2 Ethernets IPs on the same subnet ?

[Q4you]

We have a server with lan0 operates at 1GB, it is configured in 145.67.209 subnet, it is primary IP.

We recently procured another 1G card, I wish to connect it to same subnet 145.67.209 with IP 145.67.209.35, same subnet mask ?

Is it possible to configure both interfaces in the same subnet for a server ( I think it was not possible sometime back)? Any link to HP doc would be of great help ?

Is there any downside (except for LAN redundancy) ?

- Q4U

[Patrick Wallek]

No, you should not do this. I searched though the Forums with the phrase "2 IP same subnet" and got a whole list of threads on this topic. Here are 3 out the list that I got:

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0xe0e3a848deccd61190050090279cd0f9,00.html

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x10fb56bd90a9d611abdb0090277a778c,00.html

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x026650011d20d6118ff40090279cd0f9,00.html

[Steven E. Protter]

You can't do that.

On HP-UX you will bring down the network in the worst case, in the best case, one of the cards will not be up.

You can purchase Auto Port Agregation(APA) and this can provide redundancy.

You can write a script that will occaisionally ping the main card and bring up the backup if its down.

SEP

[Sunil Sharma_1]

Hi,
You should not use 2 cards in same subnet.
but you can use Hp APA software to do this in this case it will act as a single ard and will work in Load sharing or Fail over mode.
you can configure either way..

Sunil

[T G Manikandan]

You cannot do that.

For to assign ip's of same subnet you need to use the APA software.

http://www1.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000048086687

[Brian Hackley]

Folks,

Recently this issue has been revisited for HPUX 11.0 and beyond by HP Labs, Expert Center and Response Centers:

------------
The official HP policy has been, that although the functionality may
work, that it is not supported. However, due to customer request, we now
provide this document. It describes the limited support and behavior of
multiple network interfaces that are configured into the same IP subnet via the
ifconfig command on HP-UX 11.X.

To summarize, this configuration is only supported for connectivity.

It is not intended for performance improvement or load balancing over multiple
interfaces; in that case the recommendation is to use Auto Port Aggregation
("APA") product.

Statement about support of multiple I/F configured with an IP address
belonging to the same subnet:

We certify that the TCP/IP stack will have the following behavior when multiple
interfaces are configured into the same IP subnet. Please note that the use of
multiple network interfaces configured into the same subnet via the HP-UX
Transport (ie via the HP-UX ifconfig command) is for connectivity purposes to
support interface testing and system management.

It is not intended for applications requiring more than one network interface's
worth of bandwidth.

1. Inbound traffic destined for a local IP address will be received on the
interface ifconfig'd with that IP address.

2. Outbound traffic to a next-hop destination IP address in the local subnet
will be sent out via the last interface ifconfig'd into that subnet.

3. Outbound traffic to destinations for which there is a specific host route
will use the interface specified by the route, regardless of the order in which
the interfaces are configured.

4. When ndd (HP-UX 11.x only) is used to enable the "Strong ES Model,"
applications which have bound to a specific local IP address and whose next hop
destination IP address is in the local subnet, will have their traffic sent via
the interface assigned that local IP address. Note, that in the context of this
discussion, the term "local IP address" refers to one of the IP addresses
assigned to an interface on a system with multiple interfaces configured into
the same IP subnet. "Next-hop destination IP address" refers to the final
destination IP address when that destination IP address is in the same IP
subnet, or the next-hop gateway IP address when the final destination IP
address is not in the same IP subnet.

Because of this limited support, HP still does not recommend
configuring multiple LAN cards on the same IP subnet.
--------------
Hope that helps clear the air,

-> Brian Hackley