CCNA 4.3: Configure, verify and troubleshoot GRE tunnel connectivity (FREE LAB!)

Joe Barger (CCNP/CCDP)

7 years ago

Overview

Generic routing encapsulation (GRE) is a simple site-to-site VPN tunneling protocol.

Study Notes:

Nonsecure, site-to-site VPN tunneling protocol
Used to tunnel IP packets across the internet
GRE header encapsulates an IP header and packet
- The IP header and packet is the letter
- The GRE header is the envelope
GRE can encapsulate any OSI layer 3 protocol, but it’s typically IP
GRE is encapsulation not encryption. There are no strong security mechanisms
GRE is stateless and does not include flow control
GRE header plus IP header equals 24 bytes (minimum) additional overhead
GRE tunnel interface addresses will be in the same subnet

Labs:

THIS LAB FREE!
PacketTracer Lab: CCNA-4.3-Configure-verify-and-troubleshoot-GRE-tunnel-connectivity.pkt

The objective of this lab is to establish a GRE tunnel between Router1 and Router3 and verify it is working by pinging across the tunnel.

config t
    interface Tunnel 1

Set the tunnel mode to gre ip
- tunnel = protocol-over-protocol tunneling
- mode = tunnel encapsulation method
- gre = generic route encapsulation protocol
- ip = over IP

        tunnel mode gre ip

        ip address 172.16.0.1 255.255.255.252

Set the source interface of the tunnel interface. This is Serial1/0 interface on Router1.

        tunnel source s1/0

Set the destination address of the tunnel interface. This is the S1/1 interface on Router3.

        tunnel destination 192.168.1.2

        no shut
        exit

Configure a route that tells Router1 how to get to Router3. To get to 192.168.1.2/32 go to 192.168.0.2.

    ip route 192.168.1.2 255.255.255.255 192.168.0.2

Configure a default route pointing to the Tunnel interface on Router3 so that all other traffic uses the GRE tunnel.

    ip route 0.0.0.0 0.0.0.0 172.16.0.2
    exit

config t
    interface Tunnel 1

        tunnel mode gre ip

        ip address 172.16.0.2 255.255.255.252

Set the source interface of the tunnel interface. This is Serial1/01 interface on Router3.

        tunnel source s1/1

Set the destination address of the tunnel interface. This is the S1/0 interface on Router1.

        tunnel destination 192.168.0.1

        no shut
        exit

Configure a route that tells Router3 how to get to Router1. To get to 192.168.0.1/32 go to 192.168.1.1

    ip route 192.168.0.1 255.255.255.255 192.168.1.1

Configure a default route pointing to the Tunnel interface on Router1 so that all other traffic uses the GRE tunnel.

    ip route 0.0.0.0 0.0.0.0 172.16.0.1
    exit

Verification
On Router1 and Router3, verify that Serial1/0 is up/up and that Tunnel1 is up/up

show ip int brief

On Router1 and Router3, view the tunnel interfaces. In this case only 1 has been configured.

show interface Tunnel 1

ping 172.16.0.2

ping 172.16.0.1

config t
    interface loopback1
        ip address 10.0.0.1 255.255.255.255
        no shut

On Router3, traceroute to the loopback interface on Router1 and notice how it uses the GRE tunnel to get there

traceroute 10.0.0.1
Type escape sequence to abort.
Tracing the route to 10.0.0.1

1 172.16.0.1 6 msec 2 msec 2 msec