On 12/23/2024 5:33 PM, Jean Franco wrote:
I'm trying to achieve total redundancy on a multihomed environment:
ISP 1 <=> Router 1 <= X => Router 2 <=> ISP 2
Where X is my Network.
The hardest part can be handling a failure of either of the routers and
having X still be able to talk to the other in smaller networks. While
VRRP, MC-LAG, and MPLS do exist, platform, vendor, and your requirements
all make for a lot of fun. It's easy to accidentally make routers do
things the vendor hadn't intended (What do you mean subscriber services
aren't designed to work with mc-ae? Is that why dhcp sync only works
with vrrp and mpls and not mc-ae with unnumbered interfaces?)
I'll try not to cover what others have said, but there are a few things
to consider on dealing with your ISPs. They may run RPF filtering, so
even if you don't want them to route traffic for a network to you, if
you might send traffic from that network out, they'll need a route, so
always send the aggregates to everyone you send outbound traffic to. If
you have trouble getting a network added to a peer, you may have to not
send any outbound their way.
Many ISPs run local prefs to prefer directly connected networks over
more costly paths. This will override AS prepends. Some may let you
change it with a community. Some will not. If you must force traffic,
use a more specific route. Even if others filter it out, it should still
get enough distance to force traffic the way you want. If your
redundancy is slightly oversold and you need rough load balancing, more
specific routes are the way to handle that, but try and minimize their
use. We do have routing table bloat.
Jack
