CLI Reference Guide-R04

Chapter 21 | ERPS Commands
– 567 –
◆ Sub-ring with R-APS Virtual Channel – When using a virtual channel to 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 3: Sub-ring with Virtual Channel
◆ 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.
Sub-ring
with Virtual
Channel
Virtual
Channel
RPL Port
Interconnection Node
Ring Node
Major Ring