ibrahim.jamil wrote:
Hello folks
I Have Two core switches , running HSRP,how to verify wich core forward the traffic?
Ibrahim
There are a couple of things to take into account here. Lets say you have an access-layer switch connecting to core switch eg.
core switches = c1 and c2. These switches are interconnected via a L2 trunk.
access-layer switch = a1
a1 has L2 uplinks to both c1 and c2.
a client (h1) is connected to a1 in vlan 10.
c1 and c2 are running HSRP for vlan 10 with c1 being the active gateway.
In a normal network setup one of the uplinks from a1 to the core switches will be blocked by STP. So -
1) if a1 - c1 uplink is forwarding then traffic from h1 to another vlan will go from a1 to c1 and c1 will handle the forwarding of the data
2) if a1 - c2 uplink is forwarding then traffic from h1 to another vlan will go to c2. However the active gateway is on c1 so c2 simply switches the packet at L2 across the interconnect between c1 and c2 to c1. c1 will then forward the data on.
So it's a good idea to match your HSRP active switch with the STP root. The STP root setting will determine which uplink from a1 is blocked. As you can see from the above the optimal path for traffic is a1 -> c1. By setting c1 as vlan 10 for root then you ensure that the active HSRP gateway for vlan 10 is also the same switch as the STP root for vlan 10.
One final point. Usually your core switches are then connected to routers etc and you don't run HSRP as they peer with each using a routing protocol. So it's important to understand that outbound traffic from the client will go to the HSRP active gateway. If the destination however is beyond the routers then return traffic can come back using either core switch c1 or c2. If the traffic came back to c2 and the a1 - c2 uplink is blocking then again traffic would need to go across the c1 -> c2 interconnect and then be sent from c1 -> a1.
Jon
