[rbridge] Existing issues in root bridge selection for LANs.

[Greg Daley]

Hi Erik,

Erik Nordmark wrote:
> Greg Daley wrote:
> 
>> Hi,
>>
>> Here's the idea I was thinking about a while ago, which
>> looks applicable to rbridge.  More work needs to be done,
>> to ensure that the mean path cost is always lower, but
>> I'd guess it's a fairly good bet to never be worse if
>> most of the traffic exists the LAN.
> 
> 
> Given that TRILL will use a use a link-state routing protocol to find 
> the pair-wise shortest paths (without any configuration of any device), 
> I don't see how this applies to TRILL.
> 
> Are your proposing some improvements to the IEEE 802.1 protocols?

Given that each trill network interfaces with a legacy 802.1 LAN,
some work to choose a good root for the 802.1D spanning-tree in each
LAN would be useful to reduce the aggregate traffic load on the network.

This doesn't require modification to the 802.1D mechanism, except
that devices which know they are closer to the ingress/egress of
the network can make their chances better.

The original idea I had was before my rbridge awareness (notice that
the router R in the network wasn't considered).  Where trill routers
are present, they have direct knowledge that they are at the edge
of a LAN, and are potential ingress/egress points.

It's possible to use trill's routing protocol on the legacy LAN
to elect which of the trill routers will be the most preferred root
(rather than for exchange of link state).

Alternatively, if no routing protocol is explicitly used on the legacy
LAN, routers within the trill cloud which know they are attached to the
same legacy LAN can determine from their trill routing knowledge which
is the best root.

>> This means that all switches have the same default
>> priority, and MAC addresses (as provided in the
>> lower order bits of the switch's BridgeID),
>> will be used to break ties between switches.
>>
>> Therefore unless a manual priority configuration is
>> provided, MAC switch vendors with the lowest IEEE OUI
>> will win. 
> 
> 
> My understanding is that manual configuration is used in enterprise 
> Ethernets, which among other things ensure that the root bridge is close 
> to the routers.

Yes.  Terrible isn't it!?

:)

The point I was making was that manual configuration is needed,
which means that the traffic flows need to be known
(in that case by an administrator).  The same technique could be
applied automatically or heuristically.

I guess that if there's a requirement for trill/rbridge systems
to interconnect bunches of old gear, you don't want to pick a root
in each "area" manually.

The trill routers should be able to select/elect themselves as the
most likely root bridge (barring explicit manual configuration
perhaps??).  I think Radia's point was that this adds stability.
My point was that the aggregate traffic load may be smaller.

Greg