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New Member

Dual Network Core Design


Can someone please confirm if I truly understand the concept of VLANs communicating through different Switch Blocks...

VLAN A is configured on an access switch which in turn is direcltly connected to the multilayer distribution Switch1. Switch1 is directly connected to Switch2 and SW2 is directly connected to a second access switch, again configured with VLANA - all within the same Switch Block. Now, the L2 link connecting SW1 & SW2 will normally use a trunking protocol to transport many VLANs - say ISL. In other words trunking is used within a Switch Block.

I start to get troubles understanding how hosts on VLANA communicate with hosts in VLANA configured in another Switch Block. As far as I can make out L3 routing is used between the Distribution layer and the Core so does this mean that trunking is only used between distribution layer switches within the same Switch Block? How is the VLAN information preserved when routed through the Core?

Thanks for clearing this up for me

Hall of Fame Super Blue

Re: Dual Network Core Design

Hi John

access-switch connected to Switch1 = as1

access-switch connected to Switch2 = as2

If a host on as1 in vlan 10 wants to talk to a host on as2 in vlan 10 there is no routing involved at all. As long as you have a layer 2 trunk between Switch 1 and 2 then the host on as1 will simply arp out for the mac-address of the host on as2. As1 will send the traffic to switch1 which passes it on to switch2 and then on to as2. All the traffic stays within the same vlan so it is switched at layer 2.

If your distribution block is connected to the core via layer 3 routed links then if you had a vlan 10 in one switch block which connected to a pair of distribution switches, and another vlan 10 in another switch block connected to a different pair of distribution switches this will not be the same vlan.

if you route to the core you do not extend vlans across distribution switches ie. a pair of distribution switches connected via a layer 2 trunk will have the same vlan but you can't extend this vlan to another separate pair of distribution switches.

Hope this makes sense


New Member

Re: Dual Network Core Design

Thanks Jon

I realise that Cisco recommends implemnting Local VLANs design meaning that VLANs should not extend beyond the distribution switch. Also, a VLAN host might want to access remote services on a different subnet and so L3 routing might (or might not) be required to cross the core.

However, lets say the design is not so good and a VLAN does cross Switch Blocks - how would the VLAN infrmation be transported given that VLANs terminate at the distribution switch. Only L3 routing is used at the core - right?

Hall of Fame Super Blue

Re: Dual Network Core Design


I'm not sure i understand fully. Lets say you have 4 distribution switches

ds1, ds2, ds3, ds4

ds1 connects to ds2 with a layer 2 trunk.

ds3 connects to ds4 with a layer 2 trunk.

ds1/ds2 connect back to a core pair of switches using layer 3 routed links.

ds3/ds4 connect back to the same set of core switches using layer 3 routed links.

You have 10 access switches connecting into ds1/ds2.

You have another 10 access switches connecting into ds3/ds4.

The connections for the access to the distribution are layer2.

Now if you have a vlan 10 on ds1/ds2 you can also have a vlan 10 on ds3/ds4 but it will not be the same vlan ie. it will not have the same subnet range.

If you want a vlan to cross switch blocks then you cannot use layer 3 links to the core assuming your switch blocks are defined by a set of access switches and a pair of distribution switches.

You may be able to do something with bridged interfaces/GRE tunnels etc. but then if you are having to do that you should probably be thinking about a redesign.



New Member

Re: Dual Network Core Design

Hi Jon

Ok now I get it.

Good explanation!