QoS | Congestion Management

In my archieve, I ever post about Queueing Strategy and Queueing Strategy part 2. it's part of Congestion Management in QoS. Congestion management use marking on each packet to determine in which queue to place packets, mechanisms queuing algorithms each interface must have a queuing mechanism to prioritize transmission of packets. Congestion management to control congestion by determining order in which packets are sent out an interface based on priorities assigned to packets. Congestion occur at any point where there re-points of speed mismatches, aggregation or confluence, queuing manages congestion to provide bandwidth and delay guarantees.

Queueing components divide two : hardware and software. Hardware queuing always use FIFO, software queuing can be selected schedules packets into hardware queue. Software queue use if hardware queue is full. What is hardware and what is software queueing ?

- Hardware Queueing is queue happen in real interface, this handle by hardware component
- Software Queueing is queue happen in software based to schedules packet into hardware queue

Softwere queue use if hardware queue is full. A full hardware queue indicates interface congestion and software queue is used to manage it. If a packet forward that mean router bypass software queue. Router determine length hard queue based on config bandwidth interface in 'tx-ring' value. In subinterface or software interface like tunnel, dialer etc. not have separate tx-ring. You can adjust length of value with 'txt-ring-limit' command IOS. So only hardware interface have a tx-ring.

Congestion management is sophisticated queuing technology, there are algorithms of queuing or congestion management QoS features :
- FIFO (first in, first out)
- PQ (priority queuing)
- CQ (custom queuing)
- WFQ (weighted fair queuing)
- CBWFQ (class-based weighted fair queuing)
- LLQ (low latency queuing) hybrid PQ and CBWFQ

We can assign only one queueing mechanism type to an interface. A variety of queueing mechanisms can be configured using multilink, for example, Multichassis Multilink PPP (MMP). However, if only PPP is used on a tunneled interface—for example, virtual private dialup network (VPND), PPP over Ethernet (PPPoE), or PPP over Frame Relay (PPPoFR)—no queueing can be configured on the virtual interface.