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STP Doubt

hi,

If four switches are connected to each other

back to back with trunk configured.

Switches connected are like this :

Swicth 1-->2-->3-->4

And if the Switch 4 is the STP root bridge.

VLAN's are not configured on all switches are and those are running in default VLAN1.

Now if PC1 from swicth 1 pings to PC2 from switch 2.

CASE 1 . Is is so that frame from PC1 will travel upto root bridge that is switch 4, and then gets redirected back towards swicth2.

CASE 2. The frame from PC1 will travel to swicth number 2 , there is will find the PC2 mac address and swicth 2 will forward that frame to PC2.

Keeping aside the initial ARP broad cast.

Which case is correct.

Please share the experience.

Thanks

subodh

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: STP Doubt

Hello,

The Case 2 is correct. The STP does not change the way how switches forward frames. That principle remains even with STP: whenever a switch receives a frame, it tries to find its destination MAC in its switching table. If it finds it, it forwards the frame out that port only. If it does not find the destination MAC, it will forward the frame out every other port. With STP, this procedure changes only slightly: the frame will be processed only if it is received on a port in the Forwarding state, and it will be fowarded out only through ports in the Forwarding state. However, the frame absolutely does not have to go all the way to the root bridge and back.

Best regards,

Peter

2 REPLIES
Cisco Employee

Re: STP Doubt

Hello,

The Case 2 is correct. The STP does not change the way how switches forward frames. That principle remains even with STP: whenever a switch receives a frame, it tries to find its destination MAC in its switching table. If it finds it, it forwards the frame out that port only. If it does not find the destination MAC, it will forward the frame out every other port. With STP, this procedure changes only slightly: the frame will be processed only if it is received on a port in the Forwarding state, and it will be fowarded out only through ports in the Forwarding state. However, the frame absolutely does not have to go all the way to the root bridge and back.

Best regards,

Peter

New Member

Re: STP Doubt

Second case is true, its has nothing to do for a packet to move to Root Bridge and then back, what it does is that frame frm PC1 leave so Switch will see the Switching table (mac-address-table) and will forward to the specific port, if no entry is there in the table so the packet will be broadcast on all ports of the switch.

STP just had the full info, topology info etc.

You can also see a bit detail on STP on my blog, see the link

http://sohailpk.blogspot.com/2008/03/switching.html

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