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A sub-ring may be attached to a primary ring with or without a virtual
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channel. A virtual channel is used to connect two interconnection points
on the sub-ring, tunneling R-APS control messages across an arbitrary
Ethernet network topology. If a virtual channel is not used to cross the
intermediate Ethernet network, data in the traffic channel will still flow
across the network, but the all R-APS messages will be terminated at the
interconnection points.
Sub-ring with R-APS Virtual Channel – When using a virtual channel to
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tunnel R-APS messages between interconnection points on a sub-ring, the
R-APS virtual channel may or may not follow the same path as the traffic
channel over the network. R-APS messages that are forwarded over the
sub-ring's virtual channel are broadcast or multicast over the
interconnected network. For this reason the broadcast/multicast domain of
the virtual channel should be limited to the necessary links and nodes. For
example, the virtual channel could span only the interconnecting rings or
sub-rings that are necessary for forwarding R-APS messages of this sub-
ring. Care must also be taken to ensure that the local RAPS messages of the
sub-ring being transported over the virtual channel into the
interconnected network can be uniquely distinguished from those of other
interconnected ring R-APS messages. This can be achieved by, for example,
by using separate VIDs for the virtual channels of different sub-rings.
Note that the R-APS virtual channel requires a certain amount of
bandwidth to forward R-APS messages on the interconnected Ethernet
network where a sub-ring is attached. Also note that the protection
switching time of the sub-ring may be affected if R-APS messages traverse a
long distance over an R-APS virtual channel.
Figure 279: Sub-ring with Virtual Channel
RPL Port
Sub-ring without R-APS Virtual Channel – Under certain circumstances it
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may not be desirable to use a virtual channel to interconnect the sub-ring
over an arbitrary Ethernet network. In this situation, the R-APS messages
are terminated on the interconnection points. Since the sub-ring does not
provide an R-APS channel nor R-APS virtual channel beyond the
interconnection points, R-APS channel blocking is not employed on the
normal ring links to avoid channel segmentation. As a result, a failure at any
ring link in the sub-ring will cause the R-APS channel of the sub-ring to be
segmented, thus preventing R-APS message exchange between some of
the sub-ring's ring nodes.
– 431 –
Chapter 13
Interconnection Node
Sub-ring
with Virtual
Channel
| Basic Administration Protocols
Ethernet Ring Protection Switching
Ring Node
Major Ring
Virtual
Channel
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