On 2016/04/08 10:05, Doran Mori wrote: > Could you give an example of how that it could cause a routing loop?
Say you have a directly connected /27 on an interface, and a covering /22, or a default route or something. If the interface holding that /27 goes down (for example the switch fails, or somebody unplugs the cable, etc), in most setups you won't be able to reach it. Host unreachable / net unreachable. If you "fall back" to a shorter prefix or especially to a default route, in many setups, the router that you send it to will send it right back to you, causing a loop. Consider the case where you take a local interface down. Normally you want the traffic to be dropped, not sent to a transit provider. Considering phessler's suggestion, which I haven't tested but makes sense, if you would usually have another route to the *same prefix* i.e. the /27 from somewhere else (bgp, ospf, static route) then that should be used. > I'm used to the world of actual routers where the related routes from > an interface get removed from the FIB when it goes down. The kernel > already removes the UP flag. I don't understand why that's still a > valid route? > > On Fri, Apr 8, 2016 at 1:53 AM, Stuart Henderson <s...@spacehopper.org> > wrote: > > On 2016-04-07, Doran Mori <dhm...@gmail.com> wrote: > > Running 5.9. > > > > I have x.x.141.0/25 that's directly connected. With x.x.141.0/24 > reachable > > via bgp. > > > > # route -n get x.x.141.13 > > route to: x.x.141.13 > > destination: x.x.141.0 > > mask: 255.255.255.128 > > interface: em3 > > if address: x.x.141.112 > > priority: 4 (connected) > > flags: <UP,DONE,CLONING> > > > > # ifconfig em3 down > > jawaka# route -n get x.x.141.13 > > route to: x.x.141.13 > > destination: 66.117.141.0 > > mask: 255.255.255.128 > > interface: em3 > > if address: x.x.141.112 > > priority: 4 (connected) > > flags: <DONE,CLONING> > > > > I see the UP flag is removed but it has no effect. > > > > # ping x.x.141.13 > > PING x.x.141.13 (x.x.141.13): 56 data bytes > > ping: sendto: Network is down > > > > I would expect this route to be used instead: > > # route -n get x.x.141.0/24 > > route to: x.x.141.0 > > destination: x.x.141.0 > > mask: 255.255.255.0 > > gateway: x.x.144.154 > > interface: em2 > > if address: x.x.144.153 > > priority: 48 (bgp) > > flags: <UP,GATEWAY,DONE> > > > > I searched the archives but didn't see this specific case > mentioned. Can > > someone enlighten me? > > That is how things normally work. > > Sometimes the behaviour you want would be useful (especially for > machines that move between wired and wireless connectivity) but in > other > cases (e.g. the usual case with a bgp router) it would be > undesirable as > it would cause a routing loop, so there's no one-size-fits-all > answer. > > I'm not sure if there is any way to manipulate the route entry to > do > what you want. If there is, it's non-obvious at least!
