GRE
Lesson 2: GRE Tunnel Theory and Configuration
As its name suggests, a Generic Routing Encapsulation (GRE) tunnel can encapsulate nearly every type of data that you could send out of a physical router interface. In fact, GRE can encapsulate any Layer 3 protocol, which makes it very flexible.
GRE by itself does not provide any security for the data it transmits; however, a GRE packet can be sent over an IPsec VPN, causing the GRE packet (and therefore its contents) to be protected. Such a configuration is commonly used, because IPsec can only protect unicast IP packets. This limitation causes issues for routing protocols that use IP multicasts. ...
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