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I am fairly new to Juniper as an engineer and I have been doing some reading lately concerning IRB versus vlans. I have noticed we use them in most configurations throughout our environment, IRB that is. Can anyone share some of the advantages associated with both of these? From my understanding VLAN is generally used more with our EX series equipment due to it being layer 2, I do however see it on some router configurations.

When would one use one over another or both in the same configuration for that matter?

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I suspect that you probably have a mix of First Generation (EX2200, EX3200, EX3300, EX4200, EX4500, EX4550) and Second Generation (EX2300, EX3400, EX4300, EX4600, QFX5100) hardware in your network.

First generation platforms assign L3-interfaces to vlans using vlan.x, whereas the newer platforms (much like the MX-series) use the Juniper's ELS (Enhanced Layer-2 Switching) configuration style which use the irb.x interface for assigning L3 to a VLAN.

Functionally they are identical, but you don't necessarily choose one over the other, merely use the style supported by the platform you are configuring.

Juniper have a complete breakdown of all the configuration differences between traditional EX-style and ELS-style here:

http://www.juniper.net/techpubs/en_US/junos13.2/topics/task/configuration/getting-started-els.html#jd0e564

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