3Com Router User Manual

							Configure X.25207
Configure X.25 
Datagram TransmissionThe configuration of X.25 datagram transmission includes:
■Create the mapping from the protocol address to X.121 address
■Create the permanent virtual circuit
In the most frequently used X.25 service, data is transmitted remotely between 
two hosts using the X.25 protocol via X.25 public packet network. As shown in 
the figure below, LAN A and LAN B are far apart, and X.25 packet switching 
network can be used to realize information exchange between them.
Figure 63   LAN interconnection via X.25
The datagram uses IP address to communicate data and information between LAN 
A and LAN B, whereas X.121 address is used inside X.25. Therefore, we setup 
correct mapping between the IP address and X.121 address. 
1Create the mapping from the protocol address to X.121 address
An X.25 interface has its own X.121 address and inter-network protocol (such as 
IP protocol) address. When X.25 initiates a call through this interface, the source 
address (calling DTE address) it carries in the call request packet is the X.121 
address of this interface.
For a datagram with a definite destination IP address, its corresponding X.121 
destination address is located by the configured address mapping. The called 
destination, just like a calling source, also has its own protocol address and X.121 
address. Establish the mapping between the destination protocol address and the 
X.121 address at the calling source, you can find the destination X.121 address 
according to the destination protocol address, and successfully initiate a call. 
In the interface view, perform the following commands to create/delete an address 
mapping.
Ta b l e 246   Create/Delete the mapping from the protocol address to X.121 address
The protocol-address and x.121-address in the command line refer to the protocol 
address and X.121 address of the destination, not those of the source.
An address mapping should be created for every destination.
While creating an address mapping, specify its attributes with the option items. 
The meanings and specific content of these options will be described in 
subsequent sections.
2Create the permanent virtual circuit (PVC)
A permanent virtual circuit can be created for large-traffic and stable data 
transmission on leased line. Permanent virtual circuits (PVCs) do not need any call 
X.25LAN A
Router ALAN B
Router B
OperationCommand
Create the mapping from the destination 
protocol address to X.121 addressx25 map { ip | ipx } 
protocol-address x121-address 
x.121-address [ option ]
Delete the mapping from the destination 
protocol address to X.121 addressundo x25 map protocol 
protocol-address  
						

							208CHAPTER 16: CONFIGURING LAPB AND X.25
process and it always exists. An address mapping will be created implicitly while a 
permanent virtual circuit is created.
To create/delete a permanent virtual circuit, perform the following tasks in 
interface view.
Ta b l e 247   Create/Delete permanent virtual circuit
The format of this command shows that while a permanent virtual circuit is 
created, an address mapping is also created for it. Similarly, the protocol-address 
and x.121-address in the command also refer to the destination address. While 
creating a permanent virtual circuit, some attributes of the PVC can also be 
selected via the option. This [option] is a subset of [option] in the command 
x25 
map
......  [option].
For configuration example of permanent virtual circuit, refer to subsequent 
sections.
Configure Additional 
Parameters of X.25 
Datagram TransmissionThe Configuration additional parameters of X.25 datagram transmission includes:
■Specify the maximum idle time of SVC
■Specify the maximum number of SVCs that is associated with the same address 
mapping
■Specify the pre-acknowledgement of packet
■Configure X.25 user facility
■Set the length of virtual circuit queue
■Broadcast via X.25
■Restrict the use of address mapping
■Configure the interface with the standby center
The X.25 of the 3Com Router series allows adding some additional characteristics, 
including a series of optional user facilities stipulated in ITU-T Recommendation 
X.25.
This section shows how to configure such additional characteristics, including the 
options in the two commands of 
x25 map ...... and x25 pvc....... Please 
select and configure these additional characteristics according to the actual needs, 
X.25 network structure and the services provided by service provider.
1Configure SVC maximum idle time
Specify a time period, and if SVC is idle within this period (no packet interaction), 
then X.25 of the 3Com Router series will automatically clear this SVC to avoid 
unnecessary expenses. Before the data packet is sent next time, this SVC will be 
reestablished. So the activation of this characteristic will not affect data 
transmission.
In the interface view, this task can be accomplished in two different ways. For 
details, refer to the table as follows.
OperationCommand
Create a permanent virtual circuitx25 pvc pvc-number  protocol  
protocol-address x121-address 
x.121-address [ option ]
Delete a permanent virtual circuitundo x25 pvc pvc-number   
						

							Configure X.25209
Ta b l e 248   Specify/Cancel SVC maximum idle time
By default, the value of SVC maximum idle time is 0 minute, which means this 
SVC will not be disconnected for idle times out.
2Configure the maximum number of SVCs that are associated with the same 
address mapping
The maximum number of virtual circuits to be set up on the same address 
mapping can be specified. The X.25 of the 3Com Router series can establish up to 
8 virtual circuits on one address mapping. In case of large traffic and low line rate, 
this parameter can be increased properly to reduce data loss. By default, one 
address mapping is associated with only one virtual circuit.
In the interface view, perform the following commands.
Ta b l e 249   Specify/Cancel the maximum number of SVCs associated with the same 
address mapping
By default, the value of nvc is 1.
3Configure the pre-acknowledgment of packets
According to X.25 protocol, the receiving party will send an acknowledgment only 
after the receiving window is full (the number of received packets equals the 
window-size in-packets value). However, in some X.25 networks, the delays may 
be long, resulting in low efficiency of sending and receiving. Therefore, we specify 
a value. Each time the number of received packets reaches the value, the 
acknowledgment will be sent to the peer, thus improving receiving and sending 
efficiency. This value, called a receive-threshold, ranges between 0 and 
window-size in-packets. If it is set to 1, every packet will be acknowledged. If it is 
set to window-size in-packets, the acknowledgment will be sent only after the 
receiving window is full. In applications requiring a high response speed, this 
function is especially important.
In the interface view, perform the following task.
OperationCommand
Specify maximum idle time for all the SVCs on an 
interfacex25 timer idle minutes
Specify maximum idle time for SVC associated 
with an address mappingx25 map protocol 
protocol-address 
x121-address x.121-address 
timer idle minutes
Cancel specify maximum idle time for all the SVCs 
on an interfaceundo x25 timer idle
OperationCommand
Specify the maximum number of SVCs 
associated with all address mappings on an 
X.25 interfacex25 vc-per-map count
Specify the maximum number of SVCs 
associated with an address mappingx25 map protocol 
protocol-address
x121-address x.121-address 
vc-per-map count
Cancel the maximum number of SVCs 
associated with all address mappings on an 
X.25 interfaceundo x25 vc-per-map  
						

							210CHAPTER 16: CONFIGURING LAPB AND X.25
Ta b l e 250   Specify/Cancel packet pre-acknowledgement
By default, the number of pre-acknowledged packets is 0.
4Configure X.25 user facility
X.25 protocol defines various user facility options. The user can choose and 
configure the facilities. These configurations can be modified in two ways:
Configuration based on X.25 interface (use x25 call-facility..... 
command); configuration based on address mapping (use 
x25 map...... 
command).
The configuration based on X.25 interface will be effective in every call originated 
from this X.25 interface, while the configuration based on address mapping will 
be effective only in the calls originated from this address mapping.
In the interface view, perform the following task.
Ta b l e 251   Configure X.25 user facility
OperationCommand
Set packet acknowledgment valuex25 receive-threshold 
packet-count
Cancel packet acknowledgment valueundo x25 receive-threshold 
OperationCommand
Specify CUG (Closed User Group)x25 call-facility 
closed-user-group group-number
Or
x25 map protocol protocol-address 
x121-address x.121-address 
closed-user-group group_number
Cancel CUG numberundo x25 call-facility 
closed-user-group
Perform flow control parameter negotiation 
while initiating a callx25 call-facility packet-size 
in-size out-size 1
Or
x25 map protocol protocol-address 
x121-address x.121-address 
packet-size in-size out-size 
1
x25 call-facility window-size 
in-size out-size 1
Or
x25 map protocol protocol-address 
x121-address x.121-address 
window-size in-size out-size 
1
Cancel flow control parameter negotiation 
while initiating a callundo x25 call-facility packet-size 
Or
undo x25 call-facility window-size
Request reverse charging while initiating a 
callx25 call-facility 
reverse-charge-request
Or
x25 map protocol protocol-address 
x121-address x.121-address 
reverse-charge-request
Cancel the request of reverse charging 
while initiating a callundo x25 call-facility 
reverse-charge-request 
						

							Configure X.25211
window-size and packet-size options are also supported in x25 pvc command. 
However, in 
x25 pvc command, these two options specify the window size and 
maximum packet length of the set PVC. If these two options are not selected in 
the 
x25 pvc command, the set PVC will choose the default value of X.25 
interface.
name is the name of the ROA ID list configured by the command x25 roa-list in 
the system view, for example:
[Router]x25 roa-list list1 12 34 567
In the serial port view, list1 can be quoted:
[Router-Serial0]x25 call-facility roa-name list1
5Configure the sending queue length of virtual circuit
The sending and receiving queue lengths of the virtual circuit can be specified for 
the X.25 of the 3Com Router series to adapt to different network environments. 
The default queue length can contain 500 packets, but if data flow is very large, or 
the transmission rate of the X.25 network is low, the queue length can be 
increased to avoid unexpected data packet loss.
In the interface view, perform the following tasks to specify the length of virtual 
circuit queue.
Ta b l e 252   Configure the sending queue length of virtual circuit
6Broadcast via X.25
Receive calls with reverse charging requestsx25 reverse-charge-accept
Or
x25 map protocol protocol-address 
x121-address x.121-address 
reverse-charge-accept
Request throughput-level negotiation while 
initiating a callx25 call-facility threshold in out
Or
x25 map protocol protocol-address 
x121-address 
x.121-addressthreshold in out
Cancel the request of throughput-level 
negotiation while initiating a callundo x25 call-facility threshold
Carry transmission delay request while 
initiating a callx25 call-facility send-delay 
milliseconds
Or
x25 map protocol protocol-address 
x121-address x.121-address 
send-delay milliseconds
Cancel the carrying of transmission delay 
request while initiating a callundo x25 call-facility send-delay
Specify the use of ROA (Recognized 
operating Agency)x25 call-facility roa-name name 2
Or
x25 map protocol protocol-address 
x121-address x.121-address 
roa-name name 
2
Cancel the use of ROAundo x25 call-facility roa-name
OperationCommand
Set the length of X.25 virtual circuit queuex25 queue-length 
queue-size
Cancel set the length of X.25 virtual circuit queueundo x25 queue-length 
						

							212CHAPTER 16: CONFIGURING LAPB AND X.25
Generally, inter-network protocols will need to send some broadcast datagrams 
for specific purposes. On the broadcasting physical networks (such as Ethernet), 
such requirements are naturally supported. But for non-broadcasting networks like 
X.25, how to realize the broadcasting?
The X.25 of the 3Com Router series can enable this to decide if the broadcast 
packet should be duplicated and sent to a destination. This is very important. For 
instance, the broadcast-based application layer routing protocol will request 
broadcasting datagram sent by X.25 to exchange routing information on the X.25 
network.
It can be specified whether to send broadcasting data packets on the related 
virtual circuits of both SVC and PVC.
Ta b l e 253   Set broadcast via X.25
7Restrict the use of address mapping
X.25 calls are closely related to address mapping: before a destination is called, 
this destination must be found in the address mapping table. Before a call is 
received, the source of this call must also be found in the address mapping table. 
But in some cases, some address mappings are used for calling out only, while 
others are used for calling in only.
The X.25 of the 3Com Router series allows restricting the use of this address 
mapping addition by adding some option items, as shown in the following table.
Ta b l e 254   Restrict the use of address mapping
8Configure interface with standby center
The powerful standby function of the 3Com Router series is provided by the 
standby center. To add an X.25 interface into the standby center, perform the 
following task in the interface view.
Ta b l e 255   Set interface with standby center
OperationCommand
Enable to send broadcasting data packets to 
the peer of the SVC associated with this 
address mappingx25 map protocol 
protocol-address 
x121-address x.121-address 
broadcast
Enable to send broadcasting data packets to 
the peer of this PVCx25 pvc pvc-number protocol 
protocol-address x121-address 
x.121-address broadcast
OperationCommand
Inhibit outgoing call through this address 
mappingx25 map protocol protocol-address
x121-address X.121-address 
no-callout
Inhibit incoming call through this address 
mappingx25 map protocol protocol-address
x121-address X.121-address  
no-callin
OperationCommand
Set the logical interface number of this address 
mapping in the standby centerX25 map protocol 
protocol-address x121-address 
x.121-address logic-channel 
logical-interface-number 
						

							Configure X.25213
Configure X.25 
Sub-InterfaceX.25 sub-interface is a virtual interface with its own protocol address and virtual 
circuit. Multiple sub-interfaces can be created on a physical interface, so the 
networks can be interconnected via one physical interface. The sub-interface of 
X.25 falls into two types: point-to-point sub-interface, used to connect a single 
remote end and point-to-multipoint sub-interface, used to connect multiple 
remote ends in the same network segment. All the sub-interfaces under the main 
interface and the main interface share a X.121 address.
In the interface view, perform the following task to configure X.25 sub-interface.
Ta b l e 256   Configure X.25 sub-Interface
Configure X.25 
SwitchingX.25 Switching Function
A packet network consists of many nodes interconnected in a certain topological 
structure. From the source to its destination, a packet will pass through many 
nodes, each of which must have packet switching capability.
X.25 packet switching means to receive packets from one X.25 port, and send 
them out from the X.25 port selected according to related destination address 
information contained in the packets. X.25 switching enables the 3Com Router 
series to perform packet switching function in the packet layer, and to be used as 
a small packet switching exchange.
The 3Com Router series provides such X.25 switching functions as follows:
■SVC switching function
■Support parameter negotiation on window size and packet size
■PVC switching
The following describes how to configure X.25 switching tables for PVC and SVC.
OperationCommand
Enter X.25 interfaceinterface serial number
Configure X.25 protocollink-protocol x25
Create X.25 sub-interfaceinterface serial 
number.subinterface-number{multipoi
nt|point-to-point}
Configure address mapping
Or
Configure permanent virtual circuit
x25 map protocol protocol-address 
x121-address x.121-address [option]
or
x25 pvc pvc-number protocol 
protocol-address
x121-address x.121-address [option] 
						

							214CHAPTER 16: CONFIGURING LAPB AND X.25
Figure 64   X.25 switching networking diagram
1Enable or disable X.25 switching
In the system view, perform the following task to enable or disable X.25 switching.
Ta b l e 257   Enable or disable X.25 switching
Add or delete a PVC route
Ta b l e 258   Add or delete a PVC route
After configuration, the display x25 switch-vc-table pvc command can be 
used to show the virtual circuit route table.
2Add/Delete an SVC route
In the system view, the commands in the following table can be used to add or 
delete an SVC route.
Ta b l e 259   Add or delete an SVC route
After the configuration, use display x25 switch-vc-table svc command to 
display the switching route table.
Configure X.25 Load 
BalancingIntroduction to X.25 Load Balancing
Using the property of hunt group of X.25 protocol, ISPs can provide load 
balancing function in X.25 packet switching networks. X.25 load balancing can 
implement the load balancing in different DTEs or different links of a single DTE, 
PCPCQuidway Router
X.25 hostX.25 host
OperationCommand
Enable X.25 switchingx25 switching
Disable X.25 switchingundo x25 switching
OperationCommand
Add a PVC routex25 switch pvc number interface 
serial port-number pvc number
Delete a PVC routeundo x25 switch pvc number
OperationCommand
Add an SVC routex25 switch svc x.121-address [ sub-dest 
destination-address ] [ sub-source source-address 
] interface serial interface-number
Delete an SVC routeundo x25 switch svc x.121-address [ sub-dest 
destination-address ] [ sub-source source-address 
] [ interface serial interface-number ] 
						

							Configure X.25215
which guarantees no occurrence of link overload when an address is accessed by a 
large number of subscribers.
X.25 load balancing is provided by DCEs. In order to implement the load balancing 
in X.25 networks, a group of DTE/DCE interfaces (synchronous serial interfaces or 
XOT Tunnels) need to be configured at the remote DCE on the network as a hunt 
group. And it is necessary to allocate an X.121 address to such hunt group. When 
other equipment in the network accesses the DTE inside the hunt group, they 
need to call the hunt group address. After receiving the call request packets, the 
remote DCE will, according to diverse channel selection policies (round-robin or 
vc-number), select a line in the hunt group and send the incoming call packets. 
Different calls will be allocated to the lines in the hunt group, thus achieving load 
balancing.
It should be noted that X.25 hunt group can dynamically select different 
transmission lines only in the process of the establishment of virtual circuit call. 
Once the whole virtual circuit is established and enters into the stage of data 
transmission, hunt group will be ineffective and data transmission will be 
processed in accordance with the normal virtual circuit. After being established, 
PVC stays at the data transmission stage without the process of call establishment 
and call deletion, therefore X.25 load balancing is ineffective on PVC and 
functions only on SVC. 
Within a single X.25 hunt group, all DTEs hold identical status and have the same 
X.121 addresses. The DTEs in a hunt group can call other DTEs outside the hunt 
group in a normal mode. When equipment outside the hunt group access the 
hunt group, they cannot know which equipment they will access, because the line 
selection is controlled by the DCEs configured with hunt group.
DTE addresses in a hunt group may be identical or different to the hunt group 
addresses. X.25 hunt group supports the substitutions of source address and 
destination address. The function of destination address substitution enables us to 
hide the addresses of DTEs inside the hunt group, thus external DTEs only know 
the hunt group address, which enforces the security of the internal network of 
hunt group. The function of source address substitution can hide the addresses of 
DTEs outside a hunt group, therefore internal DTEs can only know the substituted 
source address instead of the source address a call is connected to, which protects 
subscribers privacy. 
						

							216CHAPTER 16: CONFIGURING LAPB AND X.25
Figure 65   Diagram of X.25 network load balancing
As shown in the above figure, Server A and Server B concurrently provide 
subscribers with identical services. They are configured as a hunt group named 
hg1. Server A and Server B have the same address of 9999 and the hunt group 
address is 8888. Enable the function of destination address substitution on Router 
RouterA for changing calls connected to address 8888 to calls connected to 
address 9999. When a subscriber processes a service, the subscriber terminal will 
send a call to the destination address 8888. The calls from various terminals will be 
substituted on the router RouterA with the calls to 9999 and transmitted to Server 
A and Server B respectively. Thus load balancing is realized between Server A and 
Server B, and the pressure on a single server is decreased.
X.25 hunt group supports two types of call channel selection policies: round-robin 
mode and vc-number mode, but a hunt group can only utilize one type of channel 
selection policy.
■In round-robin mode, cyclic selection method is adopted to select the next 
interface or the XOT Tunnel for every call request. For example, as shown in 
Figure1-1, if hunt group hg1 adopts rotary mode, calls will be sent to Server A 
and Server B by turns.
■vc-number mode selects the interfaces with the free logical channels in a hunt 
group for every call request. For example, as shown in the above Figure1-1, if 
hunt group hg1 adopts vc-number mode, there will be 500 residual logical 
channels in the lines between Server A and DCE and 300 residual logical 
channels in the lines between Server B and DCE. Thus all the first 200 calls will 
be sent to Server A, and the calls following the first 200 ones will be sent to 
Server A and Server B by turns.
X.25 hunt group supports synchronous serial interfaces and XOT Tunnels. It can 
indiscriminately select the available lines between synchronous serial interfaces 
and XOT Tunnels. But XOT Tunnels cannot calculate the number of logical 
channels, therefore it cannot be added into a hunt group adopting vc-number 
selection policy.
List of Configuration Tasks of X.25 Load Balancing 
The load balancing of X.25 networks is configured on DCE equipment. The 3Com 
Router is generally utilized as a DTE equipment in X.25 networks. If load balancing 
is provided by ISPs on packet switching exchanges, routers need no special 
configuration. The specific configuration procedure can be seen in the previous 
chapters. If the 3Com Router is used as an X.25 switching exchange (it serves as a 
X.25
packet
switching
networkUser terminal
Server A
Server B Hunt group HG1
8888
User terminal User terminal
Remote
DCE
9999
9999
QuidwayARouterA