What exactly the difference between Permanent Virual Circuit and Switched Virual Circuit? The X.25 and the Frame Relay are considered to be PVC or SVC? How?
Also how the GRE is considered a type of VPN (but ofcourse without secuirty)? What kind of those routes that are encapsulated in the GRE and how this encapsulation is useful for the communication?
How GRE offers tunnels IP over IP? What does it mean IP over IP tunnel?
SVC ( Switched Virtual Circuit ) : - An SVC is an on-demand connection that is dynamically established as per Customer requriment.
PVC ( Permanent Virtual Circuit ) : - PVC is a circuit that server until the service is terminated it is Permanent Circuit 24*7 .
GRE : Generic Routing Encapsulation
Tunneling protocol developed by Cisco that can encapsulate a wide variety of protocol packet types inside IP tunnels, creating a virtual point-to-point link to routers at remote points over an IP internetwork.
GRE is not refer as a VPN but when GRE is integrated VPN (Security ) then it is refer as GRE over IPSec or IPSec VPN.
GRE is only logical interface that's it.
Please refer for Frame Relay (PVC & SVC)
a PVC is permanent that means it is a logical circuit setup once and stays up all time regardless if traffic flows over it or not
an SVC is similar to a phone call: it is dynamically setup on demand and teared down when it is not needed anymore.
if we specify a destination that can be an X.25 address or an ATM NSAP address we are dealing with an SVC, if we just say use DLCI 40 to reach remote router R3 we are speaking of a PVC.
Most of X.25 routing uses SVCs ( with destination = X.25 address specified in x25 map command)
FR implementations most use PVCs, a specification for FR SVCs exists but it is not widely used.
GRE provides a way to hop over an IP network allowing routing of IP packets or non IP traffic that would not be possible or would create confusion otherwise.
a GRE packet is an IP packet that carries an IP packet in its payload
tunnel source = IP source of envelope
tunnel dest = IP destination of envelope
Hope to help
A PVC is a Virtual Circuit (VC) that is Permanently up and available. An SVC is a Virtual Circuit that is Switched (brought up when needed and dropped when not needed). X.25 is generally treated as SVC. Frame Relay is most often supplied as PVC but has to capability to operate as SVC also. Whether your particular Frame Relay uses PVC or SVC depends on what you ordered from your Service Provider.
GRE has been around for a long time and offers the capability to take some packet and encapsulate it, building a new IP packet. When I started in networking it was common that a company network might have some offices that were running IPX, or Appletalk, or some protocol like that. So you might have a situation that office A and office B were running that protocol and were connected by some network (perhaps even the Internet) which was IP only. You could use GRE across that network so that the IPX, or Appletalk, or whatever protocol could talk to each other but the network in the middle never saw the other protocol.
In todays networks it is rare to find protocols like IPX or Appletalk. But GRE is still a useful tool. Lets consider that company network where is has office A and office B. Lets assume that office A is using network 172.16.0.0 and lets assume that office B is using network 172.17.0.0. And lets assume that these offices are connected by DSL over the Internet. It is pretty clear that 172.16.0.0 and 172.17.0.0 can not be routed over the Internet. So the solution might be to run a GRE tunnel between office A and office B. You can run a routing protocol over the GRE tunnel so office A and office B learn each other's routes and can communicate but the Internet is not seeing 172.16.0.0 or 172.17.0.0. This is an example of tunneling IP in IP. And it also is an example where we can see that the GRE tunnel is sort of like a VPN. Both office A and office B can participate in the corporate network (they are virtually connected) even though they are not both physically connected to the corporate network.