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redundant configurations, it is possible for MAC addresses to become associated with
an incorrect TLS tunnel. To prevent these scenarios from causing lengthy connectivity
interruptions, the Extreme switch relearns source MAC addresses on all received
packets and withdraws VC labels for the associated TLS tunnels when a local TLS
VLAN port goes down. By always relearning MAC addresses, MAC addresses are more
likely to be associated with the correct TLS tunnel. Withdrawing a VC label when a
local TLS VLAN port goes down forces the remote LSR to remove stale MAC addresses
from its FDB associated with the TLS tunnel of the withdrawn VC label. Thus, all egress
LERs are assured of relearning the new location of any MAC address that may have
been previously associated with the down port. If the VC label was withdrawn due to a
down local TLS VLAN port, the VC label is immediately readvertised if at least one
other local TLS VLAN port is still active.
Spanning Tree Protocols
There is some debate as to the benefit of supporting Spanning Tree Protocols (STP)
within a Layer-2 VPN. The idea is that STPs could be used to provide redundant VPN
data paths that could be unblocked if the STP detects a spanning tree topology failure.
In general, it is believed that introducing TLS VPN STPs increases network complexity
with very little real benefit. Because each TLS tunnel is carried over an LSP, MPLS
already provides a sufficient level redundancy. For example, if a TLS tunnel is using an
LDP established LSP, provided there are parallel routed paths to the TLS tunnel
endpoint, the TLS tunnel will automatically shift from a withdrawn or failed LSP to the
next best available LSP. For tunnel LSPs established using RSVP-TE, secondary LSPs can
be configured that can be hot-swapped in the event of a primary LSP failure. Thus, even
though the underlying tunnel LSP may have changed, the Layer-2 VPN data plane
remains unaffected.
TLS VPN Characteristics
Characteristics of TLS include:
• Use of LDP or RSVP-TE and Targeted LDP to establish tunnel LSPs.
• Tunnel support for dynamic TLS tunnels using Targeted LDP sessions or static TLS
tunnels using configured VC labels.
• Tunnel endpoints are identified via configured IP addresses.
• VLAN label mappings are configured at both ends of a TLS tunnel. Support for
signalling VLAN label to VLAN ID mappings using configured VC ID and Group
MPLS Module Installation and User Guide
TLS VPN Characteristics
7-5
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