[f-nsp] Trunk / LAG and spanning-tree costs

[Franz Georg Köhler]

Am 27.05.16 um 02:59 schrieb ncutting at edgetg.com (Nick Cutting):
> How does spanning tree treat trunk links a.k.a Etherchannel/Aggreagated links?
>
> I would have thought that the pseudo 20 gig bandwidth would lower the Cost of the link ?

No, it is the same cost.

This is from the manual:
http://www.brocade.com/content/html/en/configuration-guide/NI_05800a_SWITCHING/GUID-052E20B0-FF5A-41F5-BB48-A2370F0D5C5F.html

STP in a LAG
The STP standard indicates that by default the path cost is determined 
by link speed. For a1 G port the path cost is 4 and for 10G port the 
path cost is 2. However, if a LAG is made consisting of n 1G ports, 
where n is less than 10, the path cost remains as 4. The standard does 
not indicate pathcost explicitly for LAG interfaces or for bandwidths 
between standard port bandwidth values, (for example, between 1G and 
10G). Therefore, during STP deployment you may find that though a LAG 
has greater bandwidth, its in blocking/discarding state as its pathcost 
is the same as any 1G link and the portIndex of 1G port is lower, making 
the LAG go into a blocking/discarding state. This behavior is not 
restricted to 1G or 10G link speed but span across different link 
speeds. The same behavior also holds TRUE for RSTP deployments.




Best regards,

Franz Georg Köhler