From: Arun Arumuganainar (aarumuga@hotmail.com)
Date: Thu Nov 03 2005 - 07:24:48 GMT-3
Default Behavior in BGP : When ever a prefix is received , router checks the
AS Path for its own AS number . If its AS is found in the prefix's AS path
attribute , it infers a loop and the prefix will not be imported in to its
BGP table .
With Allowas-in feature turned on this ****AS PATH LOOP DETECTION CHECK****
is not performed at all . That is all it does and nothing else !!!
To the best of my knowledge this is not at all related to SOO .
Where do we really use Allowas-in feature ?
When BGP is used as PE-CE protocol , it allows all CE sites that belongs to
single customer to use the same AS number . And nothing else .
It is also used in HUB/SPOKE VPN as two VRFs are configured on PE to same CE
device ( HUB Router ) !!!
Pls. Note : SOO is a loop detection feature that will be useful in Multi
Homed CEs or a Site with multiple CE routers .You might or might not use
allowas-in feature depending on choice of AS u use in your deployment . In
case all the CEs across all the sites uses same AS you will have to use
allowas-in . Otherwise there is no need for turning this on .
Thanks and Regards
Arun
----- Original Message -----
From: "Olopade Olorunloba" <lolopade@ipnxnigeria.net>
To: "'Scott Morris'" <swm@emanon.com>; "'Andrew Lissitz (alissitz)'"
<alissitz@cisco.com>; "'Jongsoo'" <bstrt2004@gmail.com>; "'C&S GroupStudy'"
<comserv@groupstudy.com>; "'FORUM'" <ccielab@groupstudy.com>
Sent: Thursday, November 03, 2005 12:29 PM
Subject: RE: "neighbor allowas-in" command ( SP CCIE)
> Now I'm getting confused. I thought by setting the SOO on the inbound
> updates, the Cisco IOS automatically checks for loops, and I do not have
to
> use things like communities to filer?
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Scott Morris
> Sent: 03 November 2005 03:03
> To: 'Andrew Lissitz (alissitz)'; 'Jongsoo'; 'C&S GroupStudy'; 'FORUM'
> Subject: RE: "neighbor allowas-in" command ( SP CCIE)
>
> Not to my knowledge, but I haven't really looked at that either.
>
> And yes, you'll still detect loops.
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Andrew Lissitz (alissitz)
> Sent: Wednesday, November 02, 2005 8:59 PM
> To: swm@emanon.com; Jongsoo; C&S GroupStudy; FORUM
> Subject: RE: "neighbor allowas-in" command ( SP CCIE)
>
> Thanks Scott,
>
> So ... with SoO being set, you will still have loop detection? Also I
asked
> Robert a question of whether or not SoO is set automatically. Can you
> comment on this Scott? Just quick comments, nothing lengthy needed
> ;-)
>
> Andrew
>
>
>
> -----Original Message-----
> From: Scott Morris [mailto:swm@emanon.com]
> Sent: Wednesday, November 02, 2005 8:08 PM
> To: Andrew Lissitz (alissitz); 'Jongsoo'; 'C&S GroupStudy'; 'FORUM'
> Subject: RE: "neighbor allowas-in" command ( SP CCIE)
>
> You can still use this in conjunction with SOO to determine which router
> REALLY originated it. This is also used (IMHO) when you don't entirely
> trust your SP to clear out everything necessry on the BGP feeds. ;)
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Andrew Lissitz (alissitz)
> Sent: Wednesday, November 02, 2005 7:50 PM
> To: Jongsoo; C&S GroupStudy; FORUM
> Subject: RE: "neighbor allowas-in" command ( SP CCIE)
>
> Hey Buddy,
>
> Here is a live example, I have not done the hub and spoke labs like
several
> others on this mail list have:
>
> CE ---bgp---PE ---(ISP Cloud)--- PE---bgp---CE
>
> Each CE runs AS 65000 and shares routes with the PE. The PEs share routes
> via iBGP. The remote PE shares routes with the remote CE, and the CE sees
> the routes from AS 65000.
>
> What is BGP to do? It sees its own AS number and realizes there is a
> problem.
>
> Solution: Either PE changes the AS number with as-override or the CE
allows
> its own AS number to come in via the allowed-as command. The number @ the
> end is how many times the CE will allow it's own AS number to be present
in
> the path string of the incoming route information.
>
> Concerning your gut feelings (btw ... hope you are not writing on empty
> stomach), number one sounds good, but with number 2, you are essentially
> saying that this command will override bgp split horizon. This is not
what
> it will do, if a route is already coming in, and it contains the BGP's AS
> number in the path, then let this in. Not whether or not to advertise to
> other peers. I have not seen this command change BGP split horizon
behavior
> ...
>
> BGP best path selection still occurs, it is just that the routes will not
be
> rejected because of loop detection. I have not seen multiple routes being
> accepted as best paths... Can multiple routes exist without the BGP
> multipath command?
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Jongsoo
> Sent: Wednesday, November 02, 2005 7:33 PM
> To: C&S GroupStudy; FORUM
> Subject: "neighbor allowas-in" command ( SP CCIE)
>
> I am trying to understand this command will allow to receive MP-bgp vpn
> routes with the same ASN.
>
> If I see usage guide in CCO, it says
>
> ##################################
> Usage Guidelines
> In a hub and spoke configuration, a PE router readvertises all prefixes
> containing duplicate autonomous system numbers. Use the neighbor
allowas-in
> command to configure two VRFs on each PE router to receive and readvertise
> prefixes are as follows:
>
> One Virtual Private Network routing/forwarding instance (VRF) receives
> prefixes with ASNs from all PE routers and then advertises them to
> neighboring PE routers.
>
> The other VRF receives prefixes with ASNs from the customer edge (CE)
> router and readvertises them to all PE routers in the hub and spoke
> configuration.
>
> You control the number of times an ASN is advertised by specifying a
number
> from 1 to 10. "
> ################################################
>
> In my gut feeling, basically, this command seems allow two things, 1)
> receive BGP routes with its own ASN from PE or CE, ( normal behavior of
BGP
> blocks BGP route with its own ASN in order to prevent loop) and
> 2) advertize iBGP routes to iBGP peers ( normal behavior of iBGP is not to
> advertize iBGP routes to any iBGP peers).
>
> What seems interesting is this feature will creates a lot of redundant
> routes but the length of AS path will quickly determine the best routes so
> that there won't be any loop...
>
> This will be a perfect command to make hub and spoke topology to work...
>
> The biggest question I have now is " am I right or wrong?".
> Someone please correct me if I am wrong.
>
> Thanks
>
>
> Jongsoo
>
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