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Inside Cisco IOS Software Architecture (CCIE Professional Development) by Russ CCIE #2635 White, Curtis CCIE #1521 Murphy, Vijay CCIE #1606 Bollapragada

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Modified Deficit Round Robin

Unlike other Cisco platforms, the 12000 series of routers doesn't support priority queuing, custom queuing, or weighted fair queuing. Instead, they implement Modified Deficit Round Robin (MDRR) for congestion management. MDRR can be configured on inbound interfaces for packets outbound to the switching fabric, and outbound interfaces for packets awaiting transmission.

MDRR classifies packets into queues based on the precedence bits in the IP header. These queues are serviced round-robin, with a queue being allowed to transmit a number of bytes each time it's serviced (the quantum). A deficit counter tracks how many bytes a queue has transmitted in each round.

When the queues first are configured, their deficit counters ...

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