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.
- On Router1, create the tunnel interface named Tunnel1
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
- Set the IP address of the tunnel interface
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
- Activate the tunnel interface
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
- On Router3, create the tunnel interface named Tunnel1
config t interface Tunnel 1
- Set the tunnel mode to gre ip
tunnel mode gre ip
- Set the IP address of the tunnel interface
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
- Activate the tunnel interface
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
- From Router1, ping the tunnel interface on Router3
ping 172.16.0.2
- From Router3, ping the tunnel interface on Router1
ping 172.16.0.1
- Additional verification
- Configure a loopback interface on Router1
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
MOISES CARABALLO
August 12, 2019 at 8:06 pmI cannont open or download this lab. When I tried to, it crashes.
Joe Barger (CCNP/CCDP) • Post Author •
August 12, 2019 at 9:15 pmTry opening it with Firefox. Chrome and Internet Explorer tend to not work well.
MOISES CARABALLO
August 13, 2019 at 10:06 pmFirefox worked. Thank you.
BTW. Recently passed my CCENT w/ the help of your website.
Joe Barger (CCNP/CCDP) • Post Author •
August 14, 2019 at 6:53 amThat’s great news. Congratulations on passing the CCENT!
Ilyas el Ouasghiri
February 2, 2020 at 5:01 pmcorrection on the test phase.
ip addres 172.16.0.1-2 to ping not 172.168.0.1-2
Joe Barger (CCNP/CCDP) • Post Author •
February 5, 2020 at 10:56 pmThank you. The lab has been updated.