3Com Router User Manual

							GRE Protocol Overview637
GRE ServicesGRE can fulfill the following services:
1Implement the LAN protocol communication in WAN by encapsulating all kinds of 
LAN protocols into a WAN protocol.
Figure 201   Multi-protocol local network transmitting via single-protocol backbone 
network
In Figure 201, Group1 and Group2 are the local networks running the Novell IPX 
protocol. Term1 and Term2 is the local network running the IP protocol. The tunnel 
encapsulated by the GRE protocol is created between Router A and Router B. Thus 
Group1 and Group2 can communicate without affecting each other, as can Term1 
and Term2.
2Enlarge the operating range of the hop-limited network, such as IPX.
Figure 202   Enlarge network operating range
When using RIP, if the hop count between two terminals in Figure 202 is more 
than 15, the two terminals cannot communicate with each other. If tunneling is 
used in the network, hop counts will not be incremented inside the tunnel, that is, 
hops can be hidden, which enlarges the operating range of the network.
3Connect some discontinuous sub-networks to establish a VPN.
Figure 203   Tunnel connecting discontinuous sub-networks
Internet
Novell IPX
Group1
IP
Term 1
Novell IPX
Group2
IP
Term 2Tunnel
Router B
Router A
Router
r rTunnel
Router
RouterRouter
IP networkIP network
PCPCIP network
Tunnel
group2novel
l
Router
Router
group 1 novell
IP network
VLAN 
						

							638CHAPTER 44: CONFIGURING GRE 
The two sub-networks group1 and group2 that are running the Novell IPX 
protocol are in different cities. With the tunnel available, the trans-WAN VPN can 
be established.
In addition, GRE also allows users to select and record an identification key word 
for the tunnel interface, a check of the encapsulated message, and the use of 
synchronous sequence numbers to ensure channel safety and correctness of 
transmission data.
Encapsulation and de-encapsulation on the GRE receiving side and transmitting 
side increases overhead cost and the increase in data volume caused by 
encapsulation also increases bandwidth cost. for these reasons, GRE decreases the 
forwarding rate of router data to some extent.
Configuring GREGRE configuration includes:
■Creating a Virtual Tunnel Interface
■Setting the Source Address of a Tunnel Interface
■Setting the Destination Address of a Tunnel Interface
■Setting the Network Address of the Tunnel Interface
■Setting the Identification Key Word of the Tunnel Interface
■Setting the Tunnel Interface to Check with Checksum
■Settng the Tunnel Interface to Synchronize the Datagram Sequence Number
Creating a Virtual Tunnel 
InterfacePerform the following tasks in the system view.
Ta b l e 687   Create Virtual Tunnel Interface
By default, no virtual tunnel interface is created. 
Setting the Source 
Address of a Tunnel 
InterfaceAfter a tunnel interface is created, the source address of tunnel channel must be 
configured. The source address is the address of the physical interface where the 
GRE packets are transmitted. The source address and destination address of the 
tunnel interface uniquely identifies a channel. These configurations must be 
implemented at both tunnel ends, and furthermore, the source address of one end 
must be the destination address of another end.
Perform the following settings in the tunnel interface view.
Ta b l e 688   Set the Source Address of Tunnel Interface
By default, no source address of the tunnel interface is configured. 
OperationCommand
Create virtual tunnel interface and enter 
tunnel view.interface tunnel tunnel-number
Cancel virtual tunnel interface.undo interface tunnel 
OperationCommand
Set the source address of tunnel interface.source ip-address
Delete the configured source address of 
tunnel the interface.undo source  
						

							Configuring GRE639
Setting the Destination 
Address of a Tunnel 
InterfaceAfter a tunnel interface is created, the destination address of the tunnel channel 
must be configured
The destination address is the address of the physical interface where the GRE 
packets are received. The source address and destination address of a tunnel 
interface uniquely identifies a channel. These configurations must be done at both 
tunnel ends. The source address of one end must be the destination address of the 
other end.
Perform the following settings in the tunnel interface view.
Ta b l e 689   Designate the Destination Address of Tunnel Interface
By default, no destination address of the tunnel interface is configured. 
Setting the Network 
Address of the Tunne
l 
Interface
Two private networks are interconnected by a GRE tunnel. This kind of connection 
is like a virtual “direct” connection between two private networks. To establish a 
direct route between these two networks, you must configure the network 
address of the tunnel interface and make sure that the network addresses at both 
ends of the channel are in the same network segment. Thus, the system can 
produce a direct tunnel route automatically.
Perform the following settings in the tunnel interface view.
Ta b l e 690   Set the Network Address of Tunnel Interface
By default, no network address for the tunnel interface is configured. 
Setting the 
Identification Key Word 
of the Tunnel InterfaceIt is stipulated in RFC 1701 that if the key field of the GRE header is set, the 
receiving side and transmitting side check the identification key word of the 
channel. Only when the set identification key words at both ends of the tunnel are 
totally identical can the check pass, or the message will be discarded.
Perform the configurations in the tunnel interface view.
Ta b l e 691   Set the Identification Key Word of Tunnel Interface
OperationCommand
Designate the destination address of 
tunnel interface.destination ip-address
Cancel the destination address of tunnel 
interface.undo destination 
OperationCommand
Set the IP address of tunnel interface.ip address { ip-address mask | 
unnumbered interface-type 
interface-number }
Delete the IP address of tunnel interface.undo ip address { ip-address mask | 
unnumbered interface-type 
interface-number }
Set the IPX address of tunnel interface.ipx network network-number
Delete the IPX address of tunnel interface.undo ipx network 
OperationCommand 
						

							640CHAPTER 44: CONFIGURING GRE 
By default, no identification key word of the tunnel interface is configured.
Setting the Tunnel 
Interface to Check with 
ChecksumIt is stipulated in RFC 1701 that if the checksum field of the GRE header is set, the 
checksum is valid. The transmitting side calculates the checksums of GRE header 
and payload. The receiving side calculates the checksum of the received message 
and compares it with the checksum field in the message. If the two checksums are 
identical, the message will be processed, otherwise it will be discarded.
If only one end of the tunnel is configured to check with the checksum, the 
message will not be checked with checksum. Only when both ends of the tunnel 
are configured to check the checksum, the message will be checked with the 
checksum.
Perform the following tasks in the tunnel interface view.
Ta b l e 692   Set Tunnel Interface to Check with Checksum
By default, the tunnel interface to check with the field of checksum is disabled.
Settng the Tunnel 
Interface to Synchronize 
the Datagram Sequence 
NumberIt is stipulated in RFC 1701 that if the sequence-datagram in the GRE header is set, 
both the receiving side and the transmitting side will synchronize the sequence 
numbers. The synchronized message should be further processed, or it is 
discarded.
With the sequence numbers, the message is unreliable but in order. The receiving 
end establishes sequence numbers for the message, which is received by the local 
end and successfully de-encapsulated. The sequence numbers are integers 
between 0 and 2
32–1 and the sequence number of the first packet is 0. After the 
channel is established, the sequence numbers is accumulated and cyclically 
counted. If the receiving end receives a message whose sequence number is less 
than or equal to that of the message received the last time, the packet will be 
considered illegal. If the receiving end receives an out-of-order message, the 
packet will be discarded automatically.
Only when the synchronization mechanism to enable or disable sequence 
numbers is established at both ends of the tunnel, the channel can be established.
Perform the following tasks in the tunnel interface view.
Ta b l e 693   Set the Tunnel to Synchronize Datagram Sequence Numbers
Set the identification key word of tunnel 
interface.gre key key-number
Cancel the identification key word of 
tunnel interface.undo gre key
OperationCommand
Set tunnel interface to check with check 
sum.gre checksum  
Disable tunnel interface to check with 
check sum.undo gre checksum
OperationCommand
Set tunnel interface to synchronize 
sequence numbers.gre sequence-datagrams 
						

							Displaying and Debugging GRE 641
By default, the tunnel interface to synchronize datagram sequence numbers is 
disabled.
Displaying and 
Debugging GRE
 
To view the working status of the tunnel interface, use the display command in 
all views. 
Ta b l e 694   Display and Debug GRE 
GRE Configuration 
Example
Application of IP-over-IP 
GRE
 
VPN should be built across the WAN for the operation of Novell IPXs two subnets 
group1 and group2. It can be implemented by using GRE. 
PC A communicates with PC B in GRE tunneling mode in the Internet. Router A 
and Router B are two ends of the GRE tunnel, while Router C is located in the GRE 
tunnel.
Figure 204   Networking diagram of GRE application
1Configure PC A and PC B:
aConfigure the IP address of PC_A to 10.110.24.100, add a default gateway in 
the network attribute (i.e., default route), or use the following command in 
DOS mode.
C:\WINDOWS> route add 0.0.0.0 mask 0.0.0.0 10.110.24.1
bConfigure the IP address of PC_B to 30.110.1.200, add a default gateway in 
the network attribute (i.e., default route), or use the following command in 
DOS mode.
C:\WINDOWS> route add 0.0.0.0 mask 0.0.0.0 30.110.1.1
2Configure Router A:
aConfigure the IP address of Serial0 interface.
[RouterA] interface serial 0
[RouterA-Serial0] ip address 10.1.1.1 255.255.255.0
Disable tunnel interface to synchronize 
sequence numbers.undo gre sequence-datagrams
OperationCommand
Display the working status of tunnel 
interface.display interfaces tunnel [ 
tunnel-number ]
Router B
Router C
PC APC B
Router A
10.110.24.10030.110.1.200tunnel
E0S0 S0 S1 S0
E0
Internet
10.110.24.1
10.1.1.220.1.1.120.1.1.210.1.1.1
30.110.1.1 
						

							642CHAPTER 44: CONFIGURING GRE 
bConfigure the IP address of Ethernet0 interface.
[RouterA-Serial0] exit
[RouterA] interface ethernet 0
[RouterA-Ethernet0] ip address 10.110.24.1 255.255.255.0
cCreate a virtual Tunnel interface and configure the IP address, source address 
and destination address.
[RouterA-Ethernet0] exit
[RouterA] interface tunnel 0
[RouterA-Tunnel0] ip address 1.1.1.1 255.255.255.0
[RouterA-Tunnel0] source 10.1.1.1
[RouterA-Tunnel0] destination 20.1.1.2
dConfigure the routes to 20.1.1.0 network and 30.110.1.0 network.
[RouterA] ip route-static 20.1.1.0 255.255.255.0 serial 0
[RouterA] ip route-static 30.110.1.0 255.255.255.0 tunnel 0
3Configure Router B:
aConfigure the IP address of Serial0.
[RouterB] interface serial 0
[RouterB-Serial0] ip address 20.1.1.2 255.255.255.0
bConfigure the IP address of Ethernet0 interface.
[RouterB-Serial0] exit
[RouterB] interface ethernet 0
[RouterB-Ethernet0] ip address 30.110.1.1 255.255.255.0
cCreate a virtual Tunnel interface, and configure the IP address, source address 
and destination address.
[RouterB-Ethernet0] exit
[RouterB] interface tunnel 0
[RouterB-Tunnel0] ip address 1.1.1.2 255.255.255.0
[RouterB-Tunnel0] source 20.1.1.2
[RouterB-Tunnel0] destination 10.1.1.1
dConfigure the routes to 20.1.1.0 network and 30.110.1.0 network.
[RouterB] ip route-static 10.1.1.0 255.255.255.0 Serial 0
[RouterB] ip route-static 10.110.24.0 255.255.255.0 tunnel 0
4Configure Router C:
aConfigure the IP address of Serial0 interface.
[RouterC] interface serial 0
[RouterC-Serial0] ip address 10.1.1.2 255.255.255.0
[RouterC-Serial0] interface serial 1
[RouterC-if-Serial1] ip address 20.1.1.1 255.255.255.0
Application of 
IPX-over-IP GREThe two subnets group1 and group2 that running Novell IPX protocol need to set 
up a virtual private network across a LAN using GRE technology. 
						

							GRE Configuration Example643
Figure 205   Networking of GRE
1Configure Router A:
aActivate IPX.
[RouterA] ipx enable node a.a.a
bConfigure the IP address and IPX address of Ethernet0.
[RouterA] interface ethernet 0
[RouterA-Ethernet0] ip address 10.1.1.1 255.255.255.0
[RouterA-Ethernet0] ipx network 1e
cConfigure the IP address of Serial0 interface.
[RouterA] interface serial 0
[RouterA-Serial0] ip address 192.10.1.1 255.255.255.0
dCreate a virtual tunnel interface, and configure the IP address, source address 
and destination address.
[RouterA] interface tunnel 0
[RouterA-Tunnel0] ip address 10.1.2.1 255.255.255.0
[RouterA-Tunnel0] ipx network 1f
[RouterA-Tunnel0] source 192.10.1.1
[RouterA-Tunnel0] destination 202.18.3.2
eConfigure the static route to Novell Group2.
[RouterA] ipx route 31 1f.b.b.b tick 2000 hop 15
2Configure Router B:
aActivate IPX.
[RouterB] ipx enable node b.b.b
bConfigure the IP address and IPX address of Ethernet0 interface.
[RouterB] interface ethernet 0
[RouterB-Ethernet0] ip address 10.1.3.1 255.255.255.0
[RouterB-Ethernet0] ipx network 31
cConfigure the IP address of Serial0 interface.
[RouterB] interface serial 0
[RouterB-Serial0] ip address 202.18.3.2 255.255.255.0
dCreate a virtual Tunnel interface, and configure the IP address, source address 
and destination address.
[RouterB] interface tunnel 0
[RouterB-Tunnel0] ip address 10.1.2.2 255.255.255.0
[RouterB-Tunnel0] ipx network 1f
[RouterB-Tunnel0] source 202.18.3.2
[RouterB-Tunnel0] destination 192.10.1.1
eConfigure the static route to Novell Group. 
202.18.3.2192.10.1.1
IPX Pr ot oc ol
Group1IPX Pr ot oc ol
Group2
InternetRouter A
Router B
tunnel
1e1f 1f31 
						

							644CHAPTER 44: CONFIGURING GRE 
[RouterB] ipx route 1e 1f.a.a.a tick 30000 hop 15
Troubleshooting GREThe two interfaces at both ends of the tunnel are correctly configured and 
the ping operation is successful, but the ping operation between PC A and 
PC B fails. 
Check whether there is a route passing through the Tunnel interface, that is, on 
Router A, the route to 10.2.0.0/16 passes through Tunnel0 interface; on Router B, 
the route to 10.1.0.0/16 passes through Tunnel0 interface (it is implemented by 
adding a static route).
Figure 206   Networking of troubleshooting GRE
Router B
Router C
PC APC B
Router A
10.1.1.110.2.1.1tunnel 
						

							X
RELIABILITY
Chapter 45Configuring a Standby Center
Chapter 46Configuring VRRP 
						

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