traffic issue on Cat6500 switch

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Mar 13th, 2009
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We have two Cat6500 switches (core1 and core2)interconnected with trunk port. we also have a few of access switches to uplink to two core switches with trunk port.In core switch, we use SPT (RPVST)for L2 load balancing and HSRP for L3 load balancing according to odd and even Vlan. For example,root bridge of SPT and active VIP of HSRP of odd vlan are in the core1 Otherwise, it is in the Core2. My question is the traffic would go through two core switches when even vlan would try to access odd vlan,right?


There is a backup server in the odd vlan. we need backup all of data to that server during the midnight. I suspected I should see the huge traffic in two core switches when other hosts in the even vlan tried to access odd vlan. But so far the huge traffics I only saw were in core2 switch.


Please advice. I would apprecaite it!

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glen.grant Fri, 03/13/2009 - 16:14
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I believe that would be incorrect . HSRP will force traffic to go out a particular side but if core 2 has an unblocked interface back into the other vlan it will go directly back in , hsrp only dictates which side will have the live default gateway for the vlan . Core 2 will see it has a directly connected route to that subnet and just send it back down the other vlan . If the network topology is such that spanning tree is actually blocking the side back in then it would have to travel up and over and back in . You can verify that by seeing where the SPT is blocking if at all , really depends on the topology and if spanning tree has blocked certain ports. Someone else can chime if my reasoning is incorrect , wouldn't be the first time . :-)

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