Thanks Bill for the well explanation! I'll probably will have to go into the communities then, some of tests I've done got me nowhere! I'm using VyOS (quagga) and prepending didn't help.
Best regards, On Tue, Dec 24, 2024 at 12:42 AM William Herrin <b...@herrin.us> wrote: > On Mon, Dec 23, 2024 at 4:53 PM Christopher Hawker <ch...@thesysadmin.au> > wrote: > > You can establish an iBGP session between the two routers that exchange > > either default & own routes, or they can send their own routes with > fulls and > > use local pref to preference the directly-connected transit session > before routes > > learnt from the iBGP session, depending on how you want engineer your > traffic. > > That's the easy part. If you want the ISPs to be equal with the > shortest path getting the traffic then you're done. Congratulations. > > > I would like to do a failover model, where if one ISP goes down the > other would take over. > > If you want to weigh one ISP to be "primary" and the other to be > "backup," you've a long hard road ahead of you. Localprefs can make > you prefer one ISP over the other for _outbound_ traffic but the > levers for controlling _inbound_ traffic are more complicated. > > You can get part of the way there by "prepending" your AS number > several times on the backup path. That makes the AS path longer from > the backup ISP which tends to cause BGP selection to pick the shorter > path via the primary ISP. That's basically BGP's default: shorter AS > path wins. > > Except for all the jackals out there who use a local mechanism to pick > the best path without regard to the AS path length. For those, you'll > have to learn about "communities." Communities are basically tags: you > tag a route and if your ISP understands the tag it does something > different than normal with that route. Your ISPs publish a list of > communities they understand along with what they will do differently > if you tag a route with that community. Typically you'll want to find > the community that tells your ISP to set their own localpref > differently than the default. You may even need to find the > communities that tell your ISP's ISPs to set their localprefs > differently than their defaults. It gets complicated fast. > > Regards, > Bill Herrin > > > > > -- > William Herrin > b...@herrin.us > https://bill.herrin.us/ >
