2 buildings, 2 sg300s, same vlans in both

[jesslpete]

I have an sg300 in each of two buildings. Each building has 4 networks. Each network is a conceptual extension of the corresponding network in the other building. Each network has a DHCP server in building A. How is it best to configure the two sg300s so that all hosts on each network in either building are accessible to all the other hosts on that network and all can access their corresponding DHCP server?

My first naive attempt was to set up each sg300 as a switch (mode 2) with 4 VLANS and run 4 cables between each of the 4 VLANS. This only worked for the VLAN that was set up as the management interface (with an ip); in that one network all hosts were visible from either building. It does not appear to be possible to set an ip for each VLAN in switch mode. Hosts on the other three networks could not see hosts in the other building.  With 8 switches this would presumably have been easy but I am not seeing how to get the sg300 to act as 4 different switches.

What is the proper way to set this up? Should I change to mode 3 (router) so that I can assign an ip to each VLAN in each building?  I am not trying to route between different VLANS, only the same VLAN on the two different switches.

[Mark Malone]

Hey if you want hosts in 1 vlan to talk to another vlan you need intervlan routing so 1 of the switches has to be layer 3 , I would set building A as L3 as your dhcp server is there , give each vlan an ip address on that switch then create a trunk connection between the switches back to the other building and allow each vlan on that trunk and make sure those vlans are created on building B switch as well, that will allow vlans in both buildings and also all clients in each vlan speak to each other as the L3 switch will do that

the switch in building B should only have a mgmt. ip address on it , only required for remote reachability

[jesslpete]

Thank you for the reply.

I would not say that I want hosts in one vlan to talk to another vlan.  Rather, I need hosts in one building to talk to the hosts in the other building that are on the same vlan.  This seems more like stacking or daisy-chaining than intervlan routing.  I want each vlan in building B to be an extension of the corresponding vlan in building A.

For the first vlan that I set up, with a management ip address, the described daisy-chaining worked.  All hosts could see each other.  When I added other vlans to each sg300, they did not work.  It is not clear that daisy-chaining in that manner would be a good idea anyway.

I did try to change the building A sg300 to L3 mode, although I have not tried to set up a single trunk line in place of having 4 trunk lines, one for each vlan.  Surprisingly, the same vlan that worked in switched mode continued to work but the others still did not.   I have a few things that I can try but it would be nice to know the ideal way to extend the four networks to the other building.

[Mark Malone · Accepted solution]

ok if that's what you need you just need to make sure the vlans are allowed on the trunk connection between the switches and that the vlans exists on both switches at layer 2 and are named exactly the same with same id, once hosts are in the same vlan you do not need layer 3 interface to talk to each other as your hosts are in the same broadcast domain. You should be able to check that the macs from the hosts on each switch are in the mac table on each switch.

As its L2 you dont have much option other than daisy chaining switches as there's no stacking cable that's going to reach between buildings, the other option is to route between them at L3.

A trunk line is only really a trunk when more than 1 vlan exists on it other than that its an access link , you can have multiple trunks but stp will shut them down to prevent L2 loop , to avoid this bundle them together in and etherchannel combining the 4 to act as 1 logical link which also gives you,link redundancy but daisy chaining can also result in single point of failure if multiple switches involved , again you can cable them to connect to each other and use STP to block certain links on cost value which will only become active when your primary links fail between switches this negates the daisy chain issue of single failure

[jesslpete]

Thank you for the background explanation.   The problem was the cables that daisy-chained the individual VLANS.  When I removed those and made a trunk line with all four VLANS, it all started to work.  It does not appear to matter whether either sg300 is set to L2 or L3.

[Mark Malone]

No problem glad you got it sorted :)