Symptom/Cause | Solution |
|---|---|
3-Level queuing hierarchy: • Queueing hierarchy at an interface is not known • Default configurations of queues not known | show policy-map interface IFNAME If an egress (queuing) policy-map is attached to interface, check its hierarchy and queue statistics (counters). show policy-map type queuing <name> Check the configurations of a queueing policy-map. |
L2 and L3 QoS/Trust IEEE 802.1p/ (DSCP): • Wrong queue selection | Queue selection for L2 and L3 ports are based on incoming packet’s cos and dscp value respectively. trust cos/dscp Change the queue selection based on cos or dscp value. show running config interface <name> Show if any user configured trust. |
Rate Limiting: • Packet drops ingress interface • Rate limiting not working | If a particular flow of packets are sent at rate lesser than received rate and packets are dropped at ingress interface, there is a possibility of that flow is being policed (rate limiting) by policer. Policers are part of ingress policy-map which are attached to ingress interface to limit the rate of “particular flows”. show running config interface <if-name> Show any ingress policy-maps that are attached to interface show policy-map type qos <policy-map name> Show the matching flow and the policer details |
Shaping and Bandwidth Reservation Per Queue: • Packet drop at egress queue • Packets sent at lesser rate than received | If there is no congestion at egress interface, but there is packet drop at queue and outgoing traffic rate is lesser than incoming rate, shaping would have been configured at that queue. show policy-map type queueing <egress-policy-map> Check if any shaping is configured on the queues of the interface. show policy-map type queueing <egress-policy-map> Check if there is congestion and queues are strict-priority; packets will flow according to the configured minimum bandwidth. |
WRR/WFQ/SP Scheduling Per Queue: • Different traffic flows are transmitted/processed at different rate | If there is congestion at interface, if queue configured for SP scheduling, the packets are processed according to minimum bandwidth of queue. If queue configured for WRR scheduling, packets are processed according to the WRR weight of the queue. show policy-map type queueing <egress-policy-map> Check the minimum bandwidth/weight. |
Weighted RED • Packets are dropped only at some or all queues. | WRED/taildrop is used decrease the queue size/limit resulting in loss of packet. show policy-map type queueing <egress-policy-map> Verify WRED/taildrop configuration. |
802.1p remarking: • cos / dscp value is getting remarked • Packets are going to different queues than the one selected by incoming cos/dscp value | If packets of particular flow are going to different queue than the queue for their cos/dscp value, a remarking could have happened. show policy-map type queueing <egress-policy-map> Check if remarking is done using an ingress policy-map. qos map cos <in-cos> <out-cos> Configure remarking. |