From: Ahmed Hassan (ahmed_hassan@rayatelecom.net)
Date: Tue Apr 08 2003 - 07:38:18 GMT-3
Nguyen The steps below is part Of the course configuring BGP on cisco routers Take care of step number 10
Step 1 Check if the next-hop attribute indicates an IP address which is reachable according
to the current forwarding table. It is not necessary to have a direct connection to the
next-hop. It can very well be several router-hops away and the route to it learned by
the IGP. If the next-hop is not reachable, the BGP route is not considered a
candidate to become selected the best.
Step 2 Prefer route with higher weight. The weight is not carried with the updates, it is a
value assigned to the route by the local router and considered only within the router
itself.
Step 3 If the local preference attributes are different, the route with the highest value is
selected best.
Step 4 If one of the routes is injected into the BGP table by the local router, the local
router prefers it to any routes received from other BGP routers.
Step 5 At this point, the lengths of the AS-paths are compared (the content is not checked,
only the number of ASes in each AS-path is counted). The route with the shortest
length is selected.
Step 6 If the AS-path lengths are the same, the origin code is checked. An origin code
indicating IGP is preferred before EGP. Unknown is the last alternative.
Step 7 Multi-Exit-Discriminator (MED) values are, normally, only compared if the
updates are received from the same neighboring AS. Routes with lower MED are
preferred.
Step 8 At this point it is clear that the destination network is outside the local AS and that
there is not much difference between the alternatives. Since the IP packets to the
destination network must leave the AS it is better that they do that sooner than later.
So, if any of the alternatives are received from a BGP peer in another AS, that
alternative is preferred.
Step 9 If all alternatives are received from peer routers in the local AS, each of them
indicates an exit point and the closest exit is used. Distance to the exit point is
calculated by comparing the IGP costs toward the BGP next-hops, as indicated in
the forwarding table.
Step 10 If all alternatives are received from external BGP neighbors the most stable path
(the oldest path) is preferred.
Step 11 If the router still cannot differentiate the routes, it nevertheless has to make a
decision and select the best route. It checks the BGP sessions on which it received
the updates and chooses the route received on the session where the peer router has
the lower BGP router ID.
The final test is made only after all other checks are made. This means that the
alternatives are equally good.
*************************************
Ahmed Hassan El-shinnawy
Network Planning and configuration Engineer
Raya Telecom
ahmed_hassan@rayatelecom.net
Mobile 0020 (10) 1722219
Phone 0020 (2) 7680967
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-----Original Message-----
From: Nguyen Hoang Long [mailto:ng-hlong@hn.vnn.vn]
Sent: Tuesday, April 08, 2003 12:20 PM
To: Ahmed Hassan; ccielab@groupstudy.com
Subject: Re: Lowest BGP router ID prefered ?
Ahmed,
"Time" ? is it also included in BGP calculation fo Path selection? can you
post a link ?
----- Original Message -----
From: "Ahmed Hassan" <ahmed_hassan@rayatelecom.net>
To: "Peng Zheng" <zpnist@yahoo.com>; "Jonathan V Hays" <jhays@jtan.com>;
<ccielab@groupstudy.com>
Sent: Tuesday, April 08, 2003 3:47 PM
Subject: RE: Lowest BGP router ID prefered ?
> 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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> > >
> > >
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>
>
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