Figure 4: Dual Paths And Spanning Tree - Juniper M10i Application Note

This architecture defines single points of failure, in addition to creating unnecessary MAC learning points (which lead
to additional scaling issues that require yet another extension to be addressed, as described in draft-cowburnl2vpn-
vpls-ldp-mac-hiding-01). The only way to eliminate the single points of failure in these cases is to create dual paths
that are somehow connected. Because the service is a multipoint L2 service, this means creating the conditions
where L2 broadcast storms may happen. Therefore, for this scenario, some vendors are proposing the use of
Spanning Tree Protocol (STP). Interestingly. Ethernet is coming back to its origins.
If service providers discarded STP (and the likes) as a protocol suitable for carrier-class deployments (in many cases
after painful experiences), why should they now use Spanning Tree Protocol again for resolving the L2 loops that are
created as a result of using H-VPLS (or more particularly the no-split-horizon rule that applies to the communication
between H-VPLS spokes and hubs) in a way that was not intended? Needless to say, it creates an additional state on
the network due to artificially enabling the bridging function to nodes on the network that were only supposed to do
P functionality.
If we look at this whole picture, what is being created is a plain VLAN L2 switching architecture with STP as control
protocol where the links between the bridges/switches are not Ethernet interfaces but MPLS pseudowires. So,
effectively, this means going back to the Spanning Tree days. Is this the way to go? Should service providers rely on this
architecture to transport their IPTV traffic? Each will need to make this judgment based on business requirements.
Copyright © 2010, Juniper Networks, Inc.
CPE
PE
PE
PE
MAC table
LDP active psuedowire
LDP backup psuedowire

Figure 4: Dual paths and Spanning Tree

APPLICATION NOTE - Demystifying H-VPLS
Psuedowire blocked
PE
by Spanning Tree
PE
Spanning Tree Protocol
PE
running through the
psuedowires
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