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Cata 6509 - Latency increase when 2 L3 switches default route to the same IP of the LAN.

Hi everyone

       I get a latency increase issue on the topology 1 below:

  1.       3750 connect to pc by L3 interface
  2.       3750-A and 3750-B connect to 6509 by L2 trunk interface.
  3.       3750-A, 3750-B has it own vlan interface and assign IP for the vlan interface.
  4.       Both 3750-A and 3750B default route to 65's ip
  5.        static route to, next hop is on 6509
  6.        static route to, next hop is on 65091

Now I need to send a packet from PC1 to PC2 by using hrping

packet information is transfer rate=200pps size=64byte, timeout is 20 sec

I found the respond time is get higher. when the respond time over about 12 sec, the hrping show up the request timeout.

For some reason, the 3750-A and 3750-B must be send all packet to 6509's ip and route to the destination.

The ARP learning is OK, routing table is OK, input and output queue is empty, all the interface is no error(show interface).

If I change the topology from topology 1 to topology 2, the hrping will not show the request timeout anymore. But the most important in my side is the root cause of the request timeout.

Is anyone know the root cause or guide me for some way?


topology 1

topology 2


Hello.What line card do you


What line card do you use on 6509 to connect to 3750 switches?

Please provide "show runn int vl102", "sh mls cef", "show ver" from 6509. 

Could you please provide the description of what is hrping?

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