Interface Output Drops on Cisco ASR-920-24SZ-M

[stefanschourek]

Hello together,

i would like to ask a Question if anybody could explain me an abnormally behavior we have found on our ASRs.

We have constantly growing interface output drops on our ASR Ports, but we have no problems and even the traffic is that small, that we can't have any congestion in our network.

Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 355825 ( its counting since 20 Hours ).

Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 7000 bits/sec, 9 packets/sec

The ASRs is uplinked with an SFP+ Port with 10Gbit to a Nexus 3548 and one SFP Port is connected to another  Switch ( Microsense ).

The Problems occur on the SFP Port ( 1Gbit ) and the counter is growing and growing ...

On the SFP Port is a service instance configured and 3 VLANs. Nearly the same is on the Uplink Port ( SFP+, some More VLANs ) configured.

Both interfaces are in the same Bridge Domain. We are currently using the ASR only as a Layer 2 Device to take and push some VLANs along. 

interface GigabitEthernet0/0/19
no ip address
service instance 1 ethernet
encapsulation dot1q 10-20, 200
l2protocol forward
bridge-domain 100

##########################

interface TenGigabitEthernet0/0/26
no ip address
service instance 1 ethernet
encapsulation dot1q 1-4094
l2protocol forward
bridge-domain 100

We have absolutely no Problems in the Network and all is working well, but why are the Output Drop counters on the Gigabit Interface growing and growing ? Does anybody have a hint for me ? Thank you in advance.

Kind Regards

Stefan Schourek

[pfuhrmann82]

Without having much expierence with ASR Routers but with switches, I can give you the following hints:

First of all, you have 12 VLANs configured on the 1 gi-link. (10 to 20 and 200).

Then it seems naturally that if you bring traffic from a 10 gi interface to a 1 gi interface, that there may be packet drops.

The Router takes the traffic arriving on Te0/0/26 with 10gbps and "switches" it to Gi0/0/19 which can handle only 1gbps.

The reason you don't have any problems at all is probably related to TCP. If the packets are not arriving accordingly on the receiver, the TCP windows will be adjusted and a retransmission begins (to talk simplified).

[stefanschourek]

Hi Pfuhrmann82,

thanks for the reply. Well, the VLANs are just examples ... actually there are only 3 VLANs running on the 1Gbit interface and normally i would say ... you are right ... but ... let me give you the bandwidth from both interfaces ...

Thats the 10Gbit Port from the Nexus witch is linked to the ASR:

Load-Interval #1: 30 seconds
30 seconds input rate 608 bits/sec, 0 packets/sec
30 seconds output rate 27648 bits/sec, 8 packets/sec

Thats the 10Gbit Port from the ASR witch is linked to the Nexus:

Output queue: 0/40 (size/max)
5 minute input rate 21000 bits/sec, 15 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec

Thats the 1Gbit Port on the ASR:

Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 395214
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 6000 bits/sec, 10 packets/sec

and the Outputs are growing and growing ... around 6 till 10 drops every 2 seconds.

Thank you in advance

Stefan

[Mark Malone]

Hi could you post the full show interface from the gig please

what type of application traffic is this crossing the interface , do you know? some apps can spike traffic cause burst flood the buffer and case output drops because the interface cant hold it anywhere

it doesn't have to be congested to happen it can be due to the type of traffic flow itself , you can capture it in wiresharks or other apps , you can graph it as well from the router but its difficult to capture it that way

http://www.cisco.com/c/en/us/support/docs/lan-switching/switched-port-analyzer-span/116260-technote-wireshark-00.html

https://notalwaysthenetwork.com/2014/01/06/microburst-detection-with-wireshark/

[stefanschourek]

Hi Mark,

thanks for the reply. Sure i can do:

Hardware is 24xGE-4x10GE-FIXED-S, address is 
MTU 9216 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full Duplex, 1000Mbps, link type is force-up, media type is BX10D
output flow-control is unsupported, input flow-control is on
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output 00:00:38, output hang never
Last clearing of "show interface" counters 19:20:47
Input queue: 0/375/0/0 (size/max/drops/flushes); Total output drops: 396334
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 3000 bits/sec, 6 packets/sec
102845 packets input, 8460784 bytes, 0 no buffer
Received 56 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 7055 multicast, 0 pause input
364257 packets output, 48917854 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

( We raised the MTU for testing, but it doesn't change anything )

The Constellation looks like that:

Our Network Infrastructure --> ( Nexus ) 10Gbit -> 10Gbit ( ASR ) 1Gbit -> 1Gbit ( Microsense Switch ) 1Gbit Poe -> 1Gbit Poe WLAN Access Point.

The connection from the Nexus till the Microns Switch is Fibre.

The Poe connection is copper.

Actually there is no real traffic and there is no fixed application running ( some http requests if we like to ) ... I haven't tested yet, if we disable the SSID beam from the AP and just watch what is happening on the interfaces.

The Traffic is just separated due to VLANs in Mgmt and Data Traffic to Manage the Switch and the Accesspoint. The Accesspoint  beams an SSID for WLAN to get Internet Access. All is working fine and very well, but only on the ASR on the 1Gbit Interface the Output Drop counter grows on and we don't have any Problems.

By The way this is an replica due to the live system. Same behavior on Both systems.

All Working fine but counter grows on and on ... 

Thanks again for the answers

Regards

Stefan