Re: [Isis-wg] draft-ginsberg-isis-l2bundles

[<[e-mail]>]

Hi Acee,

Some comments inline

-----Original Message-----
From: Acee Lindem (acee) [mailto:[e-mail]] 
Sent: Wednesday, August 05, 2015 19:24
To: LITKOWSKI Stephane SCE/IBNF; Ebben Aries; [e-mail] list ([e-mail])
Subject: Re: [Isis-wg] draft-ginsberg-isis-l2bundles

Hi Stephane,
I think the IS-IS advertisement is merely a consequence of the fact that we are satisfying the requirement of incorporating these L2 links in the segment routing path.
[SLI] Yes, and IMO, that's bad.
- I still have some doubt on the reason to split LAGs for TE and keeping bundles for other protocols. 
- Regarding TE, I don't really see how BW use cases can work with this, as there may be some TE tunnels using the bundle and some using specific link, so evaluating the remaining BW per link and for the bundle is hard.
- This "breaks" layers, IGP exposes Layer 3 topology by design, not layer 2 ... if we want to expose layer 2, that's not an issue, it's a kind of multilayer TE approach and BGP-LS may so come in the picture and is a good candidate to retrieve topological information. I do not want to see IS-IS or OSPF becoming a topology discovery protocol for everything : while it's related to the Layer 3 topology it's fine to me to keep it in the IGP for other informations, may be we need to find another way. 


 If we limit advertisement to BGP-LS, it will have the following impact:

     1. All routers in the IS-IS domain that use link-bundles will need some form of BGP LS peering, either to the controller directly or through some intermediary.
[SLI] Agree but I don't see this as a negative point, as I think most networks running TE, already have a BGP controlplane that can be reused.

     2. Since the link-bundle itself is an IS-IS L3 link, one would need to correlate the information with the corresponding IS-IS link state information (assuming not every IS-IS router advertises the entire LSDB).
[SLI] Agree there is a need of correlation, but correlation is required in all cases (in the current proposal, we advertise some parent link information).

Additionally, any time the information is coming from multiple sources, you are likely to trigger path computation more frequently.
[SLI] I would say that's implementation dependent.


I don’t think this added complexity warrants omitting them from the IGPs if we do, in fact, accept link bundle adjacency steering as a requirement.

Thanks,
Acee 


On 8/5/15, 4:12 AM, "Isis-wg on behalf of [e-mail]"
<[e-mail] on behalf of [e-mail]>
wrote:

>Hi,
>
>Pls find some inline comments.
>
>-----Original Message-----
>From: Ebben Aries [mailto:[e-mail]]
>Sent: Wednesday, August 05, 2015 01:39
>To: LITKOWSKI Stephane SCE/IBNF; [e-mail] list 
>([e-mail])
>Subject: Re: [Isis-wg] draft-ginsberg-isis-l2bundles
>
>I see BGP-LS extensions complementing this, not necessarily as a 
>replacement.
>[SLI] It's for sure an option, but my point is do we need to continue 
>to add extensions to both IGP and BGP LS ?
>Moreover I still have an issue with propagating L2 informations into 
>layer 3 routing protocol  (not technically ... more from a design 
>perspective).
>Let's say that tomorrow, you would like to advertise some L1 
>information under your layer 2 information ... ?? As we are breaking 
>layers, if you want to advertise some underlay topology, I would be in 
>favor to not doing it in IGP.
>
>For a use-case of a central entity learning these underlying l2 
>attributes to then do whatever you wish (impose label stacks, etc..) - 
>BGP-LS is a natural fit.
>[SLI] Nothing prevents to use BGP-LS in a distributed computation model.
>
>For this to remain in the IGP, a consideration could be the propagation 
>of these L2 attributes to then be included in TEDs for additional logic 
>from headend nodes (network elements within the IGP domain) - e.g.
>control packet per member from a remote endpoint overriding remote 
>hashing either by some policy/SLA or dynamic based off of per member 
>utilization, etc..
>
>[SLI] Even if TED was previously populated only by IGP (because there 
>was nothing else), this is not the case anymore. TED is also populated 
>by BGP-LS and we may be able to create also new processes to populate 
>the TED. So you can imagine having your process managing LAGs to add 
>those L2 TE information into the TED and then being able to export it 
>through BGP-LS to other nodes through the BGP controlplane, so every 
>one will have the same content in the TED.
>
>
>On 08/03/2015 07:02 AM, [e-mail] wrote:
>> Hi,
>> 
>>  
>> 
>> Thinking again about this draft, I wondering why not using BGP-LS for 
>> that purpose ?
>> 
>> I mean, the goal here is just to provide some topological information 
>> that are not related to IGP, as you want to keep L2 bundles and so a 
>> single IP link. If you want to expose the underlaying topology, you 
>> may be able to do it in BGP-LS rather than adding this in the IGP as 
>> the information you want to expose is not necessary for the IGP to run.
>> 
>>  
>> 
>> Thx
>> 
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