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In most of large ISP networks, physical interfaces are used as switch ports (layer 2 port) and VLAN interfaces are used as layer 3.

Could you please explain what is the main purpose to use like this?

closed as off-topic by Ron Maupin Sep 11 '17 at 22:02

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "NE is a site for to ask and provide answers about professionally managed networks in a business environment. Your question falls outside the areas our community decided are on topic. Please visit the help center for more details. If you disagree with this closure, please ask on Network Engineering Meta." – Ron Maupin
If this question can be reworded to fit the rules in the help center, please edit the question.

  • "In most of Large ISP networks, physical interfaces are used as switch ports (layer 2 port) and VLAN interfaces are used as layer 3." Why do you believe that? Each ISP does things its own way. Unless you work for a particular ISP, you have no way to know how it configures its network. – Ron Maupin Sep 11 '17 at 18:34
  • The OP is absolutely correct. Switchports are link layer (layer 2) in nature (and usually physical, but not always, e.g. vswitches or nfv). The layer 3 gateway address, providing Internet layer connectivity, is implemented back at their aggregation equipment. VLAN interfaces are simply the nomenclature used in the industry to refer to the layer 3 interface. – Ron Royston Sep 12 '17 at 2:26
  • i think layer 3 and layer 2 connectivity is also part IP network and ISP Core network managed by professional. not sure, one thing may using such addressing due to scalability. I just want to confirm. – Jatinder Brar Sep 12 '17 at 2:48
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Not sure what you're aiming at, but the main point is scalability: you bind your L3 interface to a VLAN. A VLAN is a virtual entity as large as the L2 segment - not even limited to a single switch. Essentially, you can keep adding physical interfaces to it as long as you like.

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Where does it use switch ports ? For transit routes ? for peering ? At route exchange points ? In general layer 2 ports are plug and play and do not need any explicit protocols to run. Hence preferred when the target is to lower overhead and CPU utilization caused due to various routing protocols and when unnecessary broadcass can be tolerated i.e in smaller networks. Secondly VLANs can provide isolation and basically split the LAN in different broadcast domain so can sometimes be as layer 3 instances. But these instances are logical and have obvious limitations.

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