ELANs question

A VLAN is a virtual LAN.   In its simplest form, it lets you subdivide a switch into virtual "sub switches",   which provides reduction of broadcast traffic  (normal broadcast traffic does not cross VLANs),  and provides security isolation:

One VLAN cannot communicate with another, except through a router, bridge, or multilayer switch you have setup to connect the two VLANs together.

In any case, you can define access lists or setup a firewall later at will, controlling what traffic may cross between VLANs.

"Is there a central termination point for ELANs, where all of the interVLAn routing occurs?"

No, you can design your network with a central termination point for most VLANs, and that may be how your specific network operates, but it is a design choice (possibly a choice not to spend the money on other switches with Layer 3 routing functions), any device that has an IP address on a VLAN, and an IP address on another VLAN, is potentially capable of routing or forwarding packets from one VLAN to another, if it supports routing, that function is enabled.

Your network design should indicate which multilayer switches or dedicated routers/firewalls  handle  routing between which VLANs.

There's nothing about how VLANs work that prevent you from using multiple different routers.

You could design a network that uses one router for certain VLANs, and another switch for others.

It's also possible in some designs to have redundant routers running various  protocols to deal with failure situations.

so what is the real difference between ELANs and VLANs?  Verizon offers an "ELAN" service to us. It looks like it goes over T1s:


Site1-----ELAN------Site2
Site3------|    |-------Site4


We have our Serial interfaces on each router in a private IP scheme. But I'm told we could have made it anything we wanted to, since it is internal.

Ok, so you're referring to Emulated LANs, as in the LANE spec.
The underlying technology is definitely different from straight Ethernet, and a ELAN connection does have to land on a device that can run the ELAN Client, normally this would be a router or L3 switch.

An ELAN is essentially  Ethernet being piped over ATM.
The model is client-server.

It's emulated, not really an Ethernet LAN per se, because the protocol is entirely different.

And it's definitely a different experience from having private copper, and actual VLANs.

A T1 is not very much bandwidth, and you don't want to be sending broadcast traffic over that limited link, using a router is a must.


If you had bigger pipes (like fiber),  in a pure VLAN setup (no ELANs), you would actually be able to extend some infrastructure VLANs  between two places, and have  machines a fair distance away on the very same subnet (Just switching, no routing).

And if you took reasonable care, the broadcast traffic would not be an issue.

But you won't be doing anything like that with T1s and ELANs.

can you speak more to the client / server model?  

thanks