IEEE lingo = Spanning-tree explorer. IBM lingo = Single-route explorer.
IEEE standardized source-route transparent bridging in Annex C of 802.1d.
(Don't believe the books that claim it's in 802.5; it's not). IEEE
specifies the bits in the Routing Type (RT) field slightly differently than
IBM did in their documentation on source-route bridging. Here's what the
IEEE document says:
Specifically routed frame (RT = 0XX). If the most significant RT bit is set
to 0, the RD fields contain a specific route through the network.
All routes explorer frame (RT = 10X). If the RT bits are set to 10X, the
frame will be routed along every route in the network.
Spanning tree explorer frame (RT = 11X). If the RT bits are set to 11X,
only SRT bridges with ports in the transparent-bridging forwarding state
relay the frame from one LAN to another.
Long before IEEE jumped on the bandwagon, IBM had already "standardized"
source-route bridging. IBM calls the route type 11X a "single-route
explorer" frame. With IBM bridges, the network administrator had to
manually configure bridges to make sure that no more than one redundant
bridge forwards single-route explorers. With IEEE's standard, the spanning
tree does this for you. (IBM later added support for spanning tree also.
They called it "automatic mode.")
One more picky thing regarding the "Specifically routed frames (unicast)"
and "Single route broadcast" wording: On a specifically routed frame, the
destination address could theoretically be unicast, broadcast, or
multicast. On a single route explorer, the frame could have a unicast MAC
destination address. This source routing stuff is a layer up from MAC
destination addresses, or at least a sub-layer up.
Anyway, I just stopped by my computer to see if it set the clock correctly.
I gotta get out of here! &;-)
Priscilla
At 08:06 AM 10/29/00, Nigel Taylor wrote:
>Hi All,
> As I tie up my studying in light of the big day I was looking
> for some clarity on the issue of token ring explorer frames.
>
>Within the RCF(Routing Control Field) bits 15-13 define the valid types of
>explorer frames;
>
>000 - Specifically routed frames(unicast)
>100 - All Route explorer frames
>110 - Spanning-tree or single route explorer frames.
>
>My question has to do with the fact that I'm also reading or seeing
>references to
>
>000 as a Single route explorer.
>
>My understanding if I'm not mistaken thinking that a Spanning-tree/single
>route explorer are the same.
>
>Could someone help me clear up my thinking on this issue.
>
>TIA
>
>Nigel.
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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