Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)

Susan Hares <> Mon, 09 November 2020 09:37 UTC

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From: Susan Hares <>
To: "'Pengshuping (Peng Shuping)'" <>, "'Stephane Litkowski (slitkows)'" <>,
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Subject: Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to 11/16/2020)
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A few questions about the scope of the Link MTU that


<individual contributors hat on> 


I have few questions application of the Link MTU to non-IGP situations: 

1) Can the Link MTU be passed across a local link upon with a NLRI source
(protocol ID: direct or static)?  


2) Can the Link MTU be passed if the link is a tunnel whose encapsulation is
passed by the BGP's tunnel encapsulation attribute


3) If the answer to question 2 is yes, could you provide me with an example
topology where you envision this might happen.   


4) If the answer to question 2 is yes,  it possible that the link MTU is
passed for a tunnel of SR policy tunnel type (see
draft-ietf-segment-routing-te-policy-09.txt)?   If so, is the Link MTU a
composite value that works for all segments? 


Thank you, Sue 

 <individual contributors hat off>   



will pass an SR policy tunnel type for the BGP attribute
(draft-ietf-idr-segement-routing-te-policy-09) that refers to a tunnel which




From: Idr [] On Behalf Of Pengshuping (Peng
Sent: Wednesday, November 4, 2020 9:23 AM
To: Stephane Litkowski (slitkows); Susan Hares;
Subject: Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020
to 11/16/2020)


Hi Stephane,


It has been included in this draft that "[RFC7176] specifies the ISIS
mechanism and extensions for link MTU Sub-TLV" which can feed BGP-LS. This
was actually suggested by the LSR WG. 


Regarding OSPF, people can post corresponding draft to LSR if there is a
need. There could be other ways too as you mentioned.


Best regards,



From: Idr [] On Behalf Of Stephane Litkowski
Sent: Wednesday, November 4, 2020 4:03 PM
To: Susan Hares <>;
Subject: Re: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020
to 11/16/2020)




"a) Are there ways to pass IGP link MTUs in 

the IGPs?  If so, is this needed in BGP-LS"


This is a valid point, most of the time BGP-LS is feeded by IGP LSDBs (of
course there are other ways too). While I see that IS-IS has some MTU subTLV
coming from TRILL RFC7176 (possibly never been implemented), I don't see
anything for OSPF (I'm not an OSPF expert, so I may have missed it).

Shouldn't this be checked and validated with LSR WG before adopting ? 






From: Idr <> On Behalf Of Susan Hares
Sent: lundi 2 novembre 2020 06:04
Subject: [Idr] WG Adoption for draft-zhu-idr-bgp-ls-path-mtu (11/1/2020 to


This begins a 2 week WG adoption call for 

draft-zhu-idr-bgp-ls-path-mtu-04.txt (11/1 - 11/16/2020). 


The authors should send in an IPR statement for this draft 

by 11/5 so the WG can include the IPR status in their decision. 


You can access the draft at:


Since this draft is reference by an existing IDR draft

I've included a bit of background below to help you place  

this draft into the larger context of the SR additions to BGP-LS

and the draft-ietf-idr-tunnel-encaps-19.txt.


This draft does continue BGP-LS additions.  if you 

are opposed to any BGP-LS additions rather than 

this specific addition, please make that clear in your 

comment in this discussion.   


The authors requested a WG adoption at IETF 108.  

The IDR co-chairs thank the authors for their patience.   

This draft has been delayed by process of having a 

new document shepherd (Sue Hares) come up to speed

on draft-ietf-idr-tunnel-encapsulation. 


Cheers, Sue 




Segment Routing technology creates SR tunnels that are 

directly overlaid on MPLS or SRv6.  While existing MPLS technology 

(LDP and RSV-TE) provides mechanisms to negotiate path MTU

based on individual link MTU limits, the Segment Routing (SR) 

on BGP-LS Link Attribute does not pass information on 

MTU size per link.   


draft-ietf-idr-sr-policy-path-mtu-02.txt sends PATH MTU 

information in the tunnel-encapsulation attribute for the tunnel type  

SR-Policy that handles segment routing (SR) paths.       

However, it lacks the information to create a reasonable 

Path size since the BGP-LS Link Attribute does distribute

this information. 


The draft proposes adding a new sub-TLV for MTU size 

to the BGP-LS Link Attribute TLV, and 

draft-ietf-idr-sr-policy-path-mtu-02.txt mentions this 

draft as one possible way to distribute the per link 



Questions for the authors might be: 

a) Are there ways to pass IGP link MTUs in 

the IGPs?  If so, is this needed in BGP-LS


b) What other mechanisms pass link MTU?