Layer2 or Layer3 between Core

[neilobrien]

Hi,

See attached - I have a two tier network - Access and Distro/Core.  All access switches come back to each core via 1Gb fiber.

If I have a layer2 link between the cores, STP kicks in and blocks one of the uplinks,

However, I have a layer3 link between them, obviously this breaks up the layer2 domain and allows us to use both uplinks and cores with something line GLBP.  But what are the disadvantages of this.  Why wouldn't you use a layer3 link?

Just looking for some opinions, maybe there's no right/wrong way to do it.

Thanks in advance,

Neil

[Julio Garcia]

Layer 3 design with first hop redudancy protocol  like hspr , vvrp etc definately better

With regards to disadvantages , i can only think of one, say you have svi or access port on the core itself and it wanted to talk to access vlan or svi  on other core, it will need to go via the access switches as there is no direct layer 2 between them.

there is another way of doing this , using vss ( need special supervisor) , the 2 core switches will form single virtual switch (like a stack),  you can keep everything layer 2 , and form etherchannels with the 2 uplinks from each access switch, that way there is no stp blocking  as the uplinks will be a single logical link.

[neilobrien]

THanks for the info Rob.

VSS did come to mind but I'm running 2x 4507's, I believe that VSS is only supported on 6500's.

Rgds,

Neil

[esomarriba]

Hi neilobrien,

I would definitely go with Layer 3 between the core's. I would get a IP subnet block /30 and get this rolling. HSRP/GLBP will be a good idea! I don't like having STP running on my core area's. STP brings many problems into the network, and sometime Level 1&2 don't know how to troubleshoot complex STP topology.

So, my personal opinion will be, keep the network topology simple. AND REMEMBER, simple is already better. Make sure you document the network, and think about it while you are doing it. Does this make sense to other??? If Yes, you are in the right track!

HTH,

Elyinn.-

Senior Network Eng.

[neilobrien]

thanks Elyinn,

I was thinking I was missing something implementing it this way.  I woulnd't want STP running through the core either, plus the benefits of utilising both uplinks and both cores as gateways far out-weighs anything else.

Thanks again,

Neil

[David Kosich]

Just to add what was already said--

VSS is going to be supported on the 4500's in the near future, using SUP7-Es. There's no timeline yet for that release last I heard.

Also, many customers feel the same way about trying to avoid L2 in the core. Working on the LAN Switching team in TAC, we see many cases where there are network outages due to bridging loops  A layer 3 core would definitely help avoid the spanning-tree pitfalls that often come with the loss of BPDUs.

Dave

[wandering_997]

Hi Neil,

My opinion, if you make layer2 loops, then you have to face it, and stp or vss-like tech will be good choice.

STP has only half bandwidth of uplinks for access switches, there's no effect to the etherchannel bwteen cores.

VSS or VPC gets full bandwidth but you only have 4500.

So I don't think layer-2 or layer-3 between cores is the key point in this case.

Maybe you should deploy a full layer-3 network between all switches, accesses and cores, then you have multi-homing problem.

If there's already a perfect solution, why we talk about so much?

Wandering