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Re: primary and secondary ip addresses

To: Henrik Nordstrom <hno@xxxxxxxxxxxxxxx>
Subject: Re: primary and secondary ip addresses
From: Hasso Tepper <hasso@xxxxxxxxx>
Date: Thu, 16 Dec 2004 13:02:42 +0200
Cc: Harald Welte <laforge@xxxxxxxxxxxx>, Neil Horman <nhorman@xxxxxxxxxx>, linux-net@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxx
In-reply-to: <Pine.LNX.4.61.0412161103320.30452@filer.marasystems.com>
Organization: Elion Enterprises Ltd.
References: <41912F7A.6000408@redhat.com> <200412161153.51251.hasso@estpak.ee> <Pine.LNX.4.61.0412161103320.30452@filer.marasystems.com>
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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".

> > 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 224.0.0.6 and eth1 has addresses 
192.168.0.1/24 and 10.10.10.1/24 which one choose for source address?

> > Preferred
> > =========
> >
> > 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 192.168.0.2, 192.168.0.1 is used as source; if 
packet is sent to the 10.10.10.28, 10.10.10.1 is used as source.


-- 
Hasso Tepper
Elion Enterprises Ltd.
WAN administrator

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