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Layer 2

Posted on 2013-11-12
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Last Modified: 2013-11-12
Hi All,
I know this may seem like a lame question and I sort of understand it, but I'm trying to get a "better" if you will --- understanding of L2 links across WANs and MANs.  

Frame Relay seems like an obvious example but it still uses L3 (right) so in today's networks using metro-Ethernet and fiber I'm looking for some clearer examples (explanations) and trying to better understand what the glue is that bonds L2 over WAN links.  Sure I can Google all day but I just keep seeing examples of what L2 does and what L3 does and not how L2 links across WAN links actually work.

Thanks,
R
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Question by:rotarypwr
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by:djcanter
ID: 39642829
Layer 3 = IP routing.
On Layer 2 connnections like  frame relay, and fiber an ethernet frame enters one end of the connection and exits the other in its original form. With a layer 2 connection, both sides of the connection can share the same ip subnet, but typically you will employ a router at both sides of the connection to provide segmentation and routing capabilities.

Layer 2 is the datalink (ethernet / mac address layer). Devices communicate over layer 2 via mac address. Layer 3 communication occurs via ip, then down to layer 2 mac after it reaches the correct destination subnet.
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by:rotarypwr
ID: 39642875
Thanks.

I understand what the layers do 1 - 7 but I seem to missing something that's right in front of me.

Router 1 IP : 192.168.1.1/252
Router 2 IP:  192.168.1.2/252

Router one is in Building 1 and Router 2 is in building 2 10 miles away over a fiber, long haul link.  So when they call this a layer 2 link, it's actually glued together via L3 but it's probably trunked so it can carry multiple VLANS, right?  I'm still learning so appreciated the clarification.  I think I understand it now?  But really it's not able to carry data until you configure L3?
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by:djcanter
ID: 39642891
Actually its glued with layer 2. All traffic from router 2 to router1 will occur via mac address. Assuming the fiber comes into a media converter and comes out as an etherent cable, you could unplug the ethernet from the routers on both ends and connect to a switch at either end and bridge the network just  like connecting 2 local switches together.

Layer 2 connectivity is exactly a network bridge
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djcanter earned 500 total points
ID: 39642898
To add to my previous, the provider has segmented you into a vlan on their network, but you could also run your own vlans over the bridge if they support QinQ, also called Vlan inside Vlan .
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by:rotarypwr
ID: 39642934
Thank you for explaining, much appreciated!
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