

Best regards,

Peter

View solution in original post

Highlighted
Enthusiast

Re: What is the ethertype for STP?

Thanks Peter for the clarification.

Highlighted
Beginner

Re: What is the ethertype for STP?

Thanks Peter,

for these very interresting comments.

Best regards,

Michelot

Highlighted
Beginner

Re: What is the ethertype for STP?

Thanks Ian,

for your digging, and your introduction to peter comments.

Best regards,

Michelot

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