From: Uchil Perera (uchil.groupstudy@gmail.com)
Date: Thu Mar 13 2008 - 09:53:28 ARST
Hi,
Manupilating the origin code is the best option, but if you have a
restriction in advertising prefixes using the "network" command, use the
below method.
*How BGP Selects Paths*
**
1. If the next hop is inaccessible, do not consider it.
2. If the path is internal, synchronization is enabled, and the route is not
in the IGP, do not consider the route.
3. Prefer the path with the largest weight (weight is a Cisco proprietary
parameter).
4. If the routes have the same weight, prefer the route with the largest
local preference.
5. If the routes have the same local preference, prefer the route that was
originated by the local router.
6. If the local preference is the same, or if no route was originated by the
local router, prefer the route with the shortest autonomous system path.
7. If the autonomous system path length is the same, prefer the route with
the lowest origin code (IGP < EGP < INCOMPLETE).
8. If the origin codes are the same, prefer the route with the lowest MED
metric attribute.
9. Prefer the external BGP (eBGP) path over the iBGP path.
10. Prefer the route that can be reached through the closest IGP neighbor
(the lowest IGP metric).
11. If the following conditions are all true, insert the route for this path
into the IP routing table:
Assuming all the above listed are equal (1-11), R4's decesion is based on
the lowest router id
*12. If multipath is not enabled, prefer the route with the lowest IP
address value for the BGP router ID.*
Therefore use a router id lower then R2's router id and aggregate the
172.16.2.2/32 prefix
Ex.
*R1*
router bgp 123
bgp router-id 1.1.1.1
aggregate-address 172.16.2.0 255.255.255.0 summary-only
Regards
Uchil
CCIE # 18536
On Thu, Mar 13, 2008 at 11:48 AM, YourPal <dearprudence28@gmail.com> wrote:
> Hi Group,
>
> I came across the following BGP scenario:
>
> R1, R2, and R3 are in AS 123. They are fully-meshed. R4 is in AS 456. R4
> peers with R1 and R2.
>
> R1 advertises 172.16.1.1/32 into BGP.
> R2 advertises 172.16.2.2/32 into BGP.
> R3 advertises 172.16.3.3/32 into BGP.
>
> Configure R1 so that R4 prefers R1 to reach 172.16.1.1/32 and
> 172.16.3.3/32.
> Route filtering and AS path manipulation are prohibited.
>
> The way I see it, I'm left with the option of configuring MED. However
> since
> the task restricts the config to be done on only R1, AFAIK there's no way
> to
> influence MED because both R1 and R2 will advertise the prefixes to R4
> with
> MED=0 (unless I'm allowed to configure a higher MED value on R2).
>
> Can't think of a solution. I may be missing something. Appreciate any
> help.
>
>
> Thank you.
>
> BR,
> Emil
>
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