Trunking on Home and Small Office Switches

I bought a couple of the Cisco/Linksys  SLM200x series switches (SLM2005 ,SLM2008) gig switches, for under $100 each.  I wasn't going to go for hundreds of dollars to get an IOS switch like I am used to plus the IOS stuff is still 10/100 for the bulk of the ports.

I was real happy to see the "Enable Jumbo Frames" check-box, I started to suspect that I could use one wire to connect two networks while keeping them separate.  Jumbo frames means a packet that is 4 bytes bigger than a standard Ethernet packet can still get through.

Whats 4 bytes bigger?  A packet that has been tagged with 802.1q trunking protocol headers. Whats interesting about 802.1q is that the native VLAN is still the normal size, I suspect there is a lot of equipment out there that works because the native packet makes it through and the trunked packets appear to alien too get any further.  Cisco's trunking protocol InterSwitch Linking(ISL) encapsulates every VLAN and means that you cant be flipping and trunking the interconnects between two switches without running to each side of the connection or being very careful in the order you do things.

In my case I have a business vlan I wanted separate from a test vlan and then on top of that I had VOIP. To do trunking I made sure the native vlans lined up on both ends and then selected a 2nd vlan on both sides of the link between the two switches using the vlan selector in the web based configuration.  I checked "allow jumbo frames" and for good measure I disabled the egress filters that select tagged packets, even though there was a setting for "all" packets.

Now you can break out a single port by making it be just a member of the 2nd VLAN.  In my case I spent $20 for a new gig Intel Ethernet card for the desktop and brought the trunk right into the desktop.  Again made the native line up and selected the next vlan.  My system now shows a total of three connectoids in Windows, one for natural interface and one each for each VLAN. I get IP addresses and DNS from 2 DHCP servers on two separate networks.

Oh yeah, you have to boot the switches after doing this much to them , I suspect that they need to build some forwarding tables from scratch.

Other nice things about the SLM series other than they are manageable in general: There are several ways to set up for QOS, port based or traffic type, and you can modify the priority mechanism a little or go to strict priority.  Just setting the port my phone was plugged into to be a hi priority port yielded the first 98% of the results I was looking for using strict priority.

They also do port monitoring for sniffiing and have a full multi-VLAN Spanning Tree implementation including portfast.  The device is sold as a Light Managed switch but for SoHo it's as managed as I needed, especially VLANs at gig speeds I am cutting down on a few cables by sharing.


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