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Re: primary and secondary ip addresses

To: Hasso Tepper <hasso@xxxxxxxxx>
Subject: Re: primary and secondary ip addresses
From: Andrea G Forte <andreaf@xxxxxxxxxxxxxxx>
Date: Fri, 17 Dec 2004 10:10:29 -0500
Cc: Henrik Nordstrom <hno@xxxxxxxxxxxxxxx>, Harald Welte <laforge@xxxxxxxxxxxx>, Neil Horman <nhorman@xxxxxxxxxx>, linux-net@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxx
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Yes, using the same concept used in Junos would make things much easier. On top of this, when I change the primary IP, it takes about 500ms to see the change applied which is critical for real time applications. Using more than one primary IP would solve this as well.
Also it seems that if I have a primary address and a secondary one (alias), even though I make the primary address invalid (trying to force the kernel to use the secondary first), the kernel still tries the primary first regardless of its validity (I set the primary IP to
Anybody has any comments about this?
Can anyone point out to me where is the code for primary/secondary IPs in the kernel?

Thanks a lot,

Hasso Tepper wrote:

Henrik Nordstrom wrote:

On Thu, 16 Dec 2004, Hasso Tepper wrote:

And why I can't even choose which address is primary?

It is the first you add in a subnet.

Yes, but to change the primary I have remove old primary (and therefore all secondaries) and assign new primary to the interface and then all secondaries etc. I can't just tell "make this address which is secondary at the moment, primary".


Configure this address to be the primary address of the protocol on
the interface. If the logical unit has more than one address, the
primary address is used by default as the source address when packets
originate from the interface and the destination does not indicate
the subnet (ie. multicast destination for example).

This is what the primary addresses is used for, indirectly via the
automatically created route entries.

No. There is only one primary address per interface and it is used if destination address doesn't indicate which source address to use. Ie. if packet is sent over eth1 to the address and eth1 has addresses and which one choose for source address?


Configure this address to be the preferred address on the interface.
If you configure more than one address on the same subnet, the
preferred source address is chosen by default as the source address
when you originate packets to destinations on the subnet.

What is the difference from primary here?

This has same meaning as primary in the Linux. There is as many preferred addresses on the interfaces as there is subnets - every subnet has one preferred address. Preferred aadress is used if next hop indicates subnet - if packet is sent to the, is used as source; if packet is sent to the, is used as source.

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