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...

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...

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...

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...

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...

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...

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...

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...

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

646