Recall that frames containing Bridge PDUs are addressed to the multicast address X'01-80-C2-00-00-00. These frames contain the 3-byte Logical Link Control (LLC) header that is displayed in See figure 13.8. The LLC destination service access point (DSAP) and source service access point (SSAP) addresses both are X'42. The control field value, X'03, means "unnumbered information."
Figure 13.8. LLC header in BPDU frames.
Logical Link Control headers, DSAPs, SSAPs, and control fields were discussed in Chapter 4, "The Classical Ethernet CSMA/CD Mac Protocol," in the section called "LSAPs."
The first 2 bytes of a BPDU ...