Would I be correct in assuming that HSRP would be the ideal load balancing protocol where layer 2 and layer 3 termination happens on the core (collapsed core)?
I have MST to evenly segment VLAN root bridges across each core switch so I would think that HSRP would be ideal in this situation the as root bridge and active layer3 interface would be on the same MLS.
In GBLP it could be either core switching meaning a 50% increase of traffic across the layer2 link between the cores as clients are distributed among the cores.
VLAN 10,20 [SWITCH A] <Layer2> [CSW01] <-Layer2-> [CSW02] <Layer2> [SWITCH B] VLAN 20,30
Client in VLAN10 with AVF MAC of CSW02 sends data to CLIENT in VLAN20:
Client in VLAN10 gets the MAC address for AVF on CSW02, Client attempts to send network traffic to client in VLAN20, the spanning tree root bridge for VLAN10,20 is on CSW01, so the data first flows to CSW01 then to CSW02 where the MAC address for the AVF is located, the frame is layer3 switched onto VLAN20, the frame now has to traverse back to CSW01, and then SwitchA again.
Thus placing the unneeded traffic on the link between CSW01/CSW02.
Now considering my switches support CEF, there is no benefit in GBLP? In fact it looks more detrimental due to increase of traffic on the layer2 links between the core.
In HSRP this situation is remove because the traffic for each VLAN only traverse this link if it is destined for that a vlan that has its root on the other CSW. E.g. HSRP address configured as primary on the SPT bridge for each VLAN.
Should I be worried about this increase in traffic vs the fact that my clients are split across cores?
So I assume GBLP is ideal where we have layer 3 between core and distribution, or distribution and access, but not if we have layer 2/3 on the core?
Comments / criticisms welcome.
Victor has this exactly correct. If you are using MST (and I assume you have it setup to load balance based on the tree), ensure the HSRP active router is the root of that tree.
GLBP wouldn't be good for the scenario you have described, which is exactly as you thought. It would load balance and then you would have a case where the forwarding router is not necessarily the root of the L2 tree, which leads to extra traffic on the trunk between the gateways.