campus network ,hierarchical design

Answered Question
Sep 19th, 2008

hi every body!

Campus network, according to my book is designed in hierarchical fashion.

Three layers ,access layer, distribution layer,core layer are the layers, campus network is designed around.

My question is about core layer, implemented using layer 2 switches( i understand it is very rare)

let say distribution switch(multilayer switch) has two links to core switches(layer 2).

Will distribution switch use different subnet for each links connected to core switches (layer 2)? or distribution should use same subnet on each links connected to core.

My impression is the links between distribution switch and core shold be in same subnet which requires distribution switch should same subnet for its links ,connected to core. can we assign same subnet to distribution ports say f0/1,f0/2, connected to core?

thanks a lot!

I have this problem too.
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Correct Answer by Jon Marshall about 8 years 2 months ago

Sarah

You definitely would not want to extend a vlan from the access to the core - STP etc. would make this bad pratice. If you want a L2 core you want it to switch packets as efficiently as possible so you absolutely do not want a vlan convergence in the access-layer propogating to the core.

What i was getting at is that in a traditional access-layer, distro, core setup your access-layer vlans are terminated on the distribution switches and are not present in the core.

For the distribution switches to be able to switch traffic across the core there will other vlan(s) not present in the access-layer that are used for this purpose.

The thing to understand is that you will have more than one distro switch connecting to the core. So lets say you have

core1 & core2 connected via a L2 trunk.

distro1 is connected to both core1 and core2

distro2 is connected to both core1 and core2

You can use one vlan to switch traffic across the core from distro1 to distro2 and put all distro ports into that vlan. But note that one of the links from distro1 and one from distro2 will block because of STP.

Alternatively you can use 2 vlans in the core and have one connection from distro1 and one connection from distro2 in one of the vlans and the other connection from distro1 and distro2 in the other vlan.

Each distro switch sees the other distro switch on 2 vlans - sees here being used in the context of a routing peer ie. EIGRP/OSPF neighbor.

Personally i would use 2 vlans across the core.

Jon

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Jon Marshall Fri, 09/19/2008 - 08:07

Sarah

Firstly a routed core is becoming more common these days so L2 in the core is not the only choice. L2 was often recommended before L3 switches increased performance of routing to wire-speed.

Secondly a lot of designs you will see now are collapsed core/distribution so a lot of the design element is around whether to run L3 from the access-layer to the core/distro layer rather than from the distro to the core layer.

To answer your question with L2 links it;s better to think of it in terms of vlans rather than subnets. And there are a number of ways of approaching it. You need to extend the L2 vlan across the core switches so if a distro switch has 2 links these can be in separate vlans but the other distro switch also needs it's 2 links in the same vlans so traffic can be routed on the distro switches and switched across the core.

Jon

sarahr202 Fri, 09/19/2008 - 09:35

thanks jon!

Is extending vlans from access into core not good design practise?

my point is can distribution switch , connected to layer 2 core redundantly by f0/1,f0/ use different subnet on those links or should it use same subnet on these ports?

Correct Answer
Jon Marshall Fri, 09/19/2008 - 09:48

Sarah

You definitely would not want to extend a vlan from the access to the core - STP etc. would make this bad pratice. If you want a L2 core you want it to switch packets as efficiently as possible so you absolutely do not want a vlan convergence in the access-layer propogating to the core.

What i was getting at is that in a traditional access-layer, distro, core setup your access-layer vlans are terminated on the distribution switches and are not present in the core.

For the distribution switches to be able to switch traffic across the core there will other vlan(s) not present in the access-layer that are used for this purpose.

The thing to understand is that you will have more than one distro switch connecting to the core. So lets say you have

core1 & core2 connected via a L2 trunk.

distro1 is connected to both core1 and core2

distro2 is connected to both core1 and core2

You can use one vlan to switch traffic across the core from distro1 to distro2 and put all distro ports into that vlan. But note that one of the links from distro1 and one from distro2 will block because of STP.

Alternatively you can use 2 vlans in the core and have one connection from distro1 and one connection from distro2 in one of the vlans and the other connection from distro1 and distro2 in the other vlan.

Each distro switch sees the other distro switch on 2 vlans - sees here being used in the context of a routing peer ie. EIGRP/OSPF neighbor.

Personally i would use 2 vlans across the core.

Jon

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