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Branch Connections

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 30, 2017.

Chapter Description

Broadband solutions provide teleworkers with high-speed connection options to business locations and to the Internet. In this sample chapter from Connecting Networks v6 Companion Guide, identify branch connection options for small to medium-sized businesses, basic configuration for a PPPoE connection on a client router, and more.

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GRE (3.4)

In this section, you implement a GRE tunnel.

GRE Overview (3.4.1)

In this topic, you learn about the purpose and benefits GRE tunnels.

GRE Introduction (

Generic Routing Encapsulation (GRE) is one example of a basic, nonsecure, site-to-site VPN tunneling protocol. GRE is a tunneling protocol developed by Cisco that can encapsulate a wide variety of protocol packet types inside IP tunnels. GRE creates a virtual point-to-point link to Cisco routers at remote points, over an IP internetwork.

GRE is designed to manage the transportation of multiprotocol and IP multicast traffic between two or more sites that may have only IP connectivity. It can encapsulate multiple protocol packet types inside an IP tunnel.

As shown in Figure 3-22, a tunnel interface supports a header for each of the following:

  • Passenger protocol: This is the original IPv4 or IPv6 packet that will be encapsulated by the carrier protocol. It could also be a legacy AppleTalk, DECnet, or IPX packet.

  • Carrier protocol: This is the encapsulation protocol such as GRE that encapsulates the passenger protocol.

  • Transport protocol: This is the delivery protocol such as IP that carries the carrier protocol.

Figure 3-22

Figure 3-22 Generic Routing Encapsulation

GRE Characteristics (

GRE is a 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 Cisco routers at remote points over an IP internetwork. IP tunneling using GRE enables network expansion across a single-protocol backbone environment. It does this by connecting multiprotocol subnetworks in a single-protocol backbone environment.

GRE has these characteristics:

  • GRE is defined as an IETF standard (RFC 2784).

  • GRE is identified as IP protocol 47 in the Transport protocol IP protocol field.

  • GRE encapsulation includes a protocol type field in its header to provide multiprotocol support. Protocol types are defined in RFC 1700 as “EtherTypes.”

  • GRE is stateless, which means that, by default, it does not include any flow-control mechanisms.

  • GRE does not include any strong security mechanisms to protect its payload.

  • The GRE header consumes at least 24 bytes of additional overhead for tunneled packets.

Figure 3-23 illustrates the GRE header components.

Figure 3-23

Figure 3-23 Header for GRE Encapsulated Packet Header


Activity Identify GRE Characteristics

Implement GRE (3.4.2)

In this topic, you learn how to troubleshoot a site-to-site GRE tunnel.

Configure GRE (

The topology displayed in Figure 3-24 will be used to create a GRE VPN tunnel between two sites.

Figure 3-24

Figure 3-24 GRE Tunnel Configuration Topology

To implement a GRE tunnel, the network administrator must know the reachable IP addresses of the endpoints.

There are five steps to configuring a GRE tunnel:

  • Step 1. Create a tunnel interface using the interface tunnel number global configuration command.

  • Step 2. Configure an IP address for the tunnel interface using the ip address ip-address interface configuration command. This is normally a private IP address.

  • Step 3. Specify the tunnel source IP address or source interface using the tunnel source {ip-address | interface-name} interface configuration command.

  • Step 4. Specify the tunnel destination IP address using the tunnel destination ip-address interface configuration command.

  • Step 5. (Optional) Specify GRE tunnel mode as the tunnel interface mode using the tunnel mode gre protocol interface configuration command. GRE tunnel mode is the default tunnel interface mode for Cisco IOS software.

The sample configuration in Example 3-11 illustrates a basic GRE tunnel configuration for R1 and R2.

Example 3-11 R1 and R2 GRE Tunnel Configuration

R1(config)# interface Tunnel0
R1(config-if)# ip address
R1(config-if)# tunnel source
R1(config-if)# tunnel destination
R1(config-if)# tunnel mode gre ip
R1(config-if)# exit
R1(config)# router ospf 1
R1(config-router)# network area 0
R2(config)# interface Tunnel0
R2(config-if)# ip address
R2(config-if)# tunnel source
R2(config-if)# tunnel destination
R2(config-if)# tunnel mode gre ip
R2(config-if)# exit
R2(config)# router ospf 1
R2(config-router)# network area 0

The minimum configuration requires specification of the tunnel source and destination addresses. The IP subnet must also be configured to provide IP connectivity across the tunnel link. Both tunnel interfaces have the tunnel source set using the IP address of their local serial S0/0/0 interface and the tunnel destination set to the IP address of the peer router serial S0/0/0 interface. The tunnel interface IP address is typically assigned a private IP address. Finally, OSPF is configured to advertise the tunnel network route over the GRE tunnel.

Table 3-1 provides the individual GRE tunnel command descriptions.

Table 3-1 GRE Configuration Command Syntax

Command Description
ip address ip_address mask Specifies the IP address of the tunnel interface
tunnel source ip_address Specifies the tunnel source IP address, in interface tunnel configuration mode
tunnel destination ip_address Specifies the tunnel destination IP address, in interface tunnel configuration mode
tunnel mode gre ip Specifies GRE tunnel mode as the tunnel interface mode, in interface tunnel configuration mode

Verify GRE (

Several commands can be used to monitor and troubleshoot GRE tunnels. To determine whether the tunnel interface is up or down, use the show ip interface brief and show interface tunnel number privileged EXEC commands, as demonstrated in Example 3-12.

Example 3-12 Verifying GRE

R1# show ip interface brief | include Tunnel
Tunnel0          YES manual up               up
R1# show interface Tunnel 0
Tunnel0 is up, line protocol is up
  Hardware is Tunnel
  Internet address is
  MTU 17916 bytes, BW 100 Kbit/sec, DLY 50000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation TUNNEL, loopback not set
  Keepalive not set
  Tunnel source, destination
  Tunnel protocol/transport GRE/IP
 <output omitted>

The first command verifies that the tunnel 0 interface is up and has an IP address assigned to it. The second command verifies the state of a GRE tunnel, the tunnel source and destination addresses, and the GRE mode supported. The line protocol on a GRE tunnel interface is up as long as there is a route to the tunnel destination. Before a GRE tunnel is implemented, IP connectivity must already be in effect between the IP addresses of the physical interfaces on opposite ends of the potential GRE tunnel.

A routing protocol could also be configured to exchange route information over the tunnel interface. For example if OSPF had also been configured to exchange routes over the GRE tunnel, you could verify that an OSPF adjacency had been established using the show ip ospf neighbor command.

In Example 3-13, note that the peering address for the OSPF neighbor is on the IP network created for the GRE tunnel.

Example 3-13 Verifying OSPF Adjacency via GRE Tunnel

R1# show ip ospf neighbor
Neighbor ID       Pri State        Dead Time  Address      Interface     0   FULL/  -     00:00:37  Tunnel0

GRE is considered a VPN because it is a private network that is created by tunneling over a public network. Using encapsulation, a GRE tunnel creates a virtual point-to-point link to Cisco routers at remote points over an IP internetwork.

The advantages of GRE are that it can be used to tunnel non-IP traffic over an IP network, allowing for network expansion by connecting multiprotocol subnetworks across a single-protocol backbone environment. GRE also supports IP multicast tunneling. This means that routing protocols can be used across the tunnel, enabling dynamic exchange of routing information in the virtual network. Finally, it is common practice to create IPv6 over IPv4 GRE tunnels, where IPv6 is the encapsulated protocol and IPv4 is the transport protocol. In the future, these roles will likely be reversed as IPv6 takes over as the standard IP protocol.

However, GRE does not provide encryption or any other security mechanisms. Therefore, data sent across a GRE tunnel is not secure. If secure data communication is needed, IPsec or Secure Sockets Layer (SSL) VPNs should be configured.

Troubleshoot GRE (

Issues with GRE are usually due to one or more of the following misconfigurations:

  • The tunnel interface IP addresses are not on the same network or the subnet masks do not match.

  • The interfaces for the tunnel source and/or tunnel destination are not configured with the correct IP address or are in the down state.

  • Static or dynamic routing is not properly configured.

Figure 3-25 shows the GRE configuration topology.

Figure 3-25

Figure 3-25 GRE Tunnel Configuration Topology

Use the show ip interface brief command on both routers to verify that the tunnel interface is up and configured with the correct IP addresses for the physical interface and the tunnel interface. Also, verify that the source interface on each router is up and configured with the correct IP addresses, as shown in Example 3-14.

Example 3-14 Verifying That All Necessary Interfaces Are Up

R1# show ip interface brief
<some output omitted>
Interface    IP-Address      OK?  Method  Status  Protocol
Serial0/0/0   YES  manual  up      up
Loopback0        YES  manual  up      up
Tunnel0     YES  manual  up      up
R2# show ip interface brief
<some output omitted>
Interface    IP-Address      OK?  Method  Status  Protocol
Serial0/0/0  YES  manual  up      up
Loopback0      YES  manual  up      up
Tunnel0     YES  manual  up      up

Routing can cause an issue. Both routers need a default route pointing to the Internet. Also, both routers need the correct dynamic or static routing configured. You can use the show ip ospf neighbor command to verify neighbor adjacency. Regardless of the routing used, you can also use show ip route to verify that networks are being passed between the two routers, as shown in Example 3-15.

Example 3-15 Verify That Networks Are Being Routed

R1# show ip route ospf is subnetted, 1 subnets
O [110/1001] via, 00:19:44, Tunnel0
R2# show ip route ospf is subnetted, 1 subnets
O [110/1001] via, 00:20:35, Tunnel0
8. eBGP (3.5) | Next Section Previous Section

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