From: Dmitry Volkov (dmitry.volkov@rogers.com)
Date: Sat Dec 27 2003 - 20:52:19 GMT-3
Alec,
Because it PRE pends. ASNs appending from right to left when routes are
propagating (traffic will flow in opposite direction).
my neighbor<-----neighbor of my neighbor<-----.....<------ neighbor of
origin<-------Origin
Since Origin AS on the right-most and closest to You AS on left-most it's
logically that You place bogus AS just before (to right) your own AS
(prepend) when you advertise it no neighbor. So your neighbor will see your
AS as left-most and prepended AS next to right from your AS. When You have
route-map in - You will see on each router inside of your AS, prepended AS
on left-most place (because You won't see your own ASN inside AS-path on
routers within your AS and because it's You who did prepend it), but as soon
as You advertise it to neighbor it will be just after (if read from left to
right) your AS.
Let say your neigh in AS 100 and your AS 200 and You prepend 300 so your
neighbor will see "200 300" - your neigh peering with AS 200 not AS 300 -
right ? if prepending would happen like "300 200" , your neigh AS 100, which
peering with AS 200, wouldn't accept as-path "300 200"
Dmitry
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On
> Behalf Of Pun, Alec CL
> Sent: Saturday, December 27, 2003 12:10 PM
> To: Scott Morris; 'Brown, Jim'; ccielab@groupstudy.com
> Subject: RE: BGP as-path prepend confusion !
>
>
> Scott,
>
> So why the as-prepend command (applied on R1 outbound) put
> the new AS in the
> right-most instead of left-most ? As Ozgur pointed out, the
> new AS will be
> put in the right-most if the same route-map is applied on R2
> inbound. Any
> clue ?
>
> rgds,
> alec
>
> R1
> router bgp 65400
> no synchronization
> bgp log-neighbor-changes
> network 3.3.3.0 mask 255.255.255.0
> neighbor 192.168.4.1 remote-as 1
> neighbor 192.168.4.1 route-map prepend-as out
> no auto-summary
> route-map prepend-as permit 10
> set as-path prepend 65401 65402
> !
>
> R2#sh ip bgp
> BGP table version is 8, local router ID is 204.100.100.97
> Status codes:
> s suppressed, d damped, h history, * valid, > best, i -
> internal Origin
> codes: i - IGP, e - EGP, ? - incomplete
>
> Network Next Hop Metric LocPrf Weight Path
> *> 3.3.3.0/24 192.168.4.2 0 0
> 65400 65401
> 65402 i
>
>
>
> -----Original Message-----
> From: Scott Morris [mailto:swm@emanon.com]
> Sent: Sunday, December 28, 2003 12:56 AM
> To: 'Brown, Jim'; Pun, Alec CL; ccielab@groupstudy.com
> Subject: RE: BGP as-path prepend confusion !
>
>
> I think you are thinking a little backwards.
>
> The list reads just like you would read an english sentence, from left
> to right.
>
> The closest AS (one who advertised) is "pre-pended" at the left-most
> position in the AS SET (which you have correct). Think of it like
> making a word seem longer like a super-long word instead of
> just a long
> word. I pre-pended "super-". But you still read the word the same.
>
> Per RFC 1771:
>
> b) When a given BGP speaker advertises the route to a BGP speaker
> located in a neighboring autonomous system, then the advertising
> speaker shall update the AS_PATH attribute as follows:
>
> 1) if the first path segment of the AS_PATH is of type
> AS_SEQUENCE, the local system shall prepend its own AS number
> as the last element of the sequence (put it in the leftmost
> <<<----
> position).
>
> 2) if the first path segment of the AS_PATH is of
> type AS_SET,
> the local system shall prepend a new path segment of type
> AS_SEQUENCE to the AS_PATH, including its own AS
> number in that
> segment.
>
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713,
> CISSP, JNCIS, et al.
> IPExpert CCIE Program Manager
> IPExpert Sr. Technical Instructor
> swm@emanon.com/smorris@ipexpert.net
> http://www.ipexpert.net
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> Behalf Of
> Brown, Jim
> Sent: Saturday, December 27, 2003 11:43 AM
> To: Pun, Alec CL; ccielab@groupstudy.com
> Subject: RE: BGP as-path prepend confusion !
>
>
> The AS Path List reads from right to left with the leftmost side being
> the origin.
>
> The AS number is prepended to the beginning of the list which is the
> left-hand side.
>
> -----Original Message-----
> From: Pun, Alec CL [mailto:Alec.CL.Pun@pccw.com]
> Sent: Saturday, December 27, 2003 3:01 AM
> To: ccielab@groupstudy.com
> Subject: BGP as-path prepend confusion !
>
>
> Hi group,
>
> I am confused by the command "set as-path prepend" as the new AS #
> should be appended instead of prepended to the originated AS
> #, right ?
> My assumption of prepend is that new AS # should be added to
> the left of
> the AS-Path.
>
> rgds,
> alec
>
>
> R1
> router bgp 65400
> no synchronization
> bgp log-neighbor-changes
> network 3.3.3.0 mask 255.255.255.0
> neighbor 192.168.4.1 remote-as 1
> neighbor 192.168.4.1 route-map prepend-as out
> no auto-summary
> route-map prepend-as permit 10
> set as-path prepend 65401 65402
> !
>
> R2#sh ip bgp
> BGP table version is 8, local router ID is 204.100.100.97
> Status codes:
> s suppressed, d damped, h history, * valid, > best, i -
> internal Origin
> codes: i - IGP, e - EGP, ? - incomplete
>
> Network Next Hop Metric LocPrf Weight Path
> *> 3.3.3.0/24 192.168.4.2 0 0
> 65400 65401
> 65402 i
>
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