Juniper ACX1000 Configuration Manual page 684

ACX Series Universal Access Router Configuration Guide
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A Layer 2 pseudowire terminates on one of the logical tunnel interfaces (x), defined with
the circuit cross-connect (CCC) address family configured. A Layer 3 VPN (RFC 2547)
terminates the second logical tunnel interface (y), defined with the IPv4 (inet) address
family. Logical tunnel interface (x) and (y) are paired. Layer 2 pseudowires established
between each access router and its corresponding aggregation PE devices terminate on
the logical tunnel interface defined within each PE device. This logical tunnel interface
is used to establish a Layer 2 virtual circuit (VC) toward the remote end. In consequence,
the CCC address family needs to be configured on it. The same applies to the remote
end, where an equivalent interface needs to be defined with CCC capabilities.
This CCC logical tunnel interface created in the aggregation PE devices is paired with a
second logical tunnel interface on which the INET address family is enabled. This second
logical tunnel interface is configured within the context of an RFC 2547 Layer 3 VPN.
Within the scope of this document, we refer to the CCC and INET logical tunnel interfaces
as LT(x) and LT(y), respectively.
The Junos OS routing protocol process (rpd) enables the stitching required to interconnect
the Layer 2 VC ending in LT(x) and the associated LT(y).
In the aggregation PE routers, the routing process builds a pseudowire toward access
routers, and this happens regardless of the active or standby state of the pseudowire.
The same occurs in access routers, where the control and forwarding state is
preestablished in both the Routing Engine and the Packet Forwarding Engine to mitigate
traffic disruption during convergence periods.
An attachment circuit (AC) is a physical or virtual circuit (VC) that attaches a CE device
to a PE device. Local preference is used to provide better information than the multiple
exit discriminator (MED) value provides for a packet's path selection. You can configure
the local preference attribute so that it has a higher value for prefixes received from a
router that provides a desired path than prefixes received from a router that provides a
less desirable path. The higher the value, the more preferred the route. The local
preference attribute is the metric most often used in practice to express preferences for
one set of paths over another.
If the Layer 2 circuit is primary, the corresponding PE device advertises the AC's subnet
with the higher local preference. All aggregation PE devices initially advertise the AC's
subnet with the same local preference. You can configure a routing policy to allow a
higher local preference value to be advertised if the Layer 2 VC is active.
If a pseudowire is down, LT(x) is tagged with the CCC_Down flag. When this happens,
the corresponding PE device withdraws the AC subnet that was initially advertised. The
LT(y) address is shared between the aggregation PE devices as a virtual instance port
(VIP). No VRRP hello messages are exchanged. Both PE devices assume mastership.
Both primary and standby Layer 2 VCs are kept open to reduce traffic disruption in
backup-to-primary transitions. The
manual activation.
configuration statement allows
hot-standby-vc-on
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