Restrictions
There may also be restrictions you need to be aware of when implementing features
in distributed mode.There may be other features that need to be enabled
before a particular feature is turned on, or there may be features that are disabled
necessarily when a distributed feature is used. Distributed WRED (DWRED) is
an example.WRED is used to avoid congestion by dropping packets and thereby
throttling back congestion notification compliant flows. Distributed WRED was
designed for ISP providers who would be aggregating many smaller circuits into
larger ones. Since WRED needs to monitor the current buffer state of interface
output queues, and because interfaces reside on the VIP, it makes sense that the
WRED process would run in distributed mode on the VIP. Otherwise, the RSP
would have to continually poll the status of all buffers across all VIPs. By
offloading this function to the VIP, the main processor is freed up to do other
things. However, at press time, there were some technical restrictions:
You cannot configure DWRED on the same interface as RSP-based
custom queuing, priority queuing, or weighted fair queuing (WFQ).You
can, however, configure both DWRED and DWFQ (or DCBWFQ) on
the same interface.
DWRED is available only on a per interface basis.You cannot configure
DWRED on a subinterface.
DWRED is not supported with the ATM encapsulations AAL5-MUX
and AAL5-NLPID.
DWRED is not supported on Fast EtherChannel or Tunnel interfaces.
Although the goal is to make these distributed feature differences transparent
and the restrictions as few as possible, differences and restrictions do exist. If you
have 7500 Series routers in your network, it is up to you to do a little research to
make sure the features you need can be extended to the RSP platform, and that
they will scale by running them in distributed mode.There are many tools and a
vast amount of documentation on Cisco’s Web site that can help with this.The
IOS Feature Navigator tool can help in determining if the feature you want is
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available on a particular platform, and it can tell you what IOS code and feature
set you need to run. Additionally, IOS software release notes can give you
detailed information on how to configure distributed features. Finally, you can
always contact your Cisco representative for especially difficult queries.