Symptom

"Output errors" incrementing in the "show interface " command output when Lightspeed (4th Generation) or Lightspeed Plus (5th Generation) Line Cards are hosting interfaces that are members of the same L2 Bridge Domain, and these interfaces have an MTU mismatch. This results in traffic loss for bigger frames exceeding the MTU value configured in the egress interface. Example: We are sending 1,000 pings from Host 1 to Host 2 with a size of 9000 bytes and donotfrag option set. Topology: (192.0.2.1) Host 1 --- (BE25.276 higher MTU) [ASR9000] (BE30.276 lower MTU) --- Host 2 (192.0.2.2) ``` RP/0/RP0/CPU0:Host-1#ping 192.0.2.2 timeout 0 count 1000 donotfrag size 9000 Sending 1000, 9000-byte ICMP Echos to 192.0.2.2 timeout is 0 seconds: ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... ...................................................................... .................... Success rate is 0 percent (0/1000) ``` The output errors will be seen in the egress interface that is programmed with a lower MTU value ``` RP/0/RSP0/CPU0:ASR-9000#show int Bundle-Ether30.276 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 1000 output errors, 0 underruns, 0 applique, 0 resets <<< 1000 output errors ```

Conditions

1. Lightspeed (4th Generation) or Lightspeed Plus (5th Generation) Line Cards hosting interfaces that are members of the same L2 Bridge Domain. The "show platform" command can be used to identify the Part ID of the Line Cards installed in the Cisco ASR 9000 router, this information is under the column called "Type" Example: Node Type State Config state -------------------------------------------------------------------------------- 0/0/CPU0 A9K-4HG-FLEX-TR IOS XR RUN NSHUT 0/1/CPU0 A9K-4HG-FLEX-TR IOS XR RUN NSHUT In the example above, both Line Cards are based on the Lightspeed Plus architecture (5th Generation). Refer to the link below for a complete list of the different Line Cards grouped by Generation for the Cisco ASR 9000 Series Router: https://www.cisco.com/c/en/us/support/docs/routers/asr-9000-series-aggregation-services-routers/116726-qanda-product-00.html 2. A Bridge Domain configured with two or more member interfaces that have different MTU values. Example: `` l2vpn bridge group 276 bridge-domain Vlan276 interface Bundle-Ether25.276 ! interface Bundle-Ether30.276 ! ! ! ! interface Bundle-Ether25.276 l2transport encapsulation dot1q 276 mtu 9188 ---> Configured MTU = 9188 bytes + 4 bytes for 802.1Q (VLAN Header) = 9,192 bytes ! interface Bundle-Ether30.276 l2transport encapsulation dot1q 276 ! ---> Missing mtu configuration results in the default MTU = 1514 bytes + 4 bytes for 802.1Q (VLAN Header) = 1,518 bytes ``` You can check the programmed MTU using the command "show l2vpn bridge-domain group private" and scrolling down to the section showing the Attachment Circuits we are interested in. Example: ``` show l2vpn bridge-domain group 276 private AC: Bundle-Ether25.276, state is up Type VLAN; Num Ranges: 1 Rewrite Tags: [] VLAN ranges: [276, 276] >>> MTU 9178; XC ID 0xc0000006; interworking none; MSTi 0 (unprotected), I-MSTi 0 AC: Bundle-Ether30.276, state is up Type VLAN; Num Ranges: 1 Rewrite Tags: [] VLAN ranges: [276, 276] >>> MTU 1504; XC ID 0xc0000008; interworking none; MSTi 0 (unprotected), I-MSTi 0 ```

Workaround

To avoid this issue, ensure that all the Attachment Circuit interfaces inside the same Bridge Domain have the same MTU configured.

Further Problem Description