Re: [2.0] alias interfaces and multicast

Mr. James W. Laferriere Network Engineer (
Thu, 26 Feb 1998 20:56:20 -0800 (PST)

Hello Matei,

On Wed, 25 Feb 1998, Matei Conovici wrote:
> Hello!
> I've encountered this problem with multicast and alias interfaces
> (tryed to run ospf/gated over an alias interface).
> While group membership for the alias interface is set properly (as
> /proc/net/igmr indicates), multicast packets which should be received
> by an alias interface are discarded.

The aliased interfaces do not hvae traffic transmitted thru
them only the primary interface gets traffic. There probably
will not be adjacencies maintained due to this , Although the
kermel will tell gated that the interface does exist .

> The reason for this is that the ip_rcv routine only checks the list of
> groups assigned to the interface which received the packet. Since this
> obviously can't be the alias interface, the packet will be dropped if
> the multicast group is only assigned to the alias, but not the
> real (physical) interface.

OK, you already saw this . :-)

> This seems a bit inconsistent to me (if an alias cannot receive the
> packets destined to one of the multicast groups it is a member of,
> then why bother keeping the group membership list for it).

Please see top explanation.

> Could this be changed so that ip_rcv also checks the groups assigned
> to alias interfaces ? Is there a reason for not doing it ?

I'm not sure of the reasoning for the aliased interface
not being valid for traffic, But if I remember Alexey(?)
has had discussions on this subject before.

I'd personally like to see aliased interfaces be a traffic
bearing entity as far as the Kernel & routing are concerned,
But I don't even know where to begin (yet).

Hth, JimL
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