If a firewall or other security services are enabled, the router TCP port might be blocked, and the router and the client cannot communicate.

If a firewall and other security services are disabled, a denial-of-service attack might occur on the router.

To configure quality of service (QoS) to set the rate-limit for MODBUS TCP traffic, create an access-list that only permits traffic sending to port number 502 that is reserved for MODBUS communication. Then attach the access-list to the input class-map and attach it to the interface and set the rate limit to permit traffic via default port 502 and prioritize SCADA packets.

DUT-1:
!
class-map match-any Modbus-out-Traffic
 match qos-group 1
class-map match-any Modbus-In-Traffic
 match access-group 101
!
policy-map Modbus-In
 class Modbus-In-Traffic
  set qos-group 1
policy-map Modbus-Out
 class Modbus-out-Traffic
  police 10000000
  priority
!
!
interface GigabitEthernet0/1/1
 switchport mode access
 service-policy input Modbus-In
!
interface GigabitEthernet0/1/2
 switchport mode access
 service-policy output Modbus-Out
!
!
interface Vlan1
 no ip address
 ip access-group 101 in
 rate-limit input access-group 101 8000 8000 8000 conform-action transmit exceed-action drop
!
! 
!
access-list 101 permit tcp 10.10.10.0 0.0.0.255 any eq 502
 
DUT-2:
 
interface Vlan1
 ip address 192.168.1.2 255.255.255.0

This example shows that 133 SCADA packets were classified.

DUT-1#show policy-map interface GigabitEthernet0/1/2
 GigabitEthernet0/1/2
  Service-policy output: Modbus-Out
    Class-map: Modbus-out-Traffic (match-any)
      133 packets
      Match: qos-group 1
      police cir 10000000 bc 312500
         conform-action transmit
         exceed-action drop
      conform: 133 (packets) exceed: 0 (packets)
        Priority
      Output Queue:
        Max queue-limit default threshold: 272
        Tail Packets Drop: 0

The router can receive multiple request messages from clients and respond to them simultaneously.

You can set the number of client connections from 1 to 5. The default is 1.

The router is not configured as a MODBUS TCP server.

The TCP port number is 502.

The number of simultaneous connection requests is 1.

Memory address spaces 0x0800 through 0x0FFF are system information registers. Clients use the 0x03 Read Multiple Registers MODBUS function code. The system-information register mapping is as follows:

Memory address spaces 0x1000 through 0x3FFF are read-only interface registers. Clients use the 0x03 Read Multiple Registers MODBUS function code to access the registers.

The following table shows the memory map for all interface registers, with 64-bit counters (address space 0x1000 – 0x2FFF, 8K registers):