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3750 traffic going through stack member only for fiber uplink


We have 2 3750 switches(Master & stack member) stacked using a stacking cable & configured for inter vlan routing(L3).

Each 3750 has 1 SFP fiber module(MM fiber) connected to each of the 2 3560(3560_1 & 3560_2) switch in 1st floor.

3560 _1 is connected to Master 3750 switch & 3560_2 is connected to 3750 stack member.

Problem - Though SFP fiber module in 3560_1 is coonected to master,the traffic is always going through 3560_2 connected to stack memeber.

Test done to check this - 1.Connected a desktop to 3560_2,ping a host in the network.

2,removed the fiber connected to 3560_1.No "request time out" found.

3.Inserted the fiber to 3560_1.After say 5 min,removed the fiber connected to 3560_2.Found 8 "request timed out" & the link came back.

Problem - why is the traffic going through 3560_2 fiber uplink when the fiber on 3560_1 is connected to the Master??

But other 3 ports on each Cisco 3750 are connected to SFP copper module to uplink for other floors(two from each floor connected one each to 3750 switch).

But it works opposite(which is how I feel should work) for other floors where we have copper uplinks through SFP copper module.

Config on all copper & fiber uplinks ports are same.

Pl let me know if any one has faced this problem




Re: 3750 traffic going through stack member only for fiber uplin

I think issue may be with CEF.Because CEF by default uses Per Destination Load Balancing" and if it is enabled on an interface it will forward packets based on the path to reach the destination. If there are two or more parallel paths exists for a destination, CEF will take the same path (single path) and avoid the parallel paths. This is due to the default behaviour of CEF. This is true in cases when load-sharing is done simultaneously on interfaces of different physical type, such as Serial andTunnel. This happens because of hash alogrithm which determines the path to be chosen. To utilize all the parallel paths in CEF and load balance the traffic, you have to enable when you haveto enable per packet load balancing different physical interfaces like serial and tunnel. So depending on the configuration and topology (Serial or Tunnel) Load Sharing may not work correctly with the default CEF Load Balancing mode

New Member

Re: 3750 traffic going through stack member only for fiber uplin

Have you check spanning tree to see if it is blocking the port? Also, you coud configure etherchannel and use both links as one.

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