Multiplex router device comprising a function for controlling a traffic occurrence at the time of alteration process of a plurality of router calculation units

A multiplex router device, having a multiplex configuration, is instrumental in reducing internal and external traffic flows for a routing protocol process that occurs at a time of system switchover. A data base integration module, in a route calculation unit in the active mode, stores network link-state information collected by a routing protocol packet transmission-reception module in a link-state data base, and at the same time sends the information to a route calculation unit, but does not send routing protocol information collected. In the route calculation unit in the standby mode, a data base integration module that received the network link-state information stores its contents into its own link-state data base. When a failure occurs in the route calculation unit in the active mode, the route calculation unit performs the routing protocol process by using the stored link-state data base.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a router device for routing packets to 
destinations on networks, and more particularly to system switchover 
technology in a router device having a redundant configuration. 
2. Description of Related Art 
A router, which forwards packets between the terminals of different 
networks, needs to exchange routing information with another router to 
perform dynamic routing of packets, and on the basis of routing 
information, create a routing table which shows the destination addresses 
of packets associated with transit nodes. 
Two protocols are known for exchanging routing information and for 
generating a routing table based on the information: one is a Distance 
Vector Algorithm (DVA) based protocol, such as Routing Information 
Protocol (RIP) stipulated by Request for Comments (RFC) 1058 prepared by 
Internet Engineering Task Force (IETF) and issued from the Internet 
Architecture Board (IAB), and the other is Link-State Algorithm (LSA) 
based protocol such as Open Shortest Path First (OSPF) stipulated by RFC 
1247. 
A RIP-based router exchanges routing table entries with another router and 
determines a routing path according to the number of hops (the number of 
routers to the destination), while an OSPF-based router exchanges network 
connection state information (addresses and so on) and determines a path 
based on a cost determined by considering many factors including the 
number of hops. It should be noted that, in exchanging routing information 
among routers, a particular packet called a routing protocol packet is 
used. 
More specifically, in OSPF, each router exchanges information with all 
other routers by using packets called routing protocol packets. Each 
router periodically transmits packets called Hello packets, a kind of 
routing protocol packet, to the network. A Hello packet includes the 
router's own ID, and the identity of the network to which the router is 
connected, and a list of other routers' ID's connected to the same network 
to which the router is connected. The other routers' ID's placed in the 
above list include the other routers' ID's of which the router was made 
aware by Hello packets received from other routers. 
If a router receives a Hello packet, which includes its own ID, from 
another router that the router has been aware of, on the understanding 
that the two routers have become aware of each other, the two routers 
exchange network link-state information by sending routing protocol 
packets. 
Network link-state information includes the ID of the advertising router, 
the identity of the network to which the advertising router is connected, 
the addresses of the interfaces through which the advertising router is 
connected to the networks, and the costs of the interfaces. The cost of an 
interface means the cost which is incurred when the interface is used to 
forward packets and which is set by the network administrator. 
A router which collected network link-state information from another 
network connected to the same network creates a routing table specifying 
the least-cost path as the packet route. This router forwards the packet 
according to a resulting routing table. 
Network link-state information is transmitted by a router where a change 
occurred in its configuration. A router receiving this information updates 
the routing table if an update is necessary according to contents of the 
change. 
Meanwhile, each router, while it transmits or receives Hello packets and 
network link-state information, manages the states of other routers on the 
network to which this router is connected and also manages the states of 
the interfaces through which this router is connected to networks. With 
regard to the states of routers, each router manages the routers' ID's, 
and checks if each of those routers is aware of this router, or checks if 
each of those routers has completed the transmission and reception of 
network link-state information. With regard to interface state, each 
router manages the addresses of the interfaces and other routers connected 
to a network to which an interface is connected. 
A list of other routers, which is included in a Hello packet, is prepared 
according to the states of routers and the states of interfaces mentioned 
above. 
Each router monitors the active modes of the other routers according to 
information from Hello packets it receives. More specifically, if there is 
any other router from which the router has not received Hello packets for 
longer than a fixed period, the router decides that a failure has occurred 
in this other router. Also, the router takes measures such as altering the 
contents of the routing table to establish another path to avoid the 
faulty router. 
To improve the performance of the routers, it has recently been proposed to 
separate a router into a portion for forwarding packets and a portion for 
creating a routing table. Using this configuration, it becomes possible to 
execute the packet forwarding process regardless of the load on the 
process of creating the routing table. 
This technique is described in "Packet Magazine Third Quarter 1995" 
(Cisco). 
In this specification, the portion for creating a routing table is called a 
route calculation unit and the portion for counting packets is called a 
forwarding process unit. 
To enhance the reliability of the router device, it is now common practice 
to multiplex the above-mentioned route calculation units. The multiplex 
router device includes a plurality of route calculation units, and always 
has one route calculation unit placed in the active mode to make it 
execute an ordinary process while keeping the remaining route calculation 
units in a standby mode. When the route calculation unit in the active 
mode runs into trouble, the multiplex router device brings one of the 
waiting route calculation units into the active mode (this is referred to 
as a system switchover of route calculation units), and the one other 
route calculation unit takes over and continues to execute the process 
that was previously being executed by the route calculation unit in 
trouble. 
This technique is described in Cisco's manual, "Configuration Fundamentals 
Configuration Guide Cisco Systems, Inc. 1996". 
To prevent the other routers from being affected by this system switchover 
of the route calculation units or to facilitate the system switchover, a 
thinkable method includes the steps of sending all items of information 
(network link-state information, states of routers, and states of 
interfaces), which the route calculation unit in the active mode obtained 
from the routing protocol process, from the route calculation unit in the 
active mode to the route calculation units in the standby mode, and 
storing them in the route calculation units in the standby mode. With the 
above arrangement, the same states in the route calculation unit, which 
was previously in the active mode, can be reproduced in a route 
calculation unit which is subsequently brought into the active mode, and 
the route calculation unit newly brought into the active mode can promptly 
become capable of executing the same process as did the previously 
operating route calculation unit. Consequently, the other routers are 
protected from the affects of the system switchover of the routers. 
However, according to the above-mentioned system switchover technology, the 
amount of information that must be sent from the route calculation unit 
that has been in the active mode to the route calculation unit in the 
standby mode increases as the number of other routers increases. For this 
reason, as the channels accommodated in the respective routers increase 
and the routers increase in number as the size of the network becomes 
larger, the traffic volume in the routers becomes too large due to the 
transmission of information from the operating route calculation units to 
the route calculation units in the standby mode. If such excessively large 
traffic flows occur, delays will arise in the process by the route 
calculation units themselves or enough traffic capacity cannot be secured 
for forwarding of packets due to the traffic of information from the route 
calculation units in the active mode to the route calculation units in the 
standby mode. The above-mentioned heavy traffic is likely to provide 
hindrances to the packet forwarding. From a different point of view, it is 
impossible to accommodate a large number of channels in the router device. 
On the other hand, if any information obtained by the routing protocol 
process is not transmitted from the route calculation unit in operation to 
the route calculation unit in standby mode, not only must the route 
calculation unit, which is subsequently brought into operation, execute 
the routing protocol process from the beginning, but also the other 
routers must execute the routing protocol process. Furthermore, the 
packets that have passed this router cannot be forwarded in a normal 
manner until the routing protocol process ends and the router collects 
network link-state information about the other routers and reorganizes the 
routing table. Furthermore, the traffic on the network is increased, for 
example, by exchange of network link-state information by routing protocol 
packets. 
SUMMARY OF THE INVENTION 
The present invention has as its object to prevent the occurrence of 
interruption of packet forwarding or an increase in traffic on the network 
while reducing the amount of information to be transmitted from the route 
calculation unit in operation to a route calculation unit in a standby 
mode at least in a multiplex router device, which includes a plurality of 
route calculation units. 
To achieve the above object, according to the present invention, there is 
provided a multiplex router device comprising a plurality of route 
calculation units, each performing a routing protocol process to create a 
routing table used to determine a packet route, wherein when one of the 
route calculation units is set in an active mode and at least one of other 
route calculation units is set in a standby mode, if a failure occurs in 
the route calculation unit in the active mode, the one route calculation 
unit in the standby mode is brought into the active mode. 
The route calculation unit includes a memory, process unit, notification 
unit and holding unit. 
The memory holds routing protocol information, when the route calculation 
unit concerned is in the active mode. The routing protocol information 
includes network link-state information showing connections between the 
routers and networks, the states of neighboring routers showing the 
link-states with neighboring routers, and interface states showing states 
of network interfaces to connect the multiplex router device to networks. 
The process unit executes the routing protocol process, including 
collection of the routing protocol information held in the memory means, 
according to the routing protocol information held in the memory. 
The notification unit sends, when the route calculation unit concerned is 
in the active mode, to the route calculation unit in the standby mode only 
the network link-state information out of the network link-state 
information, neighboring router states, and interface states stored in the 
memory means. 
The holding unit holds in the memory the network link-state information 
sent from the route calculation unit in the active mode when the route 
calculation unit is in the standby mode. 
According to the multiplex router device described above, because the 
information sent from the route calculation unit in the active mode to the 
route calculation unit in the standby mode is network link-state 
information only, the internal traffic is less than that in the case where 
all items of routing protocol information are sent. The route calculation 
unit shifted from the standby mode to the active mode already obtained 
network link-state information while it was in the standby mode and holds 
this information, so that it is not necessary for this route calculation 
unit to exchange information with other routers to collect the network 
link-state information over again. Consequently, the volume of traffic on 
the networks can be reduced compared to the case where no routing protocol 
information is sent to the route calculation unit in the standby mode. The 
network link-state information held while the router is in the standby 
mode matches the contents of the routing table the moment the router is 
shifted to the active mode. Therefore, on every system switchover of the 
route calculation units to be set in the active mode, the contents of the 
routing table cease to exist, and the normal packet forwarding is 
prevented from being affected by system switchover.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Description will now be given of a first embodiment of the present 
invention. Referring to a case where OSPF is used as the routing protocol, 
the router device according to the first embodiment will be explained. 
FIG. 1 shows the configuration of the network system to which the router 
device according to the first embodiment is applied. 
In FIG. 1, reference numeral 10 denotes a router device according to the 
first embodiment, which includes two route calculation units 11a, 11b, 
provided in a multiplex configuration, and a plurality of forwarding 
process units 13. 
The router with two route calculation units 11a, 11b in a multiplex 
configuration is hereunder referred to as the "multiplex router device" 
for convenience in differentiating it from other routers. 
The multiplex router device 10 is connected through the forwarding units 13 
to networks. Other routers 30 and communication terminals 31 are connected 
to the networks. A network is formed of lines that terminate at one end 
with the multiplex router 10 and terminate at the other end with a router 
30. 
FIG. 2 shows a detailed configuration of the multiplex router device 10. 
As shown in FIG. 2, the multiplex router 10 is formed in a configuration, 
including two route calculation units 11a, 11b, provided in a multiplex 
configuration, which create and distribute a routing table for use in 
packet forwarding, and the forwarding process units 13 for packet 
forwarding, and those units are interconnected by an internal bus 12 in 
the router. 
Each forwarding process unit 13 includes a routing table 19, a forwarding 
module 25, and a packet transmission-reception module 24. Each route 
calculation unit 11a or 11b includes a routing protocol (RP) packet 
transmission-reception module 14, a protocol information manager module 
15, a route calculation module 16, a data base (DB) integration module 17, 
a routing table manager module 18, a routing table 19, a state monitor 
module 20, and a routing protocol information module 21. The routing 
protocol information module 21 stores routing protocol information, such 
as a link-state data base (LSDB) 22, interface (I/F) state 23, and 
neighboring router state 24. 
Out of the two route calculation units 11, one is placed in the active mode 
and the other is placed in the standby mode. 
Description will be given of the contents of the link-state data base 22 
and the routing table 19 in the route calculation units 11a, 11b. Note 
that the routing table 19 in the forwarding process unit 13 is the same as 
the routing table 19 in the route calculation unit 11 as will be described 
later. 
Taken as an example here is a case where networks are formed by a multiplex 
router 10 and three routers 30a, 30b and 30c as shown in FIG. 3. 
Assume that the three interfaces of the multiplex router 10 are connected 
to netE, netA and netD, the two interfaces of a router 30a are connected 
to netE and netC, the three interfaces of a router 30b are connected to 
netA, netC and netB, and the two interfaces of a router 30c are connected 
to netD and netB. 
Assume that the interfaces from the multiplex router 10 and the routers 
30a, 30b, and 30c to the networks are given the addresses as shown in FIG. 
3. 
Assume that the routers are given ID's (identities) as shown in FIG. 3 and 
those ID's are called the router ID's. 
In this case, data is registered in the link-state data base 22 of the 
multiplex router 10 as shown in FIG. 4. 
As shown in FIG. 4, this data base shows information about the multiplex 
router 10 and the other routers 30 connected to the networks to which the 
multiplex router 10 is connected, more specifically, information about 
router ID's, the identity of the networks to which the routers with those 
router ID's are connected, the interfaces and the costs assigned to the 
interfaces. The costs are assigned to the interfaces by configuration 
definition and the like, and the values are decided by taking into account 
the bandwidths of the networks connected and the user policies. 
The link-state data base 22 makes it possible to understand the 
configuration of the network system from its contents. For example, in 
FIG. 3, from the entries of router ID=192.168.1.1, it is possible to know 
that the router with ID of 192.168.1.1 is connected to netA, netB and 
netC, and the connection interfaces have the addresses of 192.168.1.1, 
192.168.10.3, and 192.168.12.10, respectively. These items of data 
precisely represent the interconnections of the router 30b as shown in 
FIG. 3. 
FIG. 5 shows the contents of the routing table 19. 
The routing table 19 is created from the link-state data base 22 according 
to the predetermined procedure. This procedure is called the SPF (Shortest 
Path First) algorithm, which decides the shortest path from this router to 
a destination network by considering cost, and the shortest path is 
registered in the routing table 19. 
The routing table 19 created by the SPF algorithm in the multiplex router 
shows the identity of the networks, the interface addresses of the router 
through which a packet is to be routed in order to reach the networks 
(namely, next hop router addresses), and total costs up to the networks. 
In the case of FIG. 3, the multiplex router 10 is directly connected to 
netA, netD and netE (without the intervention of another router). 
Therefore, there are no next hop router addresses for netA, netD or netE. 
On the other hand, to send a packet from the multiplex router 10 to netB, 
there are two paths, one path passing through netA and the router 30b, and 
the other path passing through netD and the router 3c (Refer to FIG. 3). 
The cost of the former path is the sum of the cost of the multiplex 
router's interface to netA (value 1) and the cost of the router 30b's 
interface to netB (value 3), namely, 4. On the other hand, the cost of the 
latter path is the sum of the cost of the multiplex router's interface to 
netD (value 1) and the cost of the router 30c's interface to netB (value 
1), namely, 2. The latter path incurs a lower cost, and therefore the 
latter path is selected. Therefore, the next hop router address for netB 
is 192.168.11.12, which is the address of the router 30c's interface to 
netD, and the cost is 2. 
To send a packet from the multiplex router 10 to netC, there are two paths, 
one path passing through netE and the router 30a and the other path 
passing through netA and the router 30b (Refer to FIG. 3). The cost of the 
former path is the sum of the cost of the multiplex router 10's interface 
to netE (value 1) and the cost of the router 30a's interface to netC 
(value 5), namely, 6. On the other hand, the cost of the latter path is 
the sum of the multiplex router 10's interface to netA (value 1) and the 
cost of the router 30b's interface to netC, namely, 4. The cost is lower 
for the latter path, and so the latter path is selected. Therefore, the 
next hop router address for netC is 192.168.1.1 which is the address of 
the router 30b's interface to netA, and the cost is 4. 
Each of the routers 30 other than the multiplex router 10 has its own 
link-state data base and routing table. 
The operation of the multiplex router according to the first embodiment 
will be described in the following. 
In FIG. 1, when the route calculation unit 11a in the multiplex router 10 
is in the active mode and the route calculation unit 11b is in the standby 
mode, the routers 30 exchange routing protocol packets with the route 
calculation unit 11a through the forwarding process units 13. Network 
link-state information that the route calculation unit 11a received from 
the routers 30 is first held in the route calculation unit 11a and further 
sent through the internal bus 12 to the route calculation unit 11b. 
The route calculation units 11a and 11b fetch data from their own 
link-state data bases 22, perform route calculations as mentioned above, 
and hold routing tables with the same contents. The route calculation unit 
11a in the active mode sends a routing table to the forwarding process 
units 13 to have the packet forwarded according to the routing table. 
If the route calculation unit 11a becomes unable to operate due to a 
failure or the like, the route calculation unit 11b in the standby mode 
detects the failure, and on behalf of the route calculation unit 11a, goes 
into the active mode. After this, the route calculation unit 11b exchanges 
routing protocol packets with the routers 30 through the intermediary of 
the forwarding process units 13. 
After entering the active mode, the route calculation unit 11b sends and 
receives Hello packets, and generates information, such as neighboring 
router state 24 and interface state 23 in the routing protocol information 
module 21. However, the route calculation unit 11b, already holding 
network link-state information from the routers 30 as the link-state data 
base 22, does not exchange network link-state information with the routers 
30 over again. 
After being brought into the active mode, the route calculation unit 11b 
transmits Hello packets periodically. For a while immediately after the 
switchover to the active mode, the route calculation unit 11b has no 
information about the neighboring router state 24 and interface state 23. 
Therefore, Hello packets transmitted at this point in time from the route 
calculation unit 11b include information, such as the ID of the multiplex 
router 10 and the identity of the networks connected to the route 
calculation unit 11b itself. However, the packets do not include a list of 
ID's of other routers 30 connected to the same network to which the 
multiplex router 10 is connected because this list must be prepared from 
the above-mentioned information about neighboring router state 24 and 
interface state 23, which is not available at this moment in time. The 
route calculation unit 11b gradually accumulates information about the 
neighboring router state 24 and interface state 23 from Hello packets 
transmitted periodically from other routers 30, and also gradually brings 
into a complete form a list of ID's of other routers 30, which is included 
into Hello packets that the route calculation unit 11b sends out. 
Because the other routers 30 that received Hello packets from the route 
calculation unit 11b are periodically receiving Hello packets from the 
multiplex router 10, the other routers 30 do not regard the multiplex 
router 10 as having run into a failure nor do they rewrite the routing 
tables they hold, even if the ID list of other routers included in 
received packets is incomplete. Therefore, even if a system switchover 
occurs, this does not affect the packet forwarding. If other routers 30 
receive an incomplete packet from the multiplex router 10, by regarding 
the multiplex router 10 as being in a faulty state, they manage the router 
state of the multiplex router 10, and perform a routing protocol process 
specified in OSPF to cope with that router state. As described earlier, 
when subsequently receiving a complete Hello packet from the multiplex 
router 10, the routers 30 return to the ordinary routing protocol process 
they executed before the system switchover occurred. 
Meanwhile, when the route calculation unit 11b brought into the active mode 
later receives network information from the other routers 30, it performs 
the same process as the route calculation unit 11a did when the route 
calculation unit 11a in the active mode received network link-state 
information as described above. 
Note that network link-state information that a given router transmits 
includes the contents equivalent to the information registered in the 
link-state table about this router as shown in FIG. 4. 
Description will now be given of the operation of the interior of the 
multiplex router 10 that performs the operations mentioned above. 
The state monitor module 20 in each of the route calculation units 11a and 
11b in the multiplex router 10 holds state information about its own route 
calculation unit 11 and the other route calculation unit 11, and monitors 
the other route calculation unit 11. If the route calculation unit in the 
standby mode detects that the route calculation unit in the active mode is 
unable to execute the process due to a failure or the like, the state 
monitor module 20 brings its own route calculation unit into the active 
mode and starts the RP packet transmission-reception module 14. 
In the route calculation unit 11 in the active mode, the RP packet 
transmission-reception module 14 exchanges routing protocol packets with 
other routers 30 connected to the multiplex router 10, and passes the 
contents of a packet and information about the interfaces that received 
the packet to the protocol information manager module 15. The protocol 
information manager module 15 generates routing protocol information 21, 
such as link-state data base 22, interface state 23, and neighboring 
router state 24 from information received from the RP packet 
transmission-reception module 14, and retains these items of information. 
When the link-state data base 22 is updated by the operation of the 
protocol information manager module 15, the route calculation module 16 
and the data base integration module 17 are started. The route calculation 
module 16 calculates a route from the link-state data base 22, and passes 
resulting route information to the routing table manager module 18. 
On the other hand, the data base integration module 17 refers to state 
information about its own route calculation unit 11, and when it is in the 
active mode, sends update information from the link-state data base 22 to 
the other route calculation unit 11. 
When the data base integration module 17 in the route calculation unit 11 
in the standby mode receives the above-mentioned update information, the 
received information is reflected in the link-state data base 22 of its 
own unit. Thus, network link-state information independently collected by 
the route calculation unit 11 in the active mode is reflected in the 
link-state data base 22 of the other route calculation unit 11 in the 
standby mode. Interface state 23 and neighboring router state 24 are 
retained in the route calculation unit 11 in the active mode, but they are 
not retained in the route calculation unit in the standby mode. 
In the route calculation unit in the standby mode, when the link-state data 
base 22 is updated by the data base integration module 17, the route 
calculation module 16 starts to operate, and calculates a route from the 
link-state data base 22 and passes resulting route information to the 
routing table manager module 18. 
In the route calculation units in the active mode and in the standby mode, 
the routing table manager module 18, when receiving new route information, 
updates the routing table 19. The route calculation unit in the active 
mode also sends update information of the routing table to all forwarding 
process units 13 connected to the internal bus 12. The route calculation 
unit in the standby mode merely updates the routing table 19 in the module 
18 but does not send the update information to any forwarding process unit 
13. 
Detailed description will be given of the process steps executed by the 
respective modules in each route calculation unit 11 to realize the 
above-mentioned operations. 
FIG. 6 shows the flow of the process steps of the state monitor module 20 
of each route calculation unit 11 when the multiplex router 10 starts its 
operation. When the multiplex router 10 goes into the active mode, the 
state monitor module 20 monitors the processing portions in its own route 
calculation unit 11 to make sure that they have been initialized (step 
101). When the initialization step is finished, the route calculation 
units 11 have been initialized. At the end of the initialization, the 
route calculation units 11 are switched over to the standby mode, and each 
route calculation unit 11 gives notification that it has been placed in 
the standby mode to the other route calculation unit 11 (step 102). 
The state monitor module 20 monitors the state of the other route 
calculation unit 11 (step 103). If all route calculation units are in the 
standby mode, the state monitor module 20 checks the contents set in the 
configuration definition information in the route calculation unit (step 
104). If the route calculation unit of its own side is set in the active 
mode in the configuration definition information (step 105), the route 
calculation unit 11 brings itself into the active mode, and this is 
notified to the other route calculation unit 11 (step 106). Subsequently, 
the RP packet transmission-reception module 14 (step 107) is started. 
FIG. 7 shows the process to be executed after the process shown in FIG. 6. 
In this process, the state monitor module 20 gives an active mode 
notification to the other route calculation unit 11 periodically (step 
111). The state monitor module 20 checks if there is any state 
notification from the other route calculation unit 11 (step 112). If there 
is no state notification for a fixed period, the state monitor module 20 
perceives that a failure has occurred in the other route calculation unit 
11. If the route calculation unit 11, which was perceived to have a 
failure, is in the active mode and its own route calculation unit 11 is in 
the standby mode (step 113), the state monitor module 20 brings its own 
route calculation unit into the active mode (step 114), starts the RP 
packet transmission-reception module 14 (step 115), and continues to 
execute the monitoring process. 
FIG. 8 shows the procedure of the process steps of the RP packet 
transmission-reception module 14 in the route calculation unit 11 in the 
active mode. 
The RP packet transmission-reception module 14, when it is started, 
transmits routing protocol packets, such as Hello packets, onto the 
networks directly connected to the multiplex router 10, and receives 
routing protocol packets from other routers (step 121). If a received 
packet has come from a neighboring router, the module 14 checks whether 
the presence of which has been or has not been recognized (step 122). If 
the presence of which has not been recognized, the module 14 notifies the 
protocol information manager module 15 of the newly-detected neighboring 
router (step 123). If the presence of which has been recognized (step 
124), the module 14 sends this network link-state information to the 
protocol information manager module 15 (step 125). 
FIG. 9 shows the procedure of the process steps of the protocol information 
manager module 15 in the route calculation unit 11 in the active mode. 
In this process, the protocol information manager module 15 receives 
information from the RP packet transmission-reception module 14, and 
checks if information received is network link-state information (step 
131). If the information is not network link-state information, in other 
words, if the information is about a neighboring router, the module 15 
generates neighboring router state 24 and interface state 23 from 
information received (step 132). On the other hand, if the information is 
network link-state information, the module 15 checks if the information 
received agrees with the contents of the link-state data base 22 (step 
133). 
If agreement is confirmed, it is not necessary to update the link-state 
data base 22. If they disagree, in other words, if it is necessary to 
update or delete existing information or add new information, the module 
14 updates the link-state data base 22 (step 134). Then, the module 14 
sends notification that the network link-state data base 22 has been 
updated and the contents of update to the data base integration module 17 
and the routing table calculation module 16 (step 135). 
The process steps executed by the data base integration module 17 will be 
described with reference to FIGS. 10 and 11. 
FIG. 10 shows the procedure of the process steps by data base integration 
module 17 on receiving notification of update of the link-state data base 
22. 
In this process, when the data base integration module 17 receives update 
information (step 141), if its own route calculation unit is in the active 
mode (step 142), the data base integration module 17 sends notification 
that the link-state data base 22 has been updated and the contents of 
update to the other route calculation unit 11, and closes the process. 
FIG. 11 shows the procedure of the process steps by the data base 
integration module 17 in the route calculation unit 11 in the standby mode 
when the module 17 receives notification of update of network link-state 
information from the route calculation unit 11 in the active mode. 
In this process, the data base integration module 17 receives notification 
of update and obtains update information (step 151). The module 17 checks 
if update information agrees with the contents of the link-state data base 
22 retained (step 152). If agreement is confirmed, it is not necessary to 
update the link-state data base 22, and therefore the process is 
terminated. If they disagree, in other words, if existing information is 
updated or deleted or new information is added, the module 17 updates the 
link-state data base 22 (step 153). The module 17 then sends update 
notification that the link-state data base 22 has been updated and the 
contents of update to the routing table calculation module 16 (step 154), 
and closes the process. 
FIG. 12 shows the procedure of the process steps by the routing table 
calculation module 16 on receiving notification of update of the 
link-state data base 22. 
In this process, the routing table calculation module 16 reads information 
about the updated link-state data base 22 (step 161), calculates routes by 
the earlier-mentioned SPF algorithm (step 162), and sends calculation 
results to the routing table manager module 18 (step 163). 
FIG. 13 shows the procedure of the process steps by the routing table 
manager module 18 on receiving results of route calculation from the 
routing table calculation module 16. 
In this process, the routing table manager module 18 obtains calculation 
results (step 171), and updates the routing table 19 (step 172). When its 
own route calculation unit is in the active mode (step 173), the module 18 
sends notification that the routing table 19 has been updated and the 
contents of update to all forwarding process units existing in the 
multiplex router 10 (step 174) and closes the process. When the route 
calculation unit is in the standby mode, the module 18 updates the routing 
table and closes the process. 
The forwarding process unit 13 rewrites the internal routing table 19 in 
accordance with the notified contents of update. 
By the operations described above, even if there are a number of routers on 
the network to which a multiplex router is connected, at the time of a 
system switchover of the route calculating units in the multiplex router, 
it is possible to limit the traffic in the multiplex router and in the 
network. 
Meanwhile, the multiplex router 10 in FIG. 2 can be put into practice by 
applying a hardware configuration shown in FIG. 14, for example. 
As shown in FIG. 14, in this configuration, the route calculation units 11 
and the forwarding process units 13 are interconnected by the internal bus 
12 of the multiplex router. 
Each route calculation unit 11 includes a route calculation processor 40 
and a memory 41. The route calculation processor 40 transmits and receives 
routing protocol packets to and from the routers 30 connected to the 
multiplex router 10. The route calculation processor 40 also manages 
routing protocol information, and calculates and distributes routing 
tables. All of the RP packet transmission-reception module 14, the 
protocol information manager module 15, the data base integration module 
17, the routing table calculation module 16, the routing table manager 
module 18, and the state monitor module 20 can be collectively realized as 
processes in the route calculation processor 40. The memory 41 stores the 
routing protocol information 21 (link-state data base 22, interface state 
23, and neighboring router state 24) and the routing table 19. 
The forwarding process unit 13 includes a forwarding processor 42, a memory 
43 and a packet buffer 44. The forwarding processor 42 makes a decision 
about whether or not to forward packets between the communication 
terminals 31 and decides transit nodes. The memory 43 stores a routing 
table 19 distributed by the route calculation units 11, required for 
packet forwarding. The packet buffer 44 temporarily stores packets 
received by the multiplex router 10. Packets judged forwardable according 
to the routing table are forwarded to the packet buffer in the forwarding 
process unit 13 at transit nodes, and transmitted. The packets judged not 
forwardable are deleted from the packet buffer 44. 
The first embodiment of the present invention has been described. 
Description will now move on to a second embodiment of the present 
invention. 
FIG. 15 shows a configuration of the multiplex router according to a second 
embodiment of the present invention. 
According to the second embodiment, a multiplex router is formed by 
connecting a plurality of routers 10' each not including multiple route 
calculation units 11. The plurality of routers 10' are connected through 
forwarding process units 13 and networks or other transmission paths. 
In the second embodiment, out of a plurality of routers 10' connected, one 
router 10' is placed in the active mode and this router 10' obtains 
routing protocol information, while other routers are placed in the 
standby mode. 
The route calculation unit 11 of each router 10' monitors the other router 
10' through the forwarding process unit 13 of its own router 10'. The 
router 10' in the active mode holds routing protocol information 21, and 
when the link-state data base 22 has been updated, the data base 
integration module 17 executes the process shown in FIG. 10, and the data 
base integration module 17 sends update information to the router 10' in 
the standby mode through the forwarding process unit 13. In the router 10' 
in the standby mode, when it receives update information from the router 
10' in the active mode, the data base integration module 17 executes the 
process in FIG. 11, and registers the update information in the link-state 
data base 22 in its own router. 
If a failure or the like has occurred in the router 10' in the active mode, 
the whole of the router 10' shifts to the standby mode, a router 10' is 
switched to the active mode, and the route calculation unit 11 of the 
router 10' switched to the active mode sends a routing table to the 
forwarding process units connected to it, and causes the packet to be 
forwarded. Alternatively, it is possible to make such an arrangement that 
the routing table of the forwarding unit 13 in the router 10' switched to 
the standby mode should be rewritten each time the link-state data base is 
updated and that the forwarding process unit 13 of the router 10' switched 
to the standby mode should forward the packet. 
Incidentally, if there are a plurality of routers, priority is established 
as to which router is the first to enter the active mode using 
configuration definition information. When a router is going to be 
started, if there is not any other router which notifies that it is in the 
standby mode, the router brings itself into the active mode. If there is a 
router that notifies that it is in the active mode, the router which 
received the notification goes into the standby mode. If there is no 
router which notifies that it is operating and if there are a plurality of 
routers which notify that they are in the standby mode, a router that goes 
into the active mode is determined by a value set by configuration 
definition information. If a router goes into the active mode, the router 
starts its own RP packet transmission-reception module 14. 
If a route calculation unit 11 has a network interface, the route 
calculation unit 11 makes a direct connection to the network without the 
intervention of a forwarding process unit, and in this case, the router 
monitors the other router through a network to which the route calculation 
unit 11 is directly connected. Update of the link-state data base 22 is 
notified to the other router by direct communication between the route 
calculation units 11 without the intervention of the forwarding process 
units 13. 
With the second embodiment, the same effects can be obtained as in the 
first embodiment. 
As has been described, according to the present invention, using the router 
device including at least route calculation units in a multiplex 
configuration, it is possible to prevent an interruption of packet 
forwarding and an increase of traffic on the network at the time of system 
switchover, while reducing the amount of information transmitted from the 
route calculation unit in the active mode to the route calculation unit in 
the standby mode.