09-07-2007 07:21 AM - edited 03-03-2019 06:39 PM
Hello,
I have 3 upstream providers. When I make prepend on one of providers, both upload and download change significantly.
I thought that with prepend I only influence incoming traffic (download) and with the local preference outgoing traffic (upload). On MRTG I see that both in & out traffic is getting down on provider that I made prepend, and on the other two both in & out is getting up. Does anyone know what is the explenation for this?
09-07-2007 09:53 AM
Hi,
I guess you achieve the prepending by means of route-map. Have you applied both inbound and outbound or just outbound?
Can you post the config?
Krisztian
09-09-2007 11:50 PM
Yes I use route-maps applied only outbound (toward upstream providers).
Config is something like this:
router bgp 6666
neighbor 1.1.1.1 remote-as 7777
neighbor 2.2.2.2 remote-as 8888
neighbor 3.3.3.3 remote-as 9999
address-family ipv4
neighbor 1.1.1.1 route-map FIRST_OUT out
neighbor 2.2.2.2 route-map SECOND_OUT out
neighbor 3.3.3.3 route-map THIRD_OUT out
ip prefix-list PREFIX seq 10 permit x
route-map FIRST_OUT permit 10
match ip address prefix-list PREFIX
set as-path prepend 6666
route-map SECOND_OUT permit 10
match ip address prefix-list PREFIX
set as-path prepend 6666 6666
09-10-2007 02:28 AM
Hi,
Based on the config you showed. Anyone on beyond your network who wants to route to network x will aim for 1.1.1.1 which will inturn aim for the router that you applied this config [the one you pasted here] on.
Assumption is that you have a seperate physical interface peer EBGP neighbor.
The followinf link may help you:
09-10-2007 03:20 AM
Hi,
As I know in case of prepending the AS number is only added when the routing update sent an even don't see when checking what advertised to the particular neighbor therefore it shouldn't affect the backward traffic (the bgp topology you have), but for sure I always tuned the localpref if wanted to influence the outgoing route selection as well.
Krisztian
09-10-2007 04:26 AM
Hi,
I may be wrong here but I'm 99% sure that setting local prefernce on an outbound EBGP peering does nothing!
Let me explain: If you do set LP outbound across an EBGP, the prefix that the EBGP peer receives will still have the default LP of 100. LP does not cross an EBGP boundary. The BGP attributes that can be set outbound to an EBGP peer that affects 'download' or incoming traffic are: prepending AS, MED, BGP router id, Origin.
However, you can set LP inbound on the receving router of an EBGP peering.
09-10-2007 04:37 AM
Hi,
Sorry, obviously I meant to set the local preference on inbound.
The local preference is stant for tell the router _within_ the AS where to leave that.
Krisztian
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