Will Crossover cable work for SG heartbeat?

[Darrell Tschakert]

Hi,
We have two rx4640's running 11.23. They share an HP VA7410 and run HP Service Guard. We currently have our heartbeat connection running on a private network through a cheap six port hub. It currently goes like this:

rx4640#1 -> Single port NIC -> Cable -> Hub -> Cable -> Single Port NIC -> rx4640#2.

Can I dump the two cables and hub and replace them with a cross-over cable? If not, then why.

Thanks,

Darrell Tschakert

[melvyn burnard · Accepted solution]

We often get questions asking whether Crossover cables are supported for use in a ServiceGuard cluster. The short answer is YES, but there are some important issues that you should be aware of:

This solution only works in a two node cluster. There is no way to have a Standby LAN card when using a Crossover LAN cable, so it is not possible to use SG local LAN failover.

When either LAN card fails, or the crossover cable is disconnected, both LAN cards go down. This is because the electrical signals necessary for the cards to determine that a valid LAN connection exists are not present. The result is that since both nodes appear to have a bad LAN card, ServiceGuard may TOC the wrong node. If a hub was used between the two LAN cards, then the hub would provide the electrical signals to the other card, allowing it to stay up.

On multi-speed cards, such as 10/100Base-T, the cards must negotiate which speed will be used when the system boots up. If only one system is booted and the remote system is down, then the negotiation will fail, and the card will not be enabled at all. So when the second node eventually comes up, it's LAN will also be down. If a hub is used, then the negotiation will succeed, so the LAN cards will come up at bootup, even if only one node is running.
It may be possible to force some multi-speed LAN cards to bypass the negotiation at bootup and to use a predetermined fixed speed. If this is possible, then would allow the two systems to boot up at different times and still use the Crossover cable connected LAN cards once they are both booted up.

Since both cards may go down when there is a failure when a Crossover cable is used, it can be difficult to determine where the problem lies. Another problem using Crossover cables is that if they are not properly labeled, they may accidently be used in situations where they will not work.
For the reasons listed above, HP does not recommend using Crossover cables for ServiceGuard configurations. However, they are still supported as long as you are willing to accept the above limitations. Using a Crossover cables is cheaper than using a hub, but it compromises the HA solution.

[Darrell Tschakert]

Burnard,
What a great answer. Give the guy a silver dollar and a thank you very much. Very helpful. I will pass it on to my supervisor and she will say the same thing.

Enjoy,

Darrell Tschakert

[A. Clay Stephenson]

... and, one of the best reasons for not using an xover cable is that now you don't have any winky-blinky lights. In your case, with both end's of the connection showing dark LED's, (should those be called Dark Emitting Diodes -- DED's?) which end is actually bad? The hub, while seeming to add another failure point is actually a very good online diagnostic tools because now it is easy to determine which end of the connection actually has the problem. Moreover, because you should always have multiple network connects the loss of a hub doesn't significantly add to overall failure risk.

[Darrell Tschakert]

Thank you for the two replies. We will definitely not be using a cross-over cable to connect the heartbeat.

Darrell Tschakert