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> Christopher Morrow
> Sent: Tuesday, April 14, 2020 2:51 AM
> On Mon, Apr 13, 2020 at 7:38 PM Brandon Martin
> <lists.nanog at monmotha.net> wrote:
> >
> > On 4/13/20 4:31 PM, Randy Bush wrote:
> > > it seems a lot of folk think prepending acrually works.
> >
> > I mean, there's prepending and then there's prepending 50+ times...
> > Has the latter EVER been useful in any way, shape, or form?
> for ~4 yrs or so there's been possible problems with as-paths longer than ~50
> (I think, i can't recall the exact vendor bug) so, folk should have already been
> denying announcements with longer than ~soemthing-like-45 asn in the
> path.. right? :)
>From memory this was one of the two accidents (someone prepending their AS 255 times and an university announcing special unheard-of attribute) that triggered the good work around RFC 7606 - Revised Error Handling for BGP UPDATE Messages.
And why Randy and we all can enjoy messages like
Apr 12 17:57:42 r0.iad rpd[1752]: Prefix Send failed ! bgp_rt_trace_too_big_message:1209 path attribute too big. Cannot build update.
RP/0/RSP0/CPU0:Jan 18 00:22:41.029 : bgp[1058]: %ROUTING-BGP-3-MALFORM_UPDATE : Malformed UPDATE message received from neighbor x.x.x.x (VRF: INTERNET) - message length 87 bytes, error flags 0x00400000, action taken "DiscardAttr". Error details: "Error 0x00400000, Field "Attr-unexpected", Attribute 128 (Flags 0xe0, Length 18), Data [e0801200]". NLRIs: [IPv4 Unicast] <<not gonna name and shame>>
While our BGP sessions keep on working just fine and either the update is treated as withdraw or the attribute is deleted.

On the point of as-path length limit, Yes I know of at least one tier-1 that does it and since I left some 8 years back I do it everywhere I go.
In addition to the above (best common practice, id' say)
-on junos you can do community length limiting
-and on cisco you can do attribute filtering  -hence my question to this forum some time back about whether folks do filter all the "experiments" for the sake of running a successful business (paraphrasing...)