[SPAM] Re: manipulating the best path, eBGP, MED?

Michael McConnell michael at winkstreaming.com
Tue Mar 7 20:38:47 CET 2017

> On Mar 7, 2017, at 1:16 PM, Thomas at PhaseHosting <thomas at phasehosting.io> wrote:
> Supp,
> 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.

in your environment if the oldest BGP session is the inbound, if you want to make hyper1 the master again, just do a soft reset of the BGP session on hypervisor 2. you can configure this to be executed via a ucarp script when hypervisor 1 comes back online. 

outbound will always be via the ucarp floating address (the gateway)

> 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 phasehosting.io
> +32477915307

More information about the Bird-users mailing list