JUNOS Internet Software Network Operations Guide: Hardware
596 ! Understanding Redundant PCGs
Understanding Redundant PCGs
Purpose You monitor redundant PCGs to ensure that they generate a clock signal to
synchronize the modules and application-specific integrated circuits (ASICs) that
make up the Packet Forwarding Engine.
What Are Redundant
PCGs
Redundant PCGs are two PCGs installed in a router. The PCG supplies a 125-MHz
system clock to synchronize the modules and ASICs that make up the Packet
Forwarding Engine.
The M40e and M160 routers have two PCGs located at the rear of the chassis in the
slots labeled
PCG0 and PCG1, to the right of the Routing Engine slots (see
Figure 230).
Figure 230: M40e and M160 Router PCG Location
During normal operation, both PCGs generate a 125-MHz clock signal to the Packet
Forwarding Engine modules, along with a signal indicating which is the master
clock source. One PCG is designated as the master. The master Routing Engine
controls which PCG is master and which is backup.
The modules and ASICs in the Packet Forwarding Engine that use the clock signal to
gate packet processing use only the signal from the master PCG.
The PCGs are field-replaceable and hot-pluggable. You can remove and replace
them without powering down the router, but the routing functions of the system are
interrupted when a PCG is removed.
If the master PCG fails or you remove it from the chassis, the Packet Forwarding
Engine resets so that the components start using the signal from the other PCG
(which becomes the master). Packet forwarding halts while there is no clock signal
because the Packet Forwarding Engine does not accept incoming packets. If the
backup PCG fails or is removed, router function is not affected.
1780
PCG
0
SFM
0
SFM
1
M
C
S 0
RE 0
RE 1
M
CS 1
PCG
1
Do not install an SFM in this slot
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0
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1
M
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S
1
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M
2
S
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3
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PCGs
PCGs
M40e router rear M160 router rear
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