topic Re: LAN failover in Operating System - OpenVMS

LAN failover

Wim Van den Wyngaert — Thu, 13 May 2004 06:16:13 GMT

I have a GS160 with double everything. Except double network connection. I would like to plan the activation of the 2nd card (on a 2nd switch). I have VMS 7.3 and I am unable to upgrade (even install patches).

For decnet, I can enable the 2nd card and decnet will use both cards.

For LAT, I can create a link for the 2nd card.

For IP, I can assign a second IP address. AND I don't want to do that because my clients are unable to use it.

Is there a way to let IP use 2 cards with 1 IP address ? My IP version is 5.3 ECO 2.

And for those who know DSM/MUMPS : how will DDP use the second card ?

Re: LAN failover

Mobeen_1 — Thu, 13 May 2004 06:28:57 GMT

Wim,
Thats a good one. What we have been doing is, having 2 cards configured to 2 different IPs, one for production use and the other for backup network (for carrying out backups without clogging the production network).

It would be interesting for me to see if we could have one IP assigned to 2 n/w cards and know what benefits one could get in such a configuration.

regards
Mobeen

Re: LAN failover

Wim Van den Wyngaert — Thu, 13 May 2004 06:31:14 GMT

Mobeen,

FYI : 7.3-2 has LAN failover. 1 card can fail and the other takes over. But I don't know if this would also work if the switch behind it fails. But this is another subject.

Re: LAN failover

Antoniov. — Thu, 13 May 2004 07:49:44 GMT

Hello Wim,
I think you can't use same IP on two different cards; main problem is not in software but it's inside TCP/IP techinal characteristics.
When any host send a socket into network, this soccket have to reach the destination host; IP address tell to all network device where socket have to go; low level network, however doesn't use IP addres but use MAC (Medium Address Card), the hex number stored in every network device; two specific part of TCP/IP standard determine how IP address is associated to MAC and theese feature have named ARP and RARP.
If you assign same IP to 2 different NIC, some network device (mainly the routers and bridges) can be confused and can't deliver the socket.
This means, if can assign same IP to 2 different NIC you have disable one fo two devices.
I hope I can explain in simple way.

@Antoniov

Re: LAN failover

Ian Miller. — Thu, 13 May 2004 08:07:21 GMT

TCPIP V5.4 has failover for ip if you can upgrade tcpip.

Why the restrictions on upgrading?

Re: LAN failover

Mobeen_1 — Thu, 13 May 2004 08:28:38 GMT

Wim,
Yup, i am aware that 7.3-2 supports n/w failover, but i thought thats assuming that the 2 cards have different IP addresses and not a single IP assigned to your 2 network cards

regards
Mobeen

Re: LAN failover

Wim Van den Wyngaert — Thu, 13 May 2004 08:37:13 GMT

Ian,

I don't upgrade because "on paper" all applications must approve the new system.

We need to keep the platform as stable as possible. The migration of 6.2 -> 7.3 took 2 years !!!

Re: LAN failover

Jan van den Ende — Thu, 13 May 2004 08:46:33 GMT

Ha Wim,

there is a recent thread
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=385987
that is mainly concerned with IP cluster alias stuff.
The thread also discusses DNS round-robin (kind of poor-mans cluster alias, or maybe more exactly: as close as Unix can get to cluster aliasing). Using this mechanism it IS possible use both cards (we are doing it, over 2 cards each of 4 nodes).
The way it works: any existing connection is to MAC address and stays where it is. After a round-robin step the service name (functional analogous to DECnet cluster adddress, but NOT the IP cluster alias!) is changed to the next MAC address, and new connections (IF using DNS, not router-cache!) get connected to the other NIC. No real load-balance, but more or less even connection-time based spread.

hth.

Jan

Re: LAN failover

Wim Van den Wyngaert — Thu, 13 May 2004 08:54:17 GMT

Jan,

I found that one too. But DNS is a too big change for us. We are still using host files.

It is rather strange that since years redundant machines are sold while the network protocols are not able to use it.

Also : it would like that when the network goes down on 1 card that IP retransmits the packets on the other card.

Did someone test all this (network failure, card failure) ?

Excellent would be that in case of failure, all connections stay up and act as if the packets were lost.

Btw : how is decnet/lat reacting in case of a failure ? Is something lost ?

Re: LAN failover

Antoniov. — Thu, 13 May 2004 09:20:16 GMT

Wim,
I understand my answer don't like you but TCP/IP suite is based on MAC (physical address of card) so, today is not possible use a secondary network card to backup a primary card :-(
Every change of card require a manual modify or else require some minutes (from 1 to max 30 minutes, depending network complex) to upgrade all ARP table of network device.
You can use DNS to solve or you have ready to accept some minutes of waiting for activation of new card.
Ortherwise don't use TCP/IP (but I think also other protocol have same limitation).
Sorry for unhappy news.

@Antoniov

Re: LAN failover

Wim Van den Wyngaert — Thu, 13 May 2004 10:14:28 GMT

Antoniov,

Correct me if I'm wrong.

The network cards both have a an ethernet address. When decnet starts, it corrects this address to the well known AA. IP starts after decnet.

If decnet is using both cards with the same decnet address, all communications use the same ethernet address. The other nodes don't know that there are 2 cards.

So, for IP it should be possible to use either card too. Not ?

Re: LAN failover

Uwe Zessin — Thu, 13 May 2004 11:24:04 GMT

You typically use the DECnet end-node license. Last time I checked it used only one active circuit at a time. When that one went down it used the next.

Many networks are switched. If you put two NICs with the same MAC address on the same switch he at least stops traffic on one port - I have seen it and could diagnose it thanks to extensive counters within DECnet-Plus. If you put them on different switches you will screw up the internal tables, because they suddenly will see frames with the same MAC address comming in from different ports.

In that case you need to separate the segments by DECnet routers. Nothing new.

LAT works differently. It build multiple virtual circits and can cope with different MAC addresses.

Re: LAN failover

Antoniov. — Thu, 13 May 2004 11:38:47 GMT

I hope I can explain in simple way using my broken english.

All the network devices send data togheter using MAC (you have called AA). Theorically doesn't exist 2 NIC (Network Interface Card) with same MAC. All lowset network protocols use this information.
IP address is an abtraction of network to permit eterogeneous machines exchange information; however, socket need physical network to reach destination host and need MAC to make this. Conversion between IP address and MAC is automatic and is called ARP; you can see this table of conversion on your host typing
$ arp -a
if you have define unix like command or else
TCPIP>SHOW ARP
where MAC is called physical address.
ARP tables of all network device are dynamically updated, so your problem may be theorically solved; the trouble is the time-out of ARP table; using short time-out you can substitute a card with another but increase network traffic.
Usually the ARP table are valid ffor few minutes (from 1 until 30) and this is the time for complete substitution of NIC in network.

@Antoniov

Re: LAN failover

Antoniov. — Thu, 13 May 2004 11:42:32 GMT

Wim,
found this page can explain better than my post
http://members.cox.net/~ndav1/self_published/The_ARP_cache.doc

@Antoniov

Re: LAN failover

Wim Van den Wyngaert — Fri, 14 May 2004 01:09:59 GMT

Antoniov,

There is a 2nd feature : arp broadcast. When there is a change in network setup, a node can broadcast the new arp settings. In UCX 4.2 it was used in the cluster alias context.

Re: LAN failover

Wim Van den Wyngaert — Fri, 14 May 2004 01:12:19 GMT

Uwe,

If this causes problems, can it also cause problems on 7.3-2 ?

We use decnet PLUS. Last time I checked, decnet used the 2 ports simulaniously.

Is anyone using a solution that is not DNS based ?

Re: LAN failover

Antoniov. — Fri, 14 May 2004 01:30:51 GMT

There is a 2nd feature : arp broadcast. When there is a change in network setup, a node can broadcast the new arp settings. In UCX 4.2 it was used in the cluster alias context.

ARP tables of all network device are dynamically updated, so your problem may be theorically solved [...] increase network traffic.

Re: LAN failover

Uwe Zessin — Fri, 14 May 2004 01:59:35 GMT

Hello Wim,
it's been some years since I worked with this, but I recall that one could disable PHASE IV addressing within DECnet-Plus and then it did not alter the physical address of the interface.

Unfortunately I don't have the commands handy to check the counters and as I don't run DECnet-Plus I cannot find out easily, sorry.

Re: LAN failover

Wim Van den Wyngaert — Fri, 14 May 2004 02:12:00 GMT

OK.

I'm going to install a 2nd card in my station and test it. To be continued ...

Re: LAN failover

Steve Reece — Sat, 15 May 2004 03:32:21 GMT

"We use decnet PLUS. Last time I checked, decnet used the 2 ports simulaniously."

For a Phase IV installation, the two network interfaces would have to be on entirely separate physical networks.

For a Phase V (a.k.a. DECnet-Plus) installation, only ONE network interface will have the Phase IV compatible address running on it. The other network interface would either be on a completely separate network (as with Phase IV) or would not be running with a Phase IV compatible address and so probably wouldn't have the MAC address of the interface changed.

As a side issue, can LAT run on two network interfaces? I assumed it couldn't after a system that I have here used the wrong one for LAT when initially configured. Only one interface was used by default.