Access layer design with dual Core\Dist switches

[serotonin888]

Hi,

I am trying to understand the pros and cons of the two differing designs for an access layer with dual L3 switches acting as collapsed core\distribution.

For example.

If we have two layer 3 switches DIST1 and DIST2. Both running HSRP with DIST1 the active for all vlans and also configured as the root bridge. DIST2 is the standby also is also the secondary root bridge.

At the access layer in one of the remote data clsoets we have 3 Cisco 2960's (not my choice but this what I have). There are 2 uplinks available from the remote closet to DIST1\2.

My question is is it better to have one of the 2960's act as the uplink to DIST1 and DIST2 and chain the 2nd and 3rd 2960's from the first OR have the 1st 2960 uplink to DIST1, the 2nd 2960 connects to the 1st and 3rd, and the 3rd connects to the 2nd and uplinks to DIST 2.

So Option 1 looks like this

DIST1-----------DIST2

  \              /

   \            /

    \          /

     \        /

      \      /

         1st 2960

      /      \  

2nd 2960   3rd 2960

Option 2 is like this

DIST1-----------------DIST2

|                        |

|                        |

1st 2960---2nd 2960---3rd 2960

Option 2 provides the benefit that if any of the access layer switches fails then the others could still connect to the core.However, it also means that the 3rd 2960 will have use the path via DIST2 as the STP path cost is less than going via 2nd, 1st then DIST1. I suppose we could modify the path cost on the uplink from 3rd to DIST2 but I am not sure if this a good idea? Or what other implications I would need to be aware of?

Option 1 is simplier and we dont have the STP path issues, but it also means that 1st 2960 is a single point of failure.

I would be interested to know what others think and how you might have addressed this in your own networks?

Thanks

[Jon Marshall]

serotonin888 wrote:
Hi,
I am trying to understand the pros and cons of the two differing designs for an access layer with dual L3 switches acting as collapsed core\distribution.
For example.
If we have two layer 3 switches DIST1 and DIST2. Both running HSRP with DIST1 the active for all vlans and also configured as the root bridge. DIST2 is the standby also is also the secondary root bridge.
At the access layer in one of the remote data clsoets we have 3 Cisco 2960's (not my choice but this what I have). There are 2 uplinks available from the remote closet to DIST1\2.
My question is is it better to have one of the 2960's act as the uplink to DIST1 and DIST2 and chain the 2nd and 3rd 2960's from the first OR have the 1st 2960 uplink to DIST1, the 2nd 2960 connects to the 1st and 3rd, and the 3rd connects to the 2nd and uplinks to DIST 2.
So Option 1 looks like this
DIST1-----------DIST2
  \              /
   \            /
    \          /
     \        /
      \      /
         1st 2960
      /      \   
 2nd 2960   3rd 2960
Option 2 is like this
DIST1-----------------DIST2
|                        |
|                        |
1st 2960---2nd 2960---3rd 2960
Option 2 provides the benefit that if any of the access layer switches fails then the others could still connect to the core.However, it also means that the 3rd 2960 will have use the path via DIST2 as the STP path cost is less than going via 2nd, 1st then DIST1. I suppose we could modify the path cost on the uplink from 3rd to DIST2 but I am not sure if this a good idea? Or what other implications I would need to be aware of?
Option 1 is simplier and we dont have the STP path issues, but it also means that 1st 2960 is a single point of failure.
I would be interested to know what others think and how you might have addressed this in your own networks?
Thanks

Well there are 2 obvious solutions -

1) have more fibre runs

2) use a modular switch or 3750 stack in the access-layer

however i'm sure you have thought of these and i will assume they are not possible at the moment.

Do you have a single vlan on the 2960 switches or multiple vlans. If you have multiple vlans with option 2 and some careful planning you could utilise both links by having DIST1 HSRP active/STP root for some vlans and DIST2 HSRP active/STP root for the other vlans.

Another possible approach is to connect the DIST switches with a L3 link do there is no L2 loop and then both uplinks could be used for all vlans. There are considerations to take into account if you do this though, such as do you have any directly connected servers etc. to the DIST switches.

Option 2 is better if you do not have the fibre runs. It provides more redundancy as you say and this is always one of the key things you should  be looking for in a network design.

Jon

[serotonin888]

Thanks Jon,

Option 2 does seem to provide more benefit over option 1.

If I had been involved in the original design then I would also have used 3750's

Thanks for you feedback

[sachinraja]

Hi Serotonin

No doubts of the solution given by Jon. Option 2 all the way..  When designing networks, we look at scalability, reliability, high availability, capacity, performance etc, which directly relates to option 2...

option 1 has lots of downsides primarily Single Points of Failures & performance issues. with 2960's you would have 2 Gig uplinks (unless it is a all port gig switch), and these 2 Gig uplinks would be connected to the core switch. So, in Option 1, for connecting the other 2 2960's in downstream , you would have to use Fastethernet links, and thats not a good idea ....

Option 2, looks a standard design, but as mentioned, makes sure STP & other layer 2 parameters are configured fine. If you are in planning stage, as Jon mentioned, see if you have a budget for 3750. it has numerous advantages as compared to 2960..

Hope this helps.. All the best..

Raj