Posted on August 7, 2020 7:54 pm
 |  Asked by jaxk Panther
 |  93 views
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Hi Guys,

According to TCAM table available entries are running out on my switch soon. I’ve been struggling to change a TCAM template on a box 7050 for few days and try to have a look any resource of Arista ASIC architecture but they all were not matter to change a template of ASIC.   And look like Arista allows only FIB and RIB are able to be customized in this case when I tried. Take a look captures below.

Does anyone know how to deal with this one?  My purpose is to change allocation of TCAM template not FIB or RIB.

Thank you in advance!!

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Posted by Alexis Dacquay
Answered on August 11, 2020 11:25 am

Hi,

 

Well, the FIB is related to the TCAM.

Talking about "hardware tables" in general can be a long discussion, the FIB is a part of the TCAM or hardware tables. Some platforms have got dedicated tables for specific roles, to optimise the space, cost and scale, but in the case of the Trident-family of chips it is a universal hardware table; TCAM.

Sorry the term "forwarding-table" might bring confusion, but it is indeed the TCAM, the data-plane, the hardware table that also involves the ACLs, QoS, PBR, etc

 

What exact model is that? 7050s or 7050SX?

The 7050X-series uses a modular TCAM as a Unified Forwarding Table (UFT).

More details here: https://www.arista.com/assets/data/pdf/Whitepapers/Arista_7050X_Switch_Architecture.pdf

 

7050X#show platform trident forwarding-table partition
L2 Table Size: 160k
L3 Host Table Size: 144k
LPM Table Size: 16k, uRPF is disabled

7050X#conf
7050X(config)#platform trident forwarding-table partition ?
0 288k l2 entries, 16k l3 host, 16k lpm entries
1 224k l2 entries, 80k l3 host, 16k lpm entries
2 160k l2 entries, 144k l3 host, 16k lpm entries
3 96k l2 entries, 208k l3 host, 16k lpm entries
4 32k l2 entries, 16k l3 host, 128k lpm entries

7050X(config)#

 

Does the answer the question and clarifies the terminology?

 

Best regards,

Alexis

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Posted by jaxk Panther
Answered on August 12, 2020 6:20 am

Hi Alex
Yep, I knew FIB is a part of TCAM but isn't a table that typically optimized table of TCAM for fast forwarding packet?(combine routing table and ARP table)
My understand is that even I change FIB template but It won't change TCAM allocation size right?

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Posted by Alexis Dacquay
Answered on August 12, 2020 9:28 am

Good questions.

 

Nowadays most of the modern network processors or network ASICs are doing the forwarding in hardware, so "fast forwarding" is always happening in the data plane: the CPU/software is never involved in any transit forwarding decisions. So it's always "fast".

In the past, some of the tables were held in software by legacy switches, and only installing in hardware a portion of the tables. A lot of forwarding was still done one software.

Nowadays there is none, everything is in hardware. Todays' hardware line rate capacity at L2/L3 is 12.8Tbps per chip; at time of writing.

On the Trident-family (X-series switches), There is only one forwarding table, only one TCAM, one hardware table split among different features, so effectively TCAM ~= FIB ~= hardware table.

The command you mentioned DOES change the hardware table, DOES change the TCAM, DOES change the FIB.

The TCAM allocation WILL be changed by this command, reallocating the hardware resources to the different features

  • L2 MAC address
  • L3 host (exact match ARP entries)
  • L3 LPM (longest prefix match: hardware routing ; FIB ; subnet + subnet-length; routing lookup in hardware)

The allocation differences are listed in the command output you provided.

 

I hope this clarifies.

 

Regards,

Alexis

 

 

 

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Posted by jaxk Panther
Answered on August 13, 2020 1:32 am

Excellent job!! Appreciated your time,
Anyway. Is there the way to change iCAP size? I have a bunch of ACL to apply into switch but looks like TCAM was not enough. I think I have to deal with iCAP right? If take a look in this doc https://www.arista.com/assets/data/pdf/Whitepapers/Arista_7050X_Switch_Architecture.pdf

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