Subject: [Lsr] 答复: Questions on Prefix Unreachable Announcement

From: "Aijun Wang" <[email-masked]>
Date: Fri, 3 Jan 2020 10:58:16 +0800

Hi, Huaimo:

Thanks for your review.  After the holiday, let’s begin the discussion.

Your understanding for generating the PUA(Prefix Unreachable Announcement)
is correct, but the process for stop the PUA is not the way that we
proposed.

In theory, the ABR will send the PUA once when it notices the specific
prefix is unreachable. 

When the other router receives the PUA, it will generate one black hole
route, and this black hole route will be kept for a short time(configurable)
to facilitate the service convergence that depends on this failed prefix.

The PUA will be generated and announced by the ABR only on the following
circumstances:

1. The ABR do the summary work

2. The failed prefix located within the summary route range.

3. The numbered of the failed prefixes not exceed the
MAA(MAX_Address_Announcement) as defined in
https://tools.ietf.org/html/draft-wang-lsr-prefix-unreachable-annoucement-01
#section-6

More implementation consideration can be found on the above section.

We are also appreciate to get more feedbacks for the current implementation
consideration.

More specific answers are also presented inline below.

Best Regards.

Aijun Wang

China Telecom

发件人: [email-masked] [mailto:[email-masked]] 代表 Huaimo Chen
发送时间: 2019年12月20日 22:50
收件人: Aijun Wang; [email-masked]
主题: [Lsr] Questions on Prefix Unreachable Announcement

Hi Aijun,

    My understanding of your draft
https://datatracker.ietf.org/doc/draft-wang-lsr-prefix-unreachable-annouceme
nt/
<https://datatracker..ietf.org/doc/draft-wang-lsr-prefix-unreachable-annouce
ment/>  as follows: 

    After the ABR originates the Extended Prefix Opaque LSA, which contains
a prefix and its originator (i.e., the router originating the prefix), it
will announce that the prefix is unreachable when it can not reach the
prefix (i.e., the prefix becomes unreachable from reachable from the ABR's
perspective). 

     Is my understanding right?

       [Aijun Wang]: Yes

Should the ABR remove the prefix from the Extended Prefix Opaque LSA after
it announces the prefix unreachable for a give period of time?

      [Aijun Wang]: In theory, the ABR will only announce once when it
notices the disappear of the specific prefix.

After a router in another area receives the unreachable announcement of the
prefix, it installs a black hole route to the prefix, and then removes the
black hole route after a configurable time. After the black hole route
derived from the unreachable announcement of the prefix is removed, it seems
better to remove the unreachable prefix from the LSA. 

    [Aijun Wang]: the PUA will not be announced continuously.

     Can the ABR just remove the prefix (i.e., the TLV for the prefix) from
the Extended Prefix Opaque LSA when it can not reach the prefix? 

    [Aijun Wang]: When the ABR do the summary work, it will not announce the
detailed prefixes that located within the summary route range. Then the
unreachable prefixes located in the summary range should be announced
explicitly.

Best Regards,

Huaimo