[rbridge] LIDs

[Caitlin Bestler]

Larry Kreeger (kreeger) wrote:
> Hi,
> 
> I was catching up on old RBridge threads and saw this one about LIDs.
> 
> LIDs can be useful in reducing costs in large switches where
> the entire (potentially large) local MAC table would not need
> to be present in every module of the system.  In fact it
> could probably eliminate the need to keep locally learned
> MACs in hardware at all.  Assuming a switch with 16 slots and
> 48 ports per linecard, and servers running 16 virtual
> machines each connected to the ports, the number of local
> MACs would be 16*48*16= 12,288.  With 48 bits per address,
> that is ~59kbits.
> 
> With LIDs, an entry is need only per port, not per local MAC.
>  So given a 16 bit LID, only 16 cards * 48 ports * 16 bits =
> ~12kbits, a savings of ~47kbits.
> 
> The more locally attached MACs, the bigger the savings.
> 

Instead of keeping the MAC to Port mapping in the egress RBridge
you instead keep it in each ingress rbridge that has learned
of this rbridge. I'm not seeing a net savings in memory costs
anywhere. There is certainly a potential latency benefit, but
is there really one that is valid over the desired lifespan
of the protocol?