?– A failure or restoration in the interior of a client cloud does not cause any activity
in the server cloud??™s spanning tree or in any other client clouds??™ spanning trees.
?– Changes in the interconnect topology between clouds can result in changes to the
one directly affected client cloud??™s spanning tree and in the server cloud??™s VLAN
pruning, but not in the server cloud??™s spanning tree and not in any other client
clouds??™ spanning trees.
A cloud X can be both a client cloud, say to server cloud Y, and a server cloud, say to
client cloud Z. Clouds can be chained. If clouds are chained together in a circle, then
a hard forwarding loop will result because there is no over-arching Spanning Tree
Protocol running everywhere in the world. Fortunately, this is not possible as long as
the server cloud always adds an encapsulation, as for instance, if the server-client relationship
is between an 802.1ad provider bridge network (server) and an 802.1Q client
Ethernet Bridging 407
network (client), or between an 802.1ah backbone network (server) and an 802.1ad
provider bridge network (client). In such a loop, the ever-increasing mountain of tags
being added, as a frame traverses the loop, eventually causes a discard due to excessive
frame length (a crude form of TTL).
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