Patches and Taps

Patches and taps expose the ultra low latency layer 1 functionality of the Cisco Nexus 3550-H Hydra (formerly ExaLINK Hydra). Interfaces that are members of patch or tap objects have a low and deterministic interface to interface latency of under 3-7ns with virtually no jitter.

Patches are useful for creating bidirectional connections between two devices connected to the Nexus 3550-H. They can be used as an alternative to directly connecting two devices together, in cases where remote reconfiguration and patching is desired.

Taps are analogous to optical taps but with several advantages. Optical taps suffer from reduced output power and signal integrity issues as more tap outputs are introduced. Since the Nexus 3550-H incorporates advanced clock and data recovery circuitry on all inputs along with electronic drivers on all outputs, it is possible to tap one input to a large number of outputs. For example, a single input can be replicated up to 159 times with no loss of signal integrity and with no latency penalty.

Patching

Two interfaces can be connected together at the physical layer by creating a patch object. To create a patch object, first enter config mode, then use the patch command as shown:

admin@EXALINK-HYDRA(config)# patch ethernet1/1 ethernet1/2
Patch created between interface "ethernet1/1" and interface "ethernet1/2"

A layer 1, physical connection has now been created between interface ethernet1/1 and ethernet1/2. The port to port latency in this configuration will be under 5ns.

To remove an existing patch, use the no form of the patch command. For example, to remove the patch we created above:

EXALINK-HYDRA(config)# no patch ethernet1/1 ethernet1/2
Patch deleted between interface "ethernet1/1" and interface "ethernet1/2"

Note: that tab completion can be used on the interface numbers. This is especially useful when specifying the second interface in a patch to be removed, as the command line interface will resolve the second interface automatically.

Patches can be created between multiple interfaces in a single command. The user must specify an equal number of interfaces in order to do so. For example, to patch each interface on ethernet1 to an interface on ethernet2:

EXALINK-HYDRA(config)# patch ethernet1/* ethernet2/*
Patch created between interface "ethernet1/1" and interface "ethernet2/1"
Patch created between interface "ethernet1/2" and interface "ethernet2/2"
Patch created between interface "ethernet1/3" and interface "ethernet2/3"
Patch created between interface "ethernet1/4" and interface "ethernet2/4"
Patch created between interface "ethernet1/5" and interface "ethernet2/5"
Patch created between interface "ethernet1/6" and interface "ethernet2/6"
Patch created between interface "ethernet1/7" and interface "ethernet2/7"
Patch created between interface "ethernet1/8" and interface "ethernet2/8"

These patches can be removed with the equivalent no command:

EXALINK-HYDRA(config)# no patch ethernet1/* ethernet2/*
Patch deleted between interface "ethernet1/1" and interface "ethernet2/1"
Patch deleted between interface "ethernet1/2" and interface "ethernet2/2"
Patch deleted between interface "ethernet1/3" and interface "ethernet2/3"
Patch deleted between interface "ethernet1/4" and interface "ethernet2/4"
Patch deleted between interface "ethernet1/5" and interface "ethernet2/5"
Patch deleted between interface "ethernet1/6" and interface "ethernet2/6"
Patch deleted between interface "ethernet1/7" and interface "ethernet2/7"
Patch deleted between interface "ethernet1/8" and interface "ethernet2/8"

When specifying multiple patches at a time, the command will be interpreted to patch between the lowest port in both ranges first and work upwards from there. For example, creating 4 patches from ethernet1/4 to ethernet2/5-8 would allocate the patches in the following way:

EXALINK-HYDRA(config)# patch ethernet1/1-4 ethernet2/5-8
Patch created between interface "ethernet1/1" and interface "ethernet2/5"
Patch created between interface "ethernet1/2" and interface "ethernet2/6"
Patch created between interface "ethernet1/3" and interface "ethernet2/7"
Patch created between interface "ethernet1/4" and interface "ethernet2/8"

Setting up a tap

A tap object is analogous to an optical tap. Within the Nexus 3550-H, tapping is performed electronically, which has a number of benefits when compared with optical taps whilst providing the same low and deterministic latency.

By default, tap objects replicate data received on a source port to an output port. This default is useful for a number of situations, including replication of market data to multiple hosts. To tap one port to another port, enter config mode and use the tap command to create a tap object:

EXALINK-HYDRA(config)# tap ethernet1/1 ethernet1/2
Added input tap on interface "ethernet1/1", send to interface "ethernet1/2"

An input can be tapped to any number of outputs. To create multiple taps with one command, range specifiers may be use. As an example, to replicate any traffic received on interface ethernet1/1 to ethernet2/1, ethernet2/2, ethernet2/3, ethernet2/4 use:

EXALINK-HYDRA(config)# tap ethernet1/1 ethernet2/1-4
Added input tap on interface "ethernet1/1", send to interface "ethernet2/1"
Added input tap on interface "ethernet1/1", send to interface "ethernet2/2"
Added input tap on interface "ethernet1/1", send to interface "ethernet2/3"
Added input tap on interface "ethernet1/1", send to interface "ethernet2/4"

See Configuring multiple interfaces for more information on selecting interface ranges.

Sometimes it is necessary to tap the data that is transmitted out of a given interface, as opposed to data received on that interface. One potential application is to replicate data sent to an exchange out of a logging interface. To configure an output tap, simply add the output modifier to the tap command:

EXALINK-HYDRA(config)# tap output ethernet1/1 ethernet1/2
Added output tap on interface "ethernet1/1", send to interface "ethernet1/2"

To remove any tap use the no form of the tap command:

EXALINK-HYDRA(config)# no tap ethernet1/1 ethernet1/2
Removed input tap on interface "ethernet1/1", send to interface "ethernet1/2"

Similarly, for an output tap:

EXALINK-HYDRA(config)# no tap output ethernet1/1 ethernet1/2
Removed output tap on interface "ethernet1/1", send to interface "ethernet1/2"

This page was last updated on Feb-18-2021.