configure 2 interfaces on the same lan

[Antonio Cardoso_1]

Hi,

I'm trying to configure a rp4440 running HP-UX 11iv1 to have 2 network interfaces connected to the same LAN.
Both are configured with the same subnet mask.
When I run the ifconfig on lan1, the route configuration seems OK, for example (subnet mask is 139.54.129.192, hostname is 'visan'):
Destination Gateway Flags Refs Interface Pmtu
localhost localhost UH 0 lo0 4136
139.54.129.240 139.54.129.240 UH 0 lan1 4136
visan visan UH 0 lan0 4136
139.54.129.192 139.54.129.240 U 2 lan1 1500
139.54.129.192 visan U 2 lan0 1500
...
loopback localhost U 0 lo0 0
default 139.54.129.254 UG 0 lan0 0

but at this point, I can no more ping the default gateway from my server, nor connect to it from outside. It seems to jam the network service.

Can someone tell me where I am wrong ??

thanks.

[Patrick Wallek · Accepted solution]

Having 2 NICs on the same network is not supported and can lead to unpredictable behavior.

You either need to have the 2 NICs on different subnets, or utilize software like AutoPort Aggregation to combine the 2 physical NICs into a single logical NIC.

[Steven E. Protter]

HP-UX does not support two NIC cards on the same network.

Example:

nic1 192.168.0.10 netmask 255.255.255.0
nic2 192.168.1.20 netmask 255.255.255.0

netmask says that the first 3 octets are the network.

192.168.0
192.168.1

supported

nic1 192.168.1.10 netmask 255.255.255.0
nic2 192.168.1.20 netmask 255.255.255.0

unsupported.

I tried this once on HP-UX. It caused the machine to drop off the network immediately. It required console access to fix it.

SEP

[Antonio Cardoso_1]

OK, this confirms what I noticed.

[rick jones]

Configuring multiple NICs into the same IP subnet without using Auto Port Aggregation requires that all the NICs have equal connectivity to the network. By default, all NICs will accept inbound traffic, but only one NIC will be used for outbound traffic, so that NIC best be able to reach everything. For all incense and porpoises you should assume that the NIC used for outbound traffic is random (although it isn't really).

There can be ways to get outbound traffic to flow through each NIC without APA - in involves setting ip_strong_es_model to a value other than default. At that point, traffic will tend to flow out the same NIC on which it arrived. The downside is that now each NIC will only accept traffic with destination IP addresses matching those assigned to the NIC. I believe there was going to be a middle ground for ip_strong_es_model, but am not sure if that is released at present. The intent was to have source IP included in route selection without the inbound restriction.

You could also add specific static routes using specific local IPs as the gateway - assuming of course that your routers support proxy ARP.

Still, it _is_ best to use APA when you want multiple NICs on the same "lan" - it gives failover, load balancing etc all without needing to do clever things at the transport layer.