On 23/Jun/20 06:41, Saku Ytti wrote:
>
> I can't tell you how common it is, because that type of visibility is
> not easy to acquire, But I can explain at least one scenario when it
> occasionally happens.
>
> 1) Imagine a ring of L2 metro ethernet
> 2) Ring is connected to two PE routers, for redundancy
> 3) Customers are connected to ring ports and backhauled over VLAN to PE
>
> If there is very little traffic from Network=>Customer, the L2 metro
> forgets the MAC of customer subinterfaces (or VRRP) on the PE routers.
> Then when the client sends a packet to the Internet, the L2 floods it
> to all eligible ports, and it'll arrive to both PE routers, which will
> continue to forward it to the Internet.
> This requires an unfortunate (but typical) combination of ARP timeout
> and MAC timeout, so that sender still has ARP cache, while switch
> doesn't have MAC cache.
>
> In the opposite direction this same topology can cause loops, when PE
> routers still have a customer MAC in the ARP table, but L2 switch
> doesn't have the MAC.
>
> I wouldn't personally add code in applications to handle this case
> more gracefully.
My understanding of Layer 2-based Metro-E networks is that
multi-directional traffic would be prevented by way of Spanning Tree.
Mark.
