ServiceGuard heartbeat LAN configuration

[John Garrott]

We are setting up a two-node ServiceGuard cluster on 11i servers. I have helped setup a cluster once before and HP recommended using redundent switches for the heartbeat LAN. The network manager here has asked us to use a cross-over cable to spare the cost of the switches, but we have been unable to get the cluster to configure successfully using the cross-over for the heartbeat. Is it possible to do so? If so, does HP support this configuration? The HP consultant that helped with the previous cluster setup was fairly adament about using switches. Any feedback would be appreciated.

John

[curt larson_1]

well i'm sure you can make it work.

but with all the investment in system hardware and software, along with facilities, and application programming, etc.

to have all that and to have it all fail because of a single $20 network cable. that isn't a recommendation i'd ever make.

let's see, how much does a couple of switch ports cost? how much time has been invested in making this crossover cable work? how long is it going to take to fix this crossover cable should it fail? what are you doing to do when you need more network bandwith? I hope the answer isn't wire more crossover cables between servers.

if cheap is your motivation here, why are you using service guard at all. if it is uptime, then redunancy should be your concern. And you shouldn't have to explain which of those would be supported by using crossover cables.

[BradF]

We use hubs for the heartbeat LAN.

[A. Clay Stephenson]

I've certainly used a crossover cable for just this purpose; if you are not comfortable with a crossover cable just use an inexpensive hub. However, you really need 2 switches along with a NIC port on each host that connects to ports on each switch for redundancy anyway. I would really prefer two dual port NICS on each host. Each NIC would then have a connection to each switch and your network could tolerate the failure of a NIC or switch. If you then wanted to define an additional hearbeat_ip using an additional NIC with a crossover cable or an unmanaged hub that would be gravy. My current configurations use unmanaged hubs for the additional heartbeat because sometimes its nice to be able to see those blinks --- they ain't none too obvious inside a crossover cable.

[A. Clay Stephenson]

I should add that "sparing costs" and ServiceGuard don't collide too often in the same sentence. MC/SG is really the icing on the cake because by the time you have made your systems robust enough all the usual failures (disks,cables,network failures) are handled by LVM or APA long before ServiceGuard even comes into the picture. I can honestly say that I have never had a package failover (except for manually induced ones) in over 5 years.

[Steven E. Protter]

With hubs in the $30 range there is not an expense issue.

You don't need an expensive switch.

What you do need is an absolutely reliable, simple path for that heartbeat to go through. If you run it through your "public" lan, congestion can stop the heartbeat and if you lose hearbeat longer than the timeout setting in your cluster configuration....

boom.

toc one or both of the nodes will boot.

SEP

[melvyn burnard · Accepted solution]

Well I have posted this many timnes before, but here goes yet again:

Pros and Cons of using a Crossover cable with ServiceGuard


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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.