Two router ha setup questions

[Gregor Fajdiga]

Hi,

Is AS path prepending on backup router not an option?

#     example
#    if bgp_path ~ [= myas =] then
#        bgp_path.prepend(myas);
#    accept;

         if proto = "static_bgp_v4" then {
                 bgp_path.prepend(myas);
                 accept;
         }
         else reject;

Regards,

Gregor Fajdiga
Sistemski administrator, Informatika
System administrator, IT

Delo, d.o.o.
Likozarjeva 1,
SI-1000 Ljubljana

+386 1 4737 993
fajdiga at delo.si

www.delo.si <http://www.delo.si>


On 21/01/2022 15:49, Alessandro Brega wrote:
> Hello guys,
>
> I have two identically configured routers (bird2, only the local ip is 
> different), connected to one upstream and a few internal vlans. Each 
> router has a BGP session with the upstream router. I have a couple of 
> small local subnets (/27, /28, ..), which are on different vlan 
> interfaces, which I need to announce to upstream. They all belong to 
> one big network (/23). As the upstream router doesn't accept routes 
> smaller than /24, I only announce a single route to the /23 network. I 
> uploaded my bird configuration https://controlc.com/aa226135.
>
> For completeness: for first hop redundancy (default gateway of the 
> clients using the small subnets) I use keepalived on my routers, which 
> works fine so far.
>
> What's the problem:
> As both routers announce the same network to upstream, upstream sends 
> some of its traffic to router1 and some to router2 (but doesn't seem 
> balanced in any way). This seems to cause (at least) tcp ordering 
> issues (I suspect, couldn't confirm it in any way), because sometime 
> some connections from local client to clients behind the 
> upstream "hang" for a couple of seconds. If I stop bird on the backup 
> router (so all traffic only goes to the active one) no hangs occur.
>
> The question:
> Is my setup ok or is it (completely) broken? Is it ok to have both 
> routers announce the same subnets at the same time?
>
> Possible solution I'm thinking about but don't know how to do it:
> Both routers should have an active BGP sessions (for fast failover), 
> but only the router which is active (and thus has the gateway ips) 
> should announce the "aggregated" route (the /23). This way no traffic 
> would go over the backup. But I wonder how this could be configured in 
> bird? I could configure keepalived to not only create the gateway ips 
> but also the small subnets (in my current config the subnets are 
> created by bird). Would this make things easier for bird?
>
> My last resort would be to have keepalived execute scripts which 
> reconfigure bird on failover. But somehow this feels wrong? Or is this 
> the way to do it?
>
> Thank you very much for reading and any help. I'm struggling with this 
> for days now... :-(
>
> Alessandro
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