Extensions of this point-to-point mechanism could be made for multipoint-tomultipoint
networks. As the number of endpoints increases, the difficulty of finding two
independent paths to all of those endpoints increases, until at some point, you have to
run two networks in parallel. Also, two failures in the network, one in each half, can
render all services inoperable. Then, only a dynamic bridging protocol such as MSTP
or SPB can rescue the network. Fortunately, the changes to MSTP required to allow
a division of the VLAN IDs and port bandwidth allocations between PBB-TE circuits
and dynamically controlled topologies are trivial; both control techniques could coexist
in the same backbone bridged network.
The characteristics of PBB-TE make it attractive to many vendors and providers:
?– Its deployment requires no changes to the data plane frame forwarding behavior defined
for a P8021.ah backbone bridge, a project for which a number of vendors have
been developing equipment. This makes PBB-TE attractive to bridge vendors.
?– The similarity of PBB-TE to existing telephony circuit-switching techniques makes
it attractive to network operators; the cost of training large numbers of technicians
to understand dynamic control protocols such as MSTP is not trivial.
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