Using crossover cable vs. connecting switch in heartbeat comm.

[Hanry Zhou]

we are going to have 3 heartbeat connections. WE understand there are two approaches: a) using crossover able connects two nodes directly. b) all heartbeat connections is going through the switch.

I know that all these two methods are supported by hp, and the method a) is suggested in SG manual. However, as a matter of fact, the b) is better than a)

Can anybody please provide me any supporting document, or let me know why b) is better a)?

Thanks,

[Granite]

Hi Roger,

Added to your 'a'and'b', for a TWO-NODE cluster, you can make direct serial connections b/w the nodes for heart-beat.Generally serial conn. is used as stand-by heartbeat LAN.

Just look this,
http://docs.hp.com/en/B9903-90046/ch04s06.html

regds,granite

[A. Clay Stephenson]

Perhaps the best reason for not using a x-over cable is that you simply can't see the winky-blinkies. While you are adding another component that might fail (a hub or a switch), the diagnostics that a hub or switch gives is well worth it. Moreover, should you need to expand beyond a two-node cluster, it's much easier since all you have to do is plug in another cable.

[Mel Burslan]

I have to impose one question to you as an answer why:

do you really trust your cluster to a piece of (most probably custome made) $5 worth of cable while you invest thousands of dollars in serviceguard licenses, cluster compatible hardware and all that good stuff ?

Do you really want to have a piece of vital component which can not be monitored by any monitoring suite known to man kind but can make or break your cluster ? When you think about the down side of it, you will realize how ridiculous the cost savings of having a crosover cable instead of few switch ports, compared to the risk you are taking. Whereas if a switch port goes bad or shows any signs of malignancy, all monitoring software goes beserk sending alarms to high heavens if need be.

I don't know about you but I prefer to have an uninterrupted night of sleep instead of dealing with a cluster outage.

[melvyn burnard]

As requested:

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.

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.