There is confusion on this issue because there's more than one standard.
IBM "standardized" source-route bridging in their Token Ring Architectural
Reference Guide many years ago. The IEEE had nothing to do with this
weirdness until the early 1990s when they standardized Source-Route
Transparent bridging in Annex C of IEEE 802.1D. (Don't believe the books
that claim it is in 802.5.)
The IEEE document says this, where an X bit is undefined (can have any value):
0XX = specific route (non-broadcast)
10X = all routes explorer
11X = spanning-tree explorer
IBM didn't originally define a spanning-tree explorer, but they did define
the following:
110 = single-route broadcast, all routes broadcast return
111 = single-route broadcast, non-broadcast return
Without a spanning tree, administrators had to manually configure redundant
bridges to make sure a 11x explorer only took one path. Later IBM
introduced automatic and spanning-tree features.
Anyway, to make a long story short, C or A would be right according to IEEE
since the third bit is undefined.
Priscilla
At 03:32 PM 3/22/01, John Neiberger wrote:
>Okay, now I'm confused by something in the Sybex CCIE study guide in the
>token ring chapter. It says that if the first two bits of the routing
>control field are 10 the frame is an all-routes explorer. If the first
>two bits are 11 then it's a single-route explorer.
>
>In all of their examples, when they convert the first nibble to hex
>they use A for allroutes explorer and C to indicate single route
>explorer. I understand "C" because 1100 = C, but 1000 = 8, not A.
>What's the deal? Am I losing my mind or is the book wrong? I've
>noticed the book has been wrong in several other places but this is a
>new topic for me.
>
> From what I can tell, a RIF that starts with 8 is an all-routes
>explorer. If it starts with C it is a single-route explorer. If it
>starts with zero it is a nonexplorer frame. Is this correct?
>
>Thanks,
>John
>
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