Loopback addresses on a VoQ Chassis

[Sandeep Malhotra]

Hello,

In a multi-asic pizza box config (generated from the sonic-mgmt gen-mg) I see that all the frontend asics are using the same loopback Ipv4 and Ipv6 address (Loopback0) and different internal loopback address (Loopback4096) for the iBGP connections between the frontend and the backend asics.

For VoQ chassis, all asics in the linecards are front-end asics, and we have a full-mesh iBGP connectivity between the asics.

Currently, in our proposed T2 topology for sonic-mgmt tests, we are assigning a different loopback address to each asic - is that OK?

Or should:

• all the asics in a linecard have the same loopback address Loopback0 address, but each linecard could have a different loopback address.
• all asics in all the linecards have the same loopback address.
• use Loopback4096 like mechanism for the iBGP connections.

Thanks.
Sandeep

[Eswaran Baskaran]

Hi Sandeep,

Assigning a different loopback address to each asic makes sense to me. If we don't do that, we will need another mechanism like the Loopback4096 scheme to form the internal mesh.

I faintly remember there was some reason to assign the same Loopback0 address in the multi-asic pizza-box case though. I explicitly copied some multi-ASIC experts for help here. 

Thanks,

Eswaran

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[Rita Hui]

++subgroup.

 

From: Rita Hui
Sent: Friday, May 21, 2021 5:20 PM
To: eswaran <esw...@arista.com>; Malhotra, Sandeep (Nokia - US/Westford) <sandeep....@nokia.com>; Arvindsrinivasan Lakshmi Narasimhan <Arvindsriniv...@microsoft.com>; Abhishek Dosi <Abhish...@microsoft.com>; Judy Joseph <Judy....@microsoft.com>
Subject: RE: [EXTERNAL] Re: Loopback addresses on a VoQ Chassis

 

Hi,

 

Loopback0 address uniquely identifies the router globally. The multi-asic pizza box is viewed as a single BGP router externally.  But each asic inside the box also has a locally unique loopback (loopback4096) to identify it.

For chassis, to begin with, we could mimic each line card as a multi-asic pizza box.  That is, asics in one line card use same loopback0, while unique loopback0 address is assigned for each line card in chassis.

 

It is up to users how many unique loopback addresses can be allocated (as this can occupy IPv4 address space which is more scarce).  And for testing, it’s fine and users can change based on their needs later.

 

Thanks,

Rita  

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[Israel Meilik]

Hi Rita, Eswaran, Sandeep,

I wanted to make sure we have a clear distinction between:
() Recycle Interface - the physical interface that connects the Egress pipe to the Ingress pipe.
    Each Core on each device will have such an interface
() Recycle System Ports (out system ports) connected to the Recycle Interface.
   Typically there are few Recycle ports connected to the same Recycle Interface, this enables the user to distinguish between different use-cases and traffic that needs to use the Recycle Interface.

() Loopback address - address that typically "hand" the processing to different entity, it may be as simple as tunnel terminated at this node, or control plane traffic that is destined on the system's "host"

As noted some of the loopback addresses are global to the Router, while other loopback addresses have local significance.

I don't fully understand the exact issue, but if you can define (or point to) the issue, will be happy to give my view/advice.

Thanks,

Israel

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