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We have a MPLS link connected to our 3850 with voice traffic, our interface have 1Gbps and the link is limited to 200Mbps. Our interface shows output drops, and the client device shows overruns.

What would be generating these drops and what can I do to avoid it?

3850#show run int g1/0/1
Building configuration...

Current configuration : 146 bytes
!
interface GigabitEthernet1/0/1
 description ENLACE MPLS
 switchport access vlan X
 switchport mode access
 speed 1000
 duplex full

3850#show int g1/0/1
GigabitEthernet1/0/1 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is f87b.208f.4d01 (bia f87b.208f.4d01)
  Description: ENLACE MPLS
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 11/255, rxload 10/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:00, output hang never
  Last clearing of "show interface" counters 20:57:38
  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 62359
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 42056000 bits/sec, 23433 packets/sec
  5 minute output rate 45959000 bits/sec, 24126 packets/sec
     860048591 packets input, 189934625980 bytes, 0 no buffer
     Received 842518 broadcasts (1521 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 1521 multicast, 0 pause input
     0 input packets with dribble condition detected
     881783349 packets output, 209372802584 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

3850#show int vlan X
VlanX is up, line protocol is up
  Hardware is Ethernet SVI, address is f87b.208f.4d45 (bia f87b.208f.4d45)
  Description: GW-vlanX
  Internet address is 192.168.160.1/24
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 33/255, rxload 42/255
  Encapsulation ARPA, loopback not set
  Keepalive not supported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:01, output hang never
  Last clearing of "show interface" counters never
  Input queue: 5/375/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 168164000 bits/sec, 91584 packets/sec
  5 minute output rate 130720000 bits/sec, 70778 packets/sec
     313589895385 packets input, 81224026060488 bytes, 32 no buffer
     Received 0 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     245854847644 packets output, 60925586879564 bytes, 0 underruns
     0 output errors, 2 interface resets
     0 unknown protocol drops
     0 output buffer failures, 0 output buffers swapped out
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  • 1
    62k drops for 881 million packets is 0.0007%, quite literally negligible. You may want to run clear counters gig1/0/1 and check back after a typical day of usage. Plus, your config shows no sign of MPLS, none of these interfaces seems MPLS enabled (your WAN service provider probably has an MPLS enabled device). I suggest you remove the MPLS tag from your question. – Marc 'netztier' Luethi Aug 7 '18 at 20:43
  • There's little one can say about why these drops occur withouth knowing what else is going on in VLAN X and in the other VLANs (inter-VLAN routing going on?) of this switch. There might be a 10G connected server somewhere in that campus, or a few 1G-connected PCs, simply trying to talk TCP to another system somewhere beyond the WAN, and trying to maximize througput, as TCP does. That alone can be enough to (shortly) fill gig1/0/1's egress buffer and cause drops. Can you please add some more information, and possibly more config of the switch (incl ports, VLANs, SVIs, routing protocols etc). – Marc 'netztier' Luethi Aug 7 '18 at 20:46
  • 3
    Do you have a shaping policy configured on the VLAN interface? You should if you're interface BW is limited to 200mbps. – Robert Aug 7 '18 at 21:11
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin♦ Dec 25 '18 at 9:12
2

Output drops are normal if you have more traffic trying to leave the interface than the interface can handle. For example, if your switch has 48 interfaces, and each is trying to send 1 Gbps of traffic through a 1 Gbps uplink, then you will drop nearly 98% of the traffic.

In the real world, each interface will have a much lower usage over a period of time, but traffic on multiple interfaces can peak at the same time, and the uplink interface can experience output drops. The only way to fix that is to have more bandwidth on the uplink interface.

Output drops are caused by a congested interface. For example, the traffic rate on the outgoing interface cannot accept all packets that should be sent out. The ultimate solution to resolve the problem is to increase the line speed.

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