This article explains why packet drops may be seen while activating and deactivating Class of Service (CoS) configuration on ACX devices and whether any action must be taken to prevent the drops.
When Class of Service (CoS) configuration is activated or deactivated on ACX devices, packet drops may be observed for about 2 seconds as shown below:
Consider the following topology:
+------------------+ +----------------+ +--------------------+ | packet | | | | | | generator |--------------------| ACX - 1 +ge-0/0/5------| ACX - 2 | | | | | | | +------------------+ +----------------+ +--------------------+
In the above topology, the packet generator is seen generating 300 MB traffic that is destined to the ACX-2 device. Notice that there are no drops on the ge-0/0/5 interface as shown by the output of the show interfaces command:
show interfaces
lab@ACX1> show interfaces ge-0/0/5 extensive Physical interface: ge-0/0/5, Enabled, Physical link is Up Interface index: 660, SNMP ifIndex: 550, Generation: 151 Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Link-mode: Full-duplex, Speed: 1000 Mbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, Source filtering: Disabled Ethernet-Switching Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Flow control: Disabled, Auto-negotiation: Enabled, Remote fault: Online, Media type: Copper Device flags : Present Running Interface flags: SNMP-Traps Internal: 0x4000 Link flags : None CoS queues : 8 supported, 8 maximum usable queues Schedulers : 0 Hold-times : Up 0 ms, Down 0 ms Current address: 54:1e:56:d7:17:c8, Hardware address: 54:1e:56:d7:17:c8 Last flapped : 2018-09-04 13:47:06 GMT-3 (1d 17:17 ago) Statistics last cleared: 2018-09-06 07:00:06 GMT-3 (00:04:54 ago) Traffic statistics: Input bytes : 32024 0 bps Output bytes : 608992923 228579728 bps Input packets: 351 0 pps Output packets: 9515387 446445 pps IPv6 transit statistics: Input bytes : 0 Output bytes : 0 Input packets: 0 Output packets: 0 Input errors: Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Bucket drops: 0, Policed discards: 0, L3 incompletes: 0, L2 channel errors: 0, L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0 Output errors: Carrier transitions: 0, Errors: 0, Drops: 0, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0, Bucket drops: 0 Egress queues: 8 supported, 4 in use Queue counters: Queued packets Transmitted packets Dropped packets 0 10860459 10860459 0 1 0 0 0 2 0 0 0 3 274 274 0
Consider that CoS is already configured on the ge-0/0/5 interface:
{master:0}[edit] lab@ACX1# show class-of-service | display set set class-of-service classifiers dscp DSCP forwarding-class BE loss-priority high code-points be set class-of-service classifiers exp EXP forwarding-class BE loss-priority high code-points 000 set class-of-service host-outbound-traffic forwarding-class NC set class-of-service drop-profiles HIGH fill-level 15 drop-probability 0 set class-of-service drop-profiles MEDIUM fill-level 30 drop-probability 0 set class-of-service forwarding-classes class NC queue-num 6 set class-of-service forwarding-classes class BE queue-num 0 set class-of-service system-defaults classifiers exp EXP set class-of-service interfaces ge-0/0/4 classifiers dscp DSCP set class-of-service interfaces ge-0/0/5 scheduler-map S-BE set class-of-service interfaces ge-0/0/5 unit 0 rewrite-rules exp RW-EXP set class-of-service rewrite-rules exp RW-EXP forwarding-class BE loss-priority high code-point 000 set class-of-service rewrite-rules exp RW-EXP forwarding-class NC loss-priority low code-point 110 set class-of-service rewrite-rules exp RW-EXP forwarding-class NC loss-priority high code-point 111 set class-of-service scheduler-maps S-NC forwarding-class NC scheduler NC set class-of-service scheduler-maps S-BE forwarding-class BE scheduler BE set class-of-service schedulers NC transmit-rate percent 5 set class-of-service schedulers NC buffer-size percent 5 set class-of-service schedulers NC priority low set class-of-service schedulers BE transmit-rate remainder set class-of-service schedulers BE buffer-size remainder set class-of-service schedulers BE priority low
When the CoS configuration is deactivated, there are momentary drops on the ge-0/0/5 interface, as shown below:
{master:0}[edit] lab@ACX1# show | compare rollback 1 [edit] ! inactive: class-of-service { ... } {master:0}[edit] lab@ACX1# lab@ACX1# run show interfaces ge-0/0/5 extensive Physical interface: ge-0/0/5, Enabled, Physical link is Up Interface index: 660, SNMP ifIndex: 550, Generation: 151 Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Link-mode: Full-duplex, Speed: 1000 Mbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, Source filtering: Disabled Input errors: Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Bucket drops: 0, Policed discards: 0, L3 incompletes: 0, L2 channel errors: 0, L2 mismatch timeouts: 0, FIFO errors: 0, Resource errors: 0 Output errors: Carrier transitions: 0, Errors: 0, Drops: 6584, Collisions: 0, Aged packets: 0, FIFO errors: 0, HS link CRC errors: 0, MTU errors: 0, Resource errors: 0, Bucket drops: 0 Egress queues: 8 supported, 4 in use Queue counters: Queued packets Transmitted packets Dropped packets 0 84243934 84237350 6584 >>> Dropped packets 1 0 0 0 2 0 0 0 3 162 162 0 Ethernet-Switching Error: None, MAC-REWRITE Error: Non
Activating the CoS configuration after deactivating it on the ge-0/0/5 interface results in momentary packets drops again, as shown below:
{master:0}[edit] lab@ACX1# show | compare rollback 1 [edit] ! active: class-of-service { ... } {master:0}[edit] lab@ACX1> show interfaces ge-0/0/5 extensive Physical interface: ge-0/0/5, Enabled, Physical link is Up Interface index: 655, SNMP ifIndex: 550, Generation: 146 Link-level type: Ethernet, MTU: 1514, LAN-PHY mode, Speed: 1000mbps, BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Disabled, Queue counters: Queued packets Transmitted packets Dropped packets 0 433476817 433454541 22276 >>Dropped packets
No action needs to be taken.
This behavior is expected due to scheduler changes being applied to the interface. There is a temporary traffic drop when a scheduler is added to or deleted from the interface as per the hardware architecture on ACX devices.