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216 CHAPTER 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
network
User terminal
Server A
Server B
Hunt group HG1
8888
User terminal
User terminal
Remote
DCE
9999
9999
QuidwayA
RouterA