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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)#