The question was whether or not Cisco used the "standard" 0x7E as a flag in their HDLC implementation. The only WAN protocol analzer I could dig up predates Cisco HDLC by a few decades. So I did rely on an o'scope as planned. Between keepalives, a constant binary 111110011111100111111001111110 can be observed. This is the classic 7E7E scope trace. Seems to me that most HDLC implementations that I can remember do two 0x7Es and then all zeros for the remainder of the frame header (no "payload zeros"). Cisco apparently just keeps 'em a comin'. Of course, once keepalives or real data hit the line, there's no way to distinguish the 7E flag between frames. But given that 7E7E is used as an idle pattern, it isn't too much of stretch to assume they serve as frame delimiters as well. I'll try to verify that with a Cisco HDLC-capable WAN analyzer if I ever get my hands on one and can get it latched on to a Cisco serial connection.
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