manipulating the best path, eBGP, MED?

Thomas@PhaseHosting thomas at
Tue Mar 7 20:16:30 CET 2017


I want to send the med with eBGP to my hosts their routers so i can 
influence where the network traffic enters my virtual envirement. I have 
3 hypervisors that run bird.

I can only controll the traffic from my virtual envirement to the 
internet, because this runs on bird and ucarp. So through the hypervisor 
where the ucarp interface is placed does my traffic leave. this subnet 
is also distributed to the other hypervisors through iBGP.

These hypervisors have all an eBGP connection to my hosts addressing the 
subnet. So hypervisor 1 isnt the single point of failure. But know the 
best route (at my hosts to my subnet) is chosen on router age. So in 
case the first hypervisor dies and the second takes over everything 
works. but if the first hypervisor comes back alive the eBGP connection 
from hypervisor 2 stays because of its age.

I want to send a med value on all the eBGp connections from the 
hypervisors. So my hosts routes always first to hypervisor 1 and then to 
2 and so on.

so is this the right configuration to send a med value to my host with 
the subnet as static route:

# your default gateway IP below here
protocol bgp eBGP {
import none;
export  filter {
           where proto = "static_packet";
           bgp_med = 100;
local as 65000;
neighbor 10.80.x.x as 65530;
password "SECRET";

Op 3/7/2017 om 7:08 PM schreef Ondrej Zajicek:
> On Tue, Mar 07, 2017 at 03:43:13PM +0100, Thomas at PhaseHosting wrote:
>> Supp,
>> thnqx for the reply,
>> The BGP i have figured out and i have confirmation from my host they accept
>> customer-set MEDs. Only i couldn't get these working and i found 2 posts on
>> the internet with a different configuration and i don't know which one is
>> the right one, "med metric 10;" or "default bgp_med 10;"
>> Am i right that MED is the only option to influence the  eBGP and manipulate
>> the prefered path? ( Values like AS are the same and others are not
>> accepted)
> Hi
> I do not understand if you are trying to manipulate your hosts/routers (i.e.
> outgoing direction) or neighbors routers (i.e. incoming direction).
> Both could be influenced by bgp_med attribute (e.g. 'bgp_med = 100;' in
> import or export filter) assuming your neighbors are from the same AS.
> Option 'default bgp_med' is probably not what you want, that is used only if
> a route with MED is compared to a route without MED.
> Option 'med metric' could be used if you want to compare bgp_med
> attributes on routes received from different AS numbers. But it is a
> true/false option, the MED still must be set in filters.

Thomas Valgaeren
thomas at

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