C H A P T E R 4 Troubleshooting Router Switch Fabric; Understanding The Flags Field - Cisco CRS-1 - Carrier Routing System Router Troubleshooting Manual

Chapter 4 Troubleshooting Router Switch Fabric
Data is not transmitted across the fabric in packets. Packets are segmented into cells by the ingress queue
ASIC and reassembled back to packets by the fabric queue ASIC. After a cell is placed on a plane, it will
remain on that plane until it reaches the egress MSC. While the system is operational, cells are always
being sent and received even though they may not contain any data. Cells with no data are termed as idle
cells.
Cell forwarding on the S3 ASIC on both the Cisco CRS-1 4-slot, 8-slot, and 16-slot systems involves
examination of the cell header. Using the cell header, the S3 is able to determine which fabric queue
ASIC the cells should be sent to. The S3 ASIC injects cells in a round-robin fashion across the multiple
links connecting the S3 ASIC to the fabric queue ASIC.
As the cell is created, forward error correction (FEC) data is calculated and appended. The FEC code is
used on the transmission links to ensure the integrity of the data as it is transmitted and received. The
FEC data is removed, checked, recalculated, and reapplied by each SFE in the path until it is delivered
to the egress MSC. The FEC value is checked before a cell is processed by the SFE. If an error is
detected, the FEC code can be used to recover the cell. Depending on the nature of the error, recovery
may not be possible, in which case, the cell is discarded.
A system can operate with seven planes with out performance degradation. As further planes are
removed, overall capacity degrades but the system remains operational. A system requires a minimum
of two planes to maintain service. One plane must be odd numbered and the other plane must be even
numbered.
A 4-slot system operates on four planes. The loss of a plane reduces the usable forwarding capacity by
Note
approximately 6Gbps.
A plane marked MCAST_DOWN means that as far as the ingress line cards are concerned, not all
destination FabricQ ASICs can be reached via the plane. Since the destinations of multicast traffic
cannot be predetermined due to their dynamic nature, the plane is termed as MCAST_DOWN. Multicast
traffic will use the other fabric planes to deliver date to the appropriate egress line cards. Unicast traffic
will continue to use the plane if traffic is destined to a FabricQ ASIC that is still reachable. If traffic is
destined for a FabricQ ASIC that is not unreachable, the other available plane is used instead of the plane
in the MCAST_DOWN state.

Understanding the Flags Field

A Flags field is provided in some fabric-related show commands. The flag is an abbreviated reason for
a link being placed in the down state.
Table 4-1
P - plane admin down—plane has been taken out
of service by administrative action
C - card admin down—card has been taken out of
service by administrative action (reserved for
future use)
OL-21483-02
Table 4-1
Fabric show Command Flags
Cisco IOS XR Troubleshooting Guide for the Cisco CRS-1 Router
provides descriptions of the flags.
p - plane oper down—plane is not able to operate
or has been taken out of service by administrative
action
c - card oper down—flag is set in response to
system notification that a card will soon be
powered off. This is a transient condition lasting a
few seconds before power is withdrawn. Unusual
to see this flag in practice
Understanding the Flags Field
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