This article explains why a router is displaying incorrect bit per second (bps) statistics on the Layer 2 and Layer interfaces.

In some test or interoperability scenarios, the statistics of the L2 (Layer 2) and L3 (Layer 3) ingress interfaces display a lower bps (bit per second) rate than the outer device egress bps rate. This applies only to the cable for bps and not for pps (packets per second).

For example, a traffic generator is configured to send the fixed size and rate packets to a Junos interface as follows:

[edit]
root@MX480# show interfaces xe-3/0/0
enable;
vlan-tagging;
encapsulation flexible-ethernet-services;
unit 50 {
    vlan-id 50;
    family inet {
        address 11.22.33.44/24;
    }
}
unit 100 {
    encapsulation vlan-bridge;
    vlan-id 100;
}

Example 1: Sending 86-byte frames (same size and rate) to the L3 Interface (with VLAN 50)

[edit]
root@MX480# run clear interfaces statistics all
[edit]
root@MX480# run show interfaces statistics xe-3/0/0.50 detail
Logical interface xe-3/0/0.50 (Index 74) (SNMP ifIndex 570) (Generation 163)
Flags: SNMP-Traps 0x4000 VLAN-Tag [ 0x8100.50 ]  Encapsulation: ENET2
Traffic statistics:
Input  bytes  :                67200
Output bytes  :                    0
Input  packets:                 1050
Output packets:                    0
Local statistics:
Input  bytes  :                    0
Output bytes  :                    0
Input  packets:                    0
Output packets:                    0
Transit statistics:
Input  bytes  :                67200                51200 bps   <- bps 74% of what is reported by the traffic generator
Output bytes  :                    0                    0 bps
Input  packets:                 1050                  100 pps
Output packets:                    0                    0 pps
Protocol inet, MTU: 1500, Generation: 181, Route table: 0
Addresses, Flags: Is-Preferred Is-Primary
Destination: 11.22.33/24, Local: 11.22.33.44, Broadcast: 11.22.33.255, Generation: 154
Protocol multiservice, MTU: Unlimited, Generation: 182, Route table: 0
Policer: Input: __default_arp_policer__

Example 2: Sending 86-byte frames packets (same size & rate) to L2 Interface (with VLAN 100)

root@MX480# run clear interfaces statistics all

[edit]
root@MX480# run show interfaces statistics xe-3/0/0.100 detail
Logical interface xe-3/0/0.100 (Index 75) (SNMP ifIndex 151) (Generation 165)
Flags: SNMP-Traps 0x20004000 VLAN-Tag [ 0x8100.100 ]  Encapsulation: VLAN-Bridge
Traffic statistics:
Input  bytes  :               199588
Output bytes  :                    0
Input  packets:                 2434
Output packets:                    0
Local statistics:
Input  bytes  :                    0
Output bytes  :                    0
Input  packets:                    0
Output packets:                    0
Transit statistics:
Input  bytes  :               199588                65400 bps   <- bps 95% of what is reported by the traffic generator
Output bytes  :                    0                    0 bps
Input  packets:                 2434                   99 pps
Output packets:                    0                    0 pps
Protocol bridge, MTU: 1518, Generation: 184, Route table: 3
Flags: Is-Primary

A router is displaying incorrect bit per second (bps) statistics on the Layer 2 and Layer interfaces.

The bit rate and interface statistics of some other vendor's device include the cyclic redundancy check (CRC). On a Junos L2 interface, the byte count and bit rate are presented by using the packet size, after stripping the CRC; the L3 interface displays only the L3 contents as part of its statistics.

For example, in a traffic test scenario with a traffic generator that sends 86-byte frames with Ethernet Header (14 bytes), VLAN (4 bytes), IP (64 bytes), and CRC (4 bytes), the traffic generator provides the byte count and bit rate for the entire L2 frame, whereas the ingress L2 interfaces in Junos display the counters after discarding CRC.

xe-3/0/0 configured as L3

L3 Content = 64/86 = 74.4%
86 bytes - Ethernet Header 14 bytes - VLAN 4 bytes - CRC 4 bytes = 64 bytes (IP)

Bit Rate Byte Count
Traffic Generator: 68800 (8600x8) 665468
xe-3/0/0.0 51352 (74.6%) 495232 (74.4%)


xe-3/0/0 configured as L2

Payload excluding CRC = 82/86 = 95.3%
86 bytes - CRC 4 bytes = 82 bytes

Bit Rate Byte Count
Traffic Generator: 68800 (8600x8) 1,263,598
xe-3/0/0.1 65800 (95.6%) 1,204,826 (95.3%)