Re: Fwd: Loopback properties in routing on linux kernel

On 07.06.2016 12:46, Krishna Mohan wrote:

HI All,
    I have a question to determine the uses of multiple loopbacks
interfaces(int loopback) in routing(quagga) on linux kernel. Please
apologise if it is a dumb question. This is what I understand, please
clarify if my understanding is correct.

1. we need them to represent a /32 ip, normally used for router-id.
2. we can measure traffic destined to that ip, via its independent
statistics per loopback interface.
3. ip unnumbered uses loopbacks to represent that ip.

my question is, is it necessary to have ifindex per loopback interface
we create? if I create kernel loopback as lo:1 (as subinterface on
default loopback in kernel) with ip assigned to it will do 1. for
getting 2 and 3 done, loopback needs to be a separate netdev. is that
correct?

That is correct, you can terminate the ip addresses for iBGP (or
router-id in this case) connections easily on any ip address that is
bound to some interface within the kernel. Note, that you don't
necessarily get the statistics correctly updated if you short-cut the
packet internally in the kernel. So the statistics on the dummy or
loopback device might not be accurate in such a setup.

Will I be having any advantage if I configure the interface as lo:1 vs
a separate netdev per loopback interface (i mean dummy netdev) to
terminate the /32 destined traffic?

From kernel PoV it doesn't matter. It is a question of taste and what

your tools prefer.

Bye,
Hannes