BPDUs are sent out multicasted using an Ethernet multicast
address of 01-80-C2-XX-XX-XX and a SAP of 42 (bridging SAP).
They are sourced from what ever private mac address pool
the vendor chooses.
e.g. (Ethernet SAP)
Dest Source Ln DSAP SSAP CNTL INFO PAD FCS
0180C2000000 0010E7123456 26 42 42 03 [BPDU] XXXX XXXX
Since BPDUs (both TCN and Configs) are always 35 Bytes long add
3 for the SAP Header to get a total length of 38 (hex 0x26) for Length
40 for SNAP, although I do not believe there is a SNAP formation for
BPDUs. Perhaps Frame Relay bridging?
By simply looking at the destination address you should be able to
determine that it is in fact a multicast address by looking at the the
second nibble in the first octet. If it is odd (1,3,5,7,9,B,D,F) it is a
multicast address.
"Interconnections: Routers, Bridges, and Switches" by Radia Perlman is
suggested reading.
Ref: RFC 1700
Wayne
-----Original Message-----
From: Christopher Supino [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, August 29, 2001 10:04 AM
To: [EMAIL PROTECTED]
Subject: BPDUs [7:17607]
All ,
I came across this question while studying: How are BPDU's propagated
amongst switches? Broadcast, multicast, or unicast? Anyone have an
explanation?
Christopher Supino
CCNP, CCDP, MCSE, CNA5, ASE
Senior System Engineer
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=17804&t=17607
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