On Sat, 14 Nov 2020 15:00:03 +0100 Tobias Waldekranz wrote: > On Sat, Nov 14, 2020 at 12:59, Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> wrote: > > If any of the flowtable device goes down / removed, the entries are > > removed from the flowtable. This means packets of existing flows are > > pushed up back to classic bridge / forwarding path to re-evaluate the > > fast path. > > > > For each new flow, the fast path that is selected freshly, so they use > > the up-to-date FDB to select a new bridge port. > > > > Existing flows still follow the old path. The same happens with FIB > > currently. > > > > It should be possible to explore purging entries in the flowtable that > > are stale due to changes in the topology (either in FDB or FIB). > > > > What scenario do you have specifically in mind? Something like VM > > migrates from one bridge port to another? Indeed, 2 VMs A and B, talking to each other, A is _outside_ the system (reachable via eth0), B is inside (veth1). When A moves inside and gets its veth. Neither B's veth1 not eth0 will change state, so cache wouldn't get flushed, right? > This should work in the case when the bridge ports are normal NICs or > switchdev ports, right? > > In that case, relying on link state is brittle as you can easily have a > switch or a media converter between the bridge and the end-station: > > br0 br0 > / \ / \ > eth0 eth1 eth0 eth1 > / \ => / \ > [sw0] [sw1] [sw0] [sw1] > / \ / \ > A A > > In a scenario like this, A has clearly moved. But neither eth0 nor eth1 > has seen any changes in link state. > > This particular example is a bit contrived. But this is essentially what > happens in redundant topologies when reconfigurations occur (e.g. STP). > > These protocols will typically signal reconfigurations to all bridges > though, so as long as the affected flows are flushed at the same time as > the FDB it should work. > > Interesting stuff! Agreed, could be interesting for all NAT/conntrack setups, not just VMs.
