Cisco CCNA GRE Tunnels

GRE is a Cisco developed protocol which is one of many tunneling protocols.  While it is a Cisco developed protocol, it is also defined in several RFCs (1701, 1702 and 2784).

GRE encapsulation supports any OSI Layer 3 protocol.  This is a big benefit.

GRE does not include strong security to protect its payload.  So you will want to couple this with some other type of technology to protect your data.

Cisco CCNA GRE Configuration

The configuration example in the slide depicts a simple GRE tunnel configuration between routers R1 and R2.  Minimal configuration requires specifying the tunnel source and destination (public IP Addresses), a subnet for the tunnel (private IP Addresses) and encapsulation mode (GRE).

•Configuration example of a GRE tunnel is as follows:

R2(config)# interface Tunnel0

R2(config-if)# tunnel mode gre ip

R2(config-if)# ip address

R2(config-if)# tunnel source

R2(config-if)# tunnel destination

Cisco CCNA GRE Verification

Some commands the can be used to verify / troubleshoot are as follows:

show ip interface brief | include Tunnel – This command can be used to determine is up or down.

show interface Tunnel 0 – You can also verify the state of this interface with this command.  Note: The line protocol on a GRE tunnel interface is up as long as there is a router to the tunnel destination.

show ip route – This command is used to verify there is a router between the tunnel endpoints.

Cisco CCNA Summary



There are many WAN connection types:
•Circuit Switched
•Packet Switched
•There are multiple encapsulation methods:
•Point-to-point protocol (PPP)
•High-level Data Link Control (HDLC) – Cisco default encapsulation for serial links; Cisco Proprietary
•PPP provides for authentication, there are two types:
•Password Authentication Protocol (PAP)
•Challenge Handshake Authentication Protocol (CHAP)
•Frame Relay is a packet switching network.
•Frame Relay is not a particular interface, but rather an encapsulation type.



Cisco CCNA Summary


•VPNs are easy to implement and much less expensive and easier to scale than dedicated WANs
•VPNs offer the capability for secure communications
•GRE is a tunneling protocol that can encapsulate any Layer 3 protocol with an IP tunnel
•GRE does not provide encryption