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Server Load balancing issues

Unanswered Question
May 22nd, 2006
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the servers are loadbalancing between the switches '3'

and '2' and the link between the two switches is blocked.

This link was forwarding before and thus any traffic going to the server was

send to the Servers correctly no matter on which switch they are active.

However after addition of another link in between the switches 'root'

and '1', the path cost to the root has decreased and thus the link

between the '2' and '3' is Blocking and the other link between

the '2' and '1' is Forwarding as it should be ideally. But

this would be creating an issue because the trafic coming from outside i.e.

through switch '1' to the server will be correctly send to the

server if the server NIC is active on the '3' because the Virtual MAC

addresses are binded accordinlgy. In case the server falls onto the other

NIC which is on the '2' the traffic won't be able to pass because the

MAC address is not binded on the trunk connecting the switches '1'

and '2'. This binding cannot be done because the same MAC address is

being learned on the another trunk on the '1' which is connecting

to 'root'. So if we bind the same Virtual MAC on two trunks on the same

switch ('1') then this will cause MAC Address Flapping on the

switch and hence canot be done.

In another case,we can able to bind virtuak MAC on two trunks on the same

switch('1') and it's working fine.


The servers are load balancing in round robin fashion. each server has 2 NICs and work in Active-Passive mode. The servers load balance each other when all their active links are connected to '3' switch but when two of the active NICs of two servers are connected to '3' and the rest of the two active NICs from the other two servers are connected to '2' switch then only the forst 2 servers load balance and the other 2 servers do not load balance.


Please help.

Thanks in advance.



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Kevin Dorrell Mon, 05/22/2006 - 23:49
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In my experience, server load balancing is one of the most difficult things to get going properly in a switched LAN environment. Switched LANs are designed so that one MAC address can only be bound to one switch port. Therefore, if you have two NICs with the same MAC address (real or virtual), then you will get flapping somewhere.


I have seen various ways that the manufacturers try to get around this limitation of switched LANs. For example, one technique I have seen, practiced by ISA Server, is to use a multicast MAC address for the service so that frames go to both exit ports. But that does not always work well unless you tweak the network to acommodate it.


What sort of servers are they, and what system is used for the load balancing?


Kevin Dorrell

Luxembourg


Kevin Dorrell Mon, 05/22/2006 - 23:53
User Badges:
  • Green, 3000 points or more

In my experience, server load balancing is one of the most difficult things to get going properly in a switched LAN environment. Switched LANs are designed so that one MAC address can only be bound to one switch port. Therefore, if you have two NICs with the same MAC address (real or virtual), then you will get flapping somewhere.


I have seen various ways that the manufacturers try to get around this limitation of switched LANs. For example, one technique I have seen, practiced by ISA Server, is to use a multicast MAC address for the service so that frames go to both exit ports. But that does not always work well unless you tweak the network to acommodate it.


What sort of servers are they, and what system is used for the load balancing?


Kevin Dorrell

Luxembourg


sridharvaidyanathan Tue, 05/23/2006 - 01:50
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I was browsing through Cisco website for answers on this issue when i got noticed Cisco SLB (Server Load Balancing) on 6500 Switches. Can you let me know, if this could be the solution for my scenario since all my switches are of 6500 series with MSFC2.


Thanks,

Sridhar.

devang_etcom Tue, 05/23/2006 - 02:47
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  • Gold, 750 points or more

hi mevin dorrell,,,


when such a condition arise like two differnet lan (NIC) card having same MAC address???


regards

Devang

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