C
HAPTER
34
| ERPS Commands
– 1108 –
COMMAND USAGE
â—† A ring with no pending request has a logical topology with the traffic
channel blocked at the RPL and unblocked on all other ring links. In this
situation, the erps forced-switch command triggers protection
switching as follows:
a. The ring node where a forced switch command was issued blocks
the traffic channel and R-APS channel on the ring port to which the
command was issued, and unblocks the other ring port.
b. The ring node where the forced switch command was issued
transmits R-APS messages indicating FS over both ring ports. R-APS
(FS) messages are continuously transmitted by this ring node while
the local FS command is the ring node’s highest priority command
(see Table 128 on page 1108). The R-APS (FS) message informs
other ring nodes of the FS command and that the traffic channel is
blocked on one ring port.
c. A ring node accepting an R-APS (FS) message, without any local
higher priority requests unblocks any blocked ring port. This action
subsequently unblocks the traffic channel over the RPL.
d. The ring node accepting an R-APS (FS) message, without any local
higher priority requests stops transmission of R-APS messages.
e. The ring node receiving an R-APS (FS) message flushes its FDB.
â—† Protection switching on a forced switch request is completed when the
above actions are performed by each ring node. At this point, traffic
flows around the ring are resumed. From this point on the following
rules apply regarding processing of further forced switch commands:
While an existing forced switch request is present in a ring, any new
forced switch request is accepted, except on a ring node having a prior
local forced switch request. The ring nodes where further forced switch
commands are issued block the traffic channel and R-APS channel on
the ring port at which the forced switch was issued. The ring node
where the forced switch command was issued transmits an R-APS
message over both ring ports indicating FS. R-APS (FS) messages are
continuously transmitted by this ring node while the local FS command
is the ring node’s highest priority command. As such, two or more
forced switches are allowed in the ring, which may inadvertently cause
the segmentation of an ring. It is the responsibility of the operator to
prevent this effect if it is undesirable.
Ring protection requests, commands and R-APS signals have the
priorities as specified in the following table.
Table 128: ERPS Request/State Priority
Request / State and Status Type Priority
Clear local highest
FS local |
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