Some must to know information about VPNs

VPN - Virtual Private Network is most common overlay mechanism in Networking. We have many of them, GRE, mGRE, IPSEC, DMVPN, GETVPN, LISP, FlexVPNs, MPLS VPNs and so on. But what are the important and fundamentals thing about VPNs?.In this post I will explain some of them.  
Virtual Private Network is the logical entity, which is created over a physical infrastructure. It can be setup over another private network such as MPLS or public network such as Internet.

All VPN technologies add extra byte to the packet or frame, which increases the overall MTU so the network links should be accommodated to handle bigger MTU values.   VPN technologies work based on encapsulation and decapsulation.   For example GRE, mGRE and DMVPN encapsulate IP packets into another IP packet, VPLS and EVPN encapsulates Layer 2 frame into an MPLS packets.

  You can run routing protocols over some VPN technologies but not all VPN technologies allow you to run routing protocols. In order to support routing over tunnel, tunnel endpoints should be aware from each other.   For example MPLS Traffic Engineer tunnels don’t support routing protocols to run over, since the LSPs are unidirectional which mean Head-end and Tail-end routers are not associated. This will be explained in detail in MPLS chapter.

About VPNs, if you are looking for much more detail knowledge, from design and hands-on perspective, please refer to my VPN Training.
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Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021

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