AS-Path - ORF Draft

Jakob Heitz (jheitz) jheitz at
Mon Oct 23 18:24:27 CST 2017

IOS-XR does not have a pre-policy prefix limit.
When the limit is reached, the session will not automatically
re-establish. It needs to be manually cleared first.

It has the extra options:
warning-only        - does not drop the session.
discard-extra-paths - additionally, drops prefixes after the limit is reached.
restart <minutes>   - automatically re-establish the session after the timeout.

I agree with Job that the use of warning-only can lead to unexpected routing,
because there is no control over which prefixes are dropped.
This is a big hammer that only comes down when the other hammers don't work.


Date: Mon, 23 Oct 2017 06:57:19 -0400
From: Greg Hankins <ghankins at>
To: Job Snijders <job at>
Cc: nanog at
Subject: Re: AS-Path - ORF Draft
Message-ID: <20171023105719.GH27694 at>
Content-Type: text/plain; charset=us-ascii

Nokia SR OS defaults to pre-policy but can be configured to post-policy
by adding "post-import".

prefix-limit ipv4 100 // pre-policy
prefix-limit ipv6 100 post-import // post-policy


Greg Hankins <ghankins at>

-----Original Message-----
Date: Mon, 23 Oct 2017 12:37:13 +0200
From: Job Snijders <job at>
To: nanog at
Subject: Re: AS-Path - ORF Draft

On Mon, Oct 23, 2017 at 08:35:42AM +0200, Job Snijders wrote:
> > or it could compare each additional prefix received to already learned
> > prefixes and decide to drop one to make room for the new one. For
> > example you could drop the most specific routes before less specific
> > routes.
> The moment a BGP implementation can do such RIB compression, it may
> indeed make sense to offer two types of limits: a 'pre-policy maximum
> prefix limit' and a 'post-policy maximum prefix limit'. The former type
> of limit would be useful in context of route leaks, the latter in
> context of protecting against overflow of the FIB capability.

Apparently this already exists and is widely available, Saku Ytti gave
me some additional information. There are various keywords available,
and they operate at different attachment points in the conceptual model.

                         |      IOS XR      |         Junos
      pre-policy keyword |       ????       |      prefix-limit
     post-policy keyword |  maximum-prefix  |  accepted-prefix-limit

                 (????? means the keyword does not exist)

Now I wonder what Arista EOS, Nokia SR-OS, etc offer in this regard. :-)

(screenshot here
for those of you who can't easily view ASCII tables)

Kind regards,


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