From: Ahmed Hassan (ahmed_hassan@rayatelecom.net)
Date: Tue Apr 08 2003 - 05:47:07 GMT-3
Peng Zheng,
the router will prefer the Lower ID only if your two EBGP paths have the same time,
So I think normally when you reload the router or clear The BGP table you will have one of the paths older that the other so the router will prefer the older Path (I think most of the time one of the neighbors will establish BGp session before the other).
In Case Of IBGP the BGP will choose the Closest IBGP neighbor
Best regards
*************************************
Ahmed Hassan
Network Planning and configuration Engineer
Raya Telecom
ahmed_hassan@rayatelecom.net
*************************************
-----Original Message-----
From: Peng Zheng [mailto:zpnist@yahoo.com]
Sent: Monday, April 07, 2003 9:52 PM
To: Jonathan V Hays; ccielab@groupstudy.com
Subject: RE: Lowest BGP router ID prefered ?
Thanks. Yesterday, it preferred 192.168.1.249 even
after reloaded.
I just tried today, it preferred 192.168.1.241. I
don't know why.
--- Jonathan V Hays <jhays@jtan.com> wrote:
> Please verify the router IDs of those neighbors.
> Note that the router ID
> may not be the same as the Next Hop IP address. For
> example, see below
> where the next hop is 10.1.1.2 but the neighbor's
> router ID is
> 172.16.3.1.
>
> r2501a#show ip bgp neighbors
> BGP neighbor is 10.1.1.2, remote AS 2, external
> link
> BGP version 4, remote router ID 172.16.3.1
> BGP state = Established, up for 00:19:33
> [output omitted]
>
>
> > -----Original Message-----
> > From: Peng Zheng [mailto:zpnist@yahoo.com]
> > Sent: Monday, April 07, 2003 2:55 AM
> > To: Jonathan V Hays; ccielab@groupstudy.com
> > Subject: RE: Lowest BGP router ID prefered ?
> >
> >
> > But I implemented the same topology as that.
> > Sugarbush has three IP: 192.168.1.253 (to Stowe),
> > 192.168.1.238 (to Diamond) and 192.168.1.249 (to
> > Burke), so BGP ID is 192.168.1.253.
> >
> > And Diamond has three IPs too: 192.168.1.245 (to
> > Mammoth), 192.168.1.237 (to Sugarbush) and
> > 192.168.1.241 to Burke), so its BGP ID is
> > 192.168.1.245.
> >
> > And I got the same route table on burke( preferred
> > Sugarbush).
> >
> >
> > --- Jonathan V Hays <jhays@jtan.com> wrote:
> > > You are referring to BGP Best Path Selection:
> > >
> > >
> >
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios12
> > 1/121cgcr/
> > > ip_c/ipcprt2/1cdbgp.htm#1000898
> > >
> > > "12. If multipath is not enabled, prefer the
> route
> > > with the lowest IP
> > > address value for the BGP router ID.
> > > The router ID is usually the highest IP address
> on
> > > the router or the
> > > loopback (virtual) address, but might be
> implementation-specific. "
> > >
> > > I think you are mistaken on the IP addresses you
> > > gave since I don't see
> > > either 192.168.1.253 or 192.168.1.245 on page
> 191 of
> > > that book. Perhaps
> > > I have a different printing than you do. Anyway,
> I
> > > assume you are
> > > referring to Example 2-53 "Burke's BGP Table
> > > Contains Only the Aggregate
> > > Route" which shows 192.168.1.249 as having the
> best
> > > path instead of
> > > 192.168.1.241 (the lower IP address).
> > >
> > > Although the BGP router chosen as the Next Hop
> for
> > > this network happens
> > > to have the higher IP address value, you do not
> have
> > > enough information
> > > to say that it was the IP address value used by
> BGP
> > > to make that
> > > decision. There are eleven (11) previous steps
> in
> > > the BGP Best Path
> > > Selection hierarchy, any one of which takes
> > > precedence over this IP
> > > address criteria. The IP address criteria won't
> be
> > > used unless all of
> > > the other criteria are bypassed or are equal for
> > > both routers.
> > >
> > > Hope that helps.
> > >
> > >
> > >
> > >
> > > > -----Original Message-----
> > > > From: nobody@groupstudy.com
> > > [mailto:nobody@groupstudy.com] On
> > > > Behalf Of Peng Zheng
> > > > Sent: Sunday, April 06, 2003 10:21 PM
> > > > To: ccielab@groupstudy.com
> > > > Subject: Lowest BGP router ID prefered ?
> > > >
> > > >
> > > > Hi,
> > > >
> > > > It is said the route with lowest BGP router ID
> is
> > > > preferred. But I found the route with
> highest
> > > BGP
> > > > router ID os preferred.
> > > >
> > > > In Route TCP/IP V2, page 191, on Burke, the
> route
> > > with
> > > > BGP ID 192.168.1.253 is preferred to the route
> > > with
> > > > BGP ID 192.168.1.245.
> > > >
> > > > What's the reason?
> > > >
> > > > Thanks for help.
> > > >
> > > >
> > > >
> > > >
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This archive was generated by hypermail 2.1.4 : Thu May 01 2003 - 13:35:49 GMT-3