If I assign IP addresses on the same network to both ethernet ports on my SLC I get some strange behavior. Why is that?

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This behavior will occur on any Linux machine with two Ethernet ports using IP addresses on the same IP subnet. Since the SLC is a Linux based product the SLC will exhibit this behavior.

The way IP addresses are assigned to ethernet ports in Linux is indeterminate if both IP addresses are on the same IP subnet, as there is no routing required either port may be legally used. If two IP addresses on different subnets are used then the SLC will keep track of which IP is assigned to which port in it's routing table because now it makes a difference which port is used for which IP address.

Since both the E1 and E2 Ethernet ports on the SLC are controlled by a single Ethernet chip there's no real advantage to assigning 2 IP addresses on the same IP subnet. The dual Ethernet ports on the SLC were designed to allow the SLC to be connected to both a production network and a backup management network. This allows systems administrators to have access to the SLC via the management network if the production network goes down.

We recommend that you do not assign IP addresses on the same IP subnet to both Ethernet ports on an SLC.

Background:

Note:

E1 = SLC Ethernet port 1

E2 = SLC Ethernet port 2

Each SLC Ethernet port has it's own MAC address. If you configure each SLC Ethernet port with an IP address on the same subnet the SLC will initially associate each IP address with the MAC address for the port you specify.

However, if one of the Ethernet ports becomes unavailable, the underlying Linux operating system in the SLC will start using the available MAC address for both IP addresses, since they're on the same subnet.

Example:

Assume an SLC016 configured as follows:

Port E1 MAC: 00-80-A3-89-01-A7, IP 192.168.11.22

Port E2 MAC: 00-80-A3-89-01-A8, IP 192.168.11.23

Initially the SLC will associate 192.168.11.22 with MAC address 00-80-A3-89-01-A7. A host connected to 192.168.11.22 will have MAC address 00-80-A3-89-01-A7 associated with 192.168.11.22 in it's arp table.

If E1 becomes unavailable a connected host will continue to try to connect to MAC address 00-80-A3-89-01-A7. Until the host's arp table is refreshed to associate 192.168.11.22 with E2's MAC address of 00-80-A3-89-01-A8 the host won't be able to connect to the SLC on 192.168.11.22. However the host will still be able to connect on E2's IP address, 192.168.11.23.

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[Originally Published On: 10/28/2005 10:02 AM]