• Truncation on Tap and Tool Ports

 
 
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Introduction

EOS supports truncation on ingress and egress. In this article we will focus on how it can be applied in tap aggregation exclusive mode, on the Arista 7150 line of switches. Please refer to the supported features matrix for other hardware platforms.

Truncation is the ability to remove unwanted or unneeded bytes from the packet at a configurable or fixed starting byte position, it may also be referred to as ‘Packet Slicing’. This is useful in situations where the data of interest is contained within the headers or early in the packet payload. It can be used to remove all payload data in situations where this can help in part to ensure compliance for data at rest of captured traffic. Additionally, truncation enables a dramatic reduction in the volume of telemetry data stored where payload data is not required. Truncation on ingress can also be useful to reduce the volume of traffic crossing the aggregator’s fabric thus improving scalability. Tap Aggregation switches are typically deployed in inherently over-subscribed topologies; Diligent use of truncation is one aspect of a strategy to manage this oversubscription.

Care must be taken with ingress truncation since multiple tool ports and tools could be affected. As an example, an application performance tool may not need payload information to provide insights, whereas an IDS/IPS tool may need all packet data to perform its function. The operator must understand the effects and impact on tool applications before using truncation, especially on ingress.

On the Arista 7150 series switches, ingress truncation on tap ports can be applied at 100 bytes or greater, egress truncation can be set to 160 bytes and this is a fixed value. Please refer to the supported features matrix for other hardware platforms.

 

Command Examples

Example one

In the first example we will configure Ethernet 3 to be a tap port via the command line interface. Then we will add truncation at 100 bytes to the tap port (ingress truncation).

s7153#conf t
s7153(config)#int et3
s7153(config-if-Et3)#switchport mode tap
s7153(config-if-Et3)#switchport tap truncation 100

s7153(config-if-Et3)#show run int et3
interface Ethernet3
   switchport mode tap
   switchport tap truncation 100
s7153(config-if-Et3)#sh int et1-3 tap
Port      Configured  Status      Port     Allowed             Native Truncation
          Mode                    Identity Vlans               Vlan   Size
--------------------------------------------------------------------------------
Et1       tool        notconnect  0        All                 1      0
Et2       tool        notconnect  0        All                 1      0
Et3       tap         notconnect  0        All                 1      100

 

Example 2

In the second example we will configure Ethernet 4 to be a tool port via the command line interface. Then we will add truncation at the fixed limit of 160 bytes to the tool port (egress truncation).

s7153(config-if-Et4)#switchport mode tool
s7153(config-if-Et4)#switchport tool truncation ?
    egress packet truncation size in bytes

s7153(config-if-Et4)#switchport tool truncation 160
s7153(config-if-Et4)#show run int et4
interface Ethernet4
   switchport mode tool
   switchport tool truncation 160

Using “show interfaces tool detail we can see the truncation applied to Ethernet 4

s7153(config-if-Et4)#sh int tool detail
Port      Configured  Status      Id       Allowed             Timestamp
          Mode                    Tag      Vlans               Mode
--------------------------------------------------------------------------------
Et4       tool        tool        Off      All                 None
Port Truncation Size
--------------------------------------------------------------------------------
Et4 160

 

GUI examples

Example 1

In the first example we will configure Ethernet 17 to be a tap port via the graphical user interface. Then we will add truncation at 100 bytes to the tap port (ingress truncation).

  • Select the tap port to configure for truncation

tap agg gui truncation.001

  • Expand the Options menu

tap agg gui truncation.002

  • Check the selector box to enable truncation and enter a valid size greater than or equal to 100 bytes

tap agg gui truncation.003

  • Click “Set” once we have entered the proper value for our environment

tap agg gui truncation.004

Example 2

In the second example we will configure Ethernet 4 to be a tool port via the command line interface. Then we will add truncation at the fixed limit of 160 bytes to the tool port (egress truncation).

  • Select the Tool port to configure for truncation

tap agg gui truncation.005

  • Expand the Options menu

tap agg gui truncation.006

  • Toggle the truncation slider, Note: Egress truncation is fixed at 160 bytes

tap agg gui truncation.007

  • Don’t forget to save the configuration to make our changes permanent

tap agg gui truncation.008

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