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Chapter 35: ERPS Commands
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 35-2: Sub-ring with Virtual Channel
RPL Port
Sub-ring without R-APS Virtual Channel – Under certain circumstances
•
it 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.
No R-APS messages are inserted or extracted by other rings or sub-
rings at the interconnection nodes where a sub-ring is attached. Hence
there is no need for either additional bandwidth or for different VIDs/
Ring IDs for the ring interconnection. Furthermore, protection switching
time for a sub-ring is independent from the configuration or topology of
the interconnected rings. In addition, this option always ensures that
an interconnected network forms a tree topology regardless of its
interconnection configuration. This means that it is not necessary to
take precautions against forming a loop which is potentially composed
of a whole interconnected network.
– 1065 –
Interconnection Node
Major Ring
Sub-ring
with Virtual
Channel
Virtual
Channel
Ring Node
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