Routing With no Gateway?
We have some older model Siemens PLCs that I'm told have an IP address, but no gateway. Can anyone confirm if they have seen something like that?
Second and more important is my part in this challenge: If it is possible that a device has no gateway, how to get it to talk/route to a different subnet?
Edit: I see some Siemens documentation that talks about using subnets and supernets to get around the issue. So if it helps, my destination subnet is 10.1.179.0/24 and my PLC is 10.107.151.10.
Second and more important is my part in this challenge: If it is possible that a device has no gateway, how to get it to talk/route to a different subnet?
Edit: I see some Siemens documentation that talks about using subnets and supernets to get around the issue. So if it helps, my destination subnet is 10.1.179.0/24 and my PLC is 10.107.151.10.
yo_bee
Without a gateway there is no ability to route traffic off that subnet hence the term Gateway.
Yes, Siemens documentation is correct. You can go around gateway, but few conditions must met.
- PLC needs to believe that destination device is in the same IP address range in your case 10.107.151.10/8 or 10.107.151.10/9 will do the trick
- on router default gateway interface for PLC need to be configured to do proxy ARP and have route to 10.1.179.0/24
- 10.1.179.0/24 end host need to be configured with correct gateway
What will happen:
- PLC will ARP for 10.1.179.x device since it believes that host is part of same subnet
- (although it is not configured as default gateway on device itself) interface that is default gateway for PLC subnet configured with proxy ARP will respond to any ARP request outside it's own subnet if it has route to packet destination and PLC device will not even be aware that end host is in different subnet
Test configuration:
Host 1 (PLC)
10.107.151.10/8
no default gateway
Host 2 (other subnet)
10.1.179.2/24
default gateway 10.1.179.1
Router1 (for simplicity directly connected):
interface GigabitEthernet0/0
description PLC
ip address 10.107.151.1 255.255.255.0
ip proxy-arp
!
interface GigabitEthernet0/1
description Host2
ip address 10.1.179.1 255.255.255.0
There are other similar ways to resolve this, but all solutions will relay on proxy arp to provide communication outside of local subnet for host that has no default gateway configured.
- PLC needs to believe that destination device is in the same IP address range in your case 10.107.151.10/8 or 10.107.151.10/9 will do the trick
- on router default gateway interface for PLC need to be configured to do proxy ARP and have route to 10.1.179.0/24
- 10.1.179.0/24 end host need to be configured with correct gateway
What will happen:
- PLC will ARP for 10.1.179.x device since it believes that host is part of same subnet
- (although it is not configured as default gateway on device itself) interface that is default gateway for PLC subnet configured with proxy ARP will respond to any ARP request outside it's own subnet if it has route to packet destination and PLC device will not even be aware that end host is in different subnet
Test configuration:
Host 1 (PLC)
10.107.151.10/8
no default gateway
Host 2 (other subnet)
10.1.179.2/24
default gateway 10.1.179.1
Router1 (for simplicity directly connected):
interface GigabitEthernet0/0
description PLC
ip address 10.107.151.1 255.255.255.0
ip proxy-arp
!
interface GigabitEthernet0/1
description Host2
ip address 10.1.179.1 255.255.255.0
There are other similar ways to resolve this, but all solutions will relay on proxy arp to provide communication outside of local subnet for host that has no default gateway configured.
I can’t speak to how your PLC's behave, but it is possible to communicate off net without a gateway defined on the host.
In some cases a host can ARP for all addresses (local and non-local). The router is configured to perform “proxy ARP” and will respond to requests for non-local addresses with its own MAC.
These days it’s pretty unusual, but I’ve run into function specific hosts where this is the only way to make it communicate with hosts on different networks.
In some cases a host can ARP for all addresses (local and non-local). The router is configured to perform “proxy ARP” and will respond to requests for non-local addresses with its own MAC.
These days it’s pretty unusual, but I’ve run into function specific hosts where this is the only way to make it communicate with hosts on different networks.
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