Many implementations (including optional modes on both the Arista 7150 and 7280 series) take an existing field within a frame header or trailer and replace that value with the hardware timestamp.

Another implementation to apply the timestamp is to expand the original packet with a set of custom fields. When such an implementation is used the FCS field must be updated to ensure the packet that leaves the device is a valid Ethernet frame that can be forwarded normally by subsequent transit devices.

These L1+ devices support sub-nanosecond timestamp precision via the MetaWatch application and are typically implemented as an inline active tap device performing tapping/replication of the production links, or as physically close to the passive tap performing the replication. The active tap function replicates traffic through the system in 5 ns while simultaneously replicating the packets to the MetaWatch application.

Timestamping is supported on traffic replicated via a mirror session when the device is used as a production switch, or on ingress traffic when deployed in Tap Aggregation mode.

The 31 bit 7150 timestamp header can be used in both self-contained and overwrite modes. Specifically, the timestamp can be inserted before the FCS in the ethernet trailer or can be configured to overwrite the FCS.

mac timestamp replace-fcs

switch(config)#interface ethernet 44
switch(config-if-Et44)#mac timestamp before-fcs
switch(config-if-Et44)#show active
interface Ethernet44
   mac timestamp before-fcs
switch(config-if-Et44)#

switch(config-if-Et44)#no mac timestamp
switch(config-if-Et44)#show active
interface Ethernet44
switch(config-if-Et44)#

• An Overview of Arista Ethernet Capture Timestamps