[Idr] 答复: pre-WG LC Shepherd's report for draft-ietf-idr-bgpls-sr-vtn-mt-07

[Aijun Wang <[e-mail]>]

Hi, Sue:

 

For your mentioned #3,  ¡°3. One EBGP session is established using loopback
addresses, and there are multiple layer-3 inter-domain links between the
ASBRs.¡±, the draft
https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgpls-inter-as-topology
-ext-17 can be used to solve the detection of state of the inter-as
link-------Only when the inter-as link is up, it will be advertised by each
ASBR within the ¡°Stub Link NLRI¡± that is defined in this document.

All the related ¡°BGP Peer-Adj-SID/End.X SID and the MT-ID of the NRP¡° can
be advertised with this Stub Link NLRI.

 

Best Regards

 

Aijun Wang

China Telecom

 

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´ú±í Susan Hares
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Ö÷Ìâ: [Idr] pre-WG LC Shepherd's report for
draft-ietf-idr-bgpls-sr-vtn-mt-07

 

IDR: 

 

The IDR chairs did a pre-WG LC review for draft-ietf-idr-bgpls-sr-vtn-mt-07.
The review is included below. Since the issues raised in the review do not
change the WG LC status, the WG LC will start today. 

 

Cheerily, Sue 

 

PS ¨C Any errors in this review write-up are probably mine. 

 

 

Pre-WG LC Shepherd Report for draft-ietf-idr-bgpls-sr-vtn-mt-07 

 

1) The following 3 drafts listed as normative are only WG drafts: 

draft-ietf-spring-resource-aware-segment-10, 

draft-ietf-spring-sr-for-enhanced-vpn-08 and 

   draft-ietf-teas-enhanced-vpn-20,  

 

2) The review points out a potential issue with RFC9552 described below. The
IDR chairs feel this should be addressed as a correction to RFC9522. 

 

Desription: 

It is possible for a single eBGP session to be established between the two
ASes and BGP-LS could signal for all links between those ASes that each side
is up.  However, this can have issues previously identified for the base
BGP-LS specification when this is done.  Consider this small example:

 

     +------+

     |  L1  |

   --+      +---

R1               R2

   --+      +---

     |  L2  |

     +------+

 

R1 and R2 peer via eBGP over L1.

R1 advertises to the rest of the network links L1 and L2 at R1

R1 propagates the state for links L1 and L2 received from R2.

 

R1 and R2 do not peer over L2.

 

If the link status of L2 at R1 and R2 is Up, but there is no connectivity
between them (connectivity is down), it is possible that a controller will
compute traffic over the down L2.

 

This issue was covered in RFC 9952, Section 5.2.2 - Link Descriptors:

 

: An implementation MAY suppress the advertisement of a Link NLRI,

: corresponding to a half-link, from a link-state IGP unless the IGP has

: verified that the link is being reported in the IS-IS LSP or OSPF Router
LSA

: by both the nodes connected by that link. This 'two-way connectivity
check'

: is performed by link-state IGPs during their computation and can be

: leveraged before passing information for any half-link that is reported
from

: these IGPs into BGP-LS. This ensures that only those link-state IGP

: adjacencies that are established get reported via Link NLRIs. Such a

: 'two-way connectivity check' could also be required in certain cases
(e.g.,

: with OSPF) to obtain the proper link identifiers of the remote node.

 

The issue with RFC9552 here that is that the inter-domain case doesn't have
an IGP on both sides to do the check.

 

For typical eBGP, if the router advertised its connectivity at a link only
when the session is up, we're good.  R1 would get each half side only if the
session is up on each link.

 

As Jie Dong point out, draft-ietf-idr-bgpls-sr-vtn-mt-07 covers the
following cases: 

 

1. Multiple eBGP sessions, each over an inter-domain link. As Jeff said,
this case should be good. 

2. One EBGP session over an L2 bundle, which consists of multiple layer-2
member links. This should also be good as there is a layer-2 detection
mechanism for the liveness of the bundle members.  

3. One EBGP session is established using loopback addresses, and there are
multiple layer-3 inter-domain links between the ASBRs. 

 

Note that only #3, may have the problem above. However, in deployments there
is some detection mechanism (e.g. BFD) to check the availability of the
connection. 

 

 3. Editorial issues 

3.1  Section 2.1, paragraph 2, block quote ¨C the block quote is not
indented.  Typically, block quotes are indented.  Please indent in your next
version. 

 

Cheerily, Sue Hares