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STP vs routing


 I have two switches running as HSRP and one of L2 switch connected to ASA. Currenty L3 Primary switch is a root bridge. so, in terms of STP, the link between L2 switch and Secondary is not active. I like to  change a root bridge to L2, then from server side, it makes more sense that even though the link between Pri and server is disconnected, packet can go through server -> Sec -> L2 -> ASA -> Internet. Unfortunately, if server is sending through traffic through Sec due to VM load balancing algorithm or whatever reason, traffic will go through like that (Server -> Sec -> Pri -> L2 -> ASA -> Internet) That is why I am trying to make L2 switch as root. In that case, which way server is sending (whatever load balance algorithm is used), traffic will not detour to Primary switch. 

Now, from L3 perspective. Let us say that I put L2 switch as root, not Primary.  I have one more L3 switch (routing) next to primary switch (name: HQ) Then If server is going to reach this HQ switch, how does traffic go?  Does traffic go from server to this HQ switch like 

1. Server -> Pri -> L2 (because of STP) -> Pri -> HQ  


2. Server -> Pri -> HQ  (just following routing protocol preference)


I am confused between STP priority vs routing priority when mixed. Thanks. 

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Community Member

Would say 2 is right.The L3

Would say 2 is right.

The L3 IP address maps to a L2 Mac Adresss using ARP protocol. You can trace the path on every switch using the "show mac-address | inc xxxx.xxxx.xxxx" command, in order to find the interface where the switch will send out the frame targeted for certain mac address.


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