Core Switches Connectivity Issue

Unanswered Question
Feb 5th, 2007

Setup is like below: -

Two Core Layer 3 switches A & B. Two Access Layer 2 switches C & D.

Each access switch C & D has two individual links connecting to two core switches for redundancy.

A & B are running HSRP in a way that A is HSRP Active for VLAN 10 and B is HSRP Active for VLAN 50. Also A is STP Root for VLAN 10 and B is STP Root for VLAN 50.

My question is: - In my Setup, do I need to have a dedicated link between the two Core switches??

And if I don't have the dedicated link, then what could be the implications in any kind of failure scenario.

I have this problem too.
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Jon Marshall Mon, 02/05/2007 - 01:59

Hi

Assuming your links from the access-layer to the core switches are layer 2 links then yes it would work without a connection between the core switches. HSRP traffic would go over the access-layer links which might or might not be what you want.

Where there would be an issue would be you could end up with suboptimal paths.

eg. Switch C's path to Switch A is forwarding and a client on switch C in vlan 10 wants to talk to a client on switch D in vlan 50.

Traffic goes from switch C to A (HSRP active for vlan 10) is routed onto vlan 50 and down to switch D.

If switch C to A link goes down then switch C now sends traffic to switch B but your'e active gateway is still on switch A. So B sends traffic to D which is then sent to A. Then traffic is routed onto vlan 50 and sent back from A to D.

If you had a link between A & B switch C would send traffic to B which would send it to A which would then send it to D.

Of course the con is that if you connect your core switches together you now have created layer 2 loops in your networks and spanning-tree now has to step in and block some of your links.

HTH

Jon

rohit_s Wed, 02/07/2007 - 08:44

Hi Jon,

Thanks for the nice explanation. So as per your description, link between core switches is only required to have a direct path between the core and to have optimal flow of traffic. If required then it may be avoided (but not recommended)

Hope I have correctly understood your explanation...

Thanks

Rohit

Jon Marshall Wed, 02/07/2007 - 09:49

Hi Rohit

As long as your trunk links allow all the vlans that you have SVI's for then HSRP should just switch via the access-layer.

I probably wouldn't design a solution like that though :-)

Jon

mahmoodmkl Mon, 02/05/2007 - 02:06

Hi

I would suggest you to configure u r core swithes one a primary and other as secondry for u r vlans.

As u said that u have redundant links to both the core switches anyhow one link will be in blocking state.

its better to have link between u r switches i would prefer a L3 link between u r core switches.As HSRP it would need to exchange hello messages to maintain active/standby relationship.

Thanks

Mahmood

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