This is acceptable in simple topologies, but becomes hard to manage
in larger and more complex networks. Spanning tree timers have to be increased
as the topology grows, in order to prevent stability problems (the default timers
impose a maximum diameter of seven hops). Disabling links may also be unacceptable
as a protection mechanism in the network core. Deploying MPLS in the
metro core enables carriers to constrain spanning tree to the edge of the network,
enabling networks with larger numbers of nodes to be deployed and to make use
of all available core links. MPLS also offers protection mechanisms that converge
faster than Rapid Spanning Tree Protocol.
MPLS 451
?– VLANs 802.1Q has a 12-bit VLAN space, allowing 4094 VLANs. Since most
Ethernet switches are unable to translate VLANs, this creates a fixed limit on the
number of VLANs, and hence on the number of services per network. By deploying
MPLS in the metro core, using PE devices that have per-port VLAN scopes, carriers
can constrain the scope of VLAN uniqueness to much smaller domains??”removing
the restriction on the number of services supported. Another issue with VLAN
switching is that in order to provision a VLAN through the network, multiple
switches must be configured, or the provider must run protocols such as GVRP to
propagate VLAN knowledge.
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