Subject: [pim] PIM-SM - IP Address of Register messages

From: "Dev" <[email-masked]>
Date: Mon, 9 Jan 2006 19:57:09 -0000

Hello

I have a question about the IP address of a register message with
multicast payload.

Consider this.
One router with the interfaces IF_A and IF_B. It
receives a multicast stream from a directly connected neighbor on IF_A. The
router has been chosen to be the DR on this interface and encapsulates the
stream and forwards it onto IF_B, which is in the direction towards the
RP.

Now my to my question.
Which is the correct IP source address of the
Register packet. (The external IP header). Is it IPADDR(IF_A) or
IPADDR(IF_B)? Or maybe it doesn't matter?

Best regards
David

_______________________________________________
pim mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/pim

---

From: "Jonathan Natale" <[email-masked]>
Date: Mon, 9 Jan 2006 15:01:00 -0500

IPADDR(IF_B)
  -----Original Message-----
  From: [email-masked] [mailto:[email-masked]]On Behalf Of Dev
  Sent: Monday, January 09, 2006 2:57 PM
  To: [email-masked]
  Subject: [pim] PIM-SM - IP Address of Register messages

  Hello

  Consider this.
  One router with the interfaces IF_A and IF_B. It receives a multicast
stream from a directly connected neighbor on IF_A. The router has been
chosen to be the DR on this interface and encapsulates the stream and
forwards it onto IF_B, which is in the direction towards the RP.

  Now my to my question.
  Which is the correct IP source address of the Register packet. (The
external IP header). Is it IPADDR(IF_A) or IPADDR(IF_B)? Or maybe it doesn't
matter?

  Best regards
  David

---

From: Alexander Gall <[email-masked]>
Date: Tue, 10 Jan 2006 18:01:08 +0100

> IPADDR(IF_B)

Why?  I think it doesn't matter.  Cisco's IPv6 implementaion lets me
chose it with

ipv6 pim register-source <if>

This can be useful for exmple if

- the unicast address of the outgoing interface towards the RP is not
  reachable by the RP (and hence register-stop messages couldn't be
  received by the DR).

- the RP uses a fine-grained ACL to restrict the sources of register
  messages (e.g. the loopback addresses of the DRs)

--
Alex

---

From: "Jonathan Natale" <[email-masked]>
Date: Tue, 10 Jan 2006 12:32:25 -0500

Hello.  because the outgoing interface IS the src.  Sure you can change it,
but that was not the question; defaulting to loopback0 MIGHT make sense, but
why default to the address of the interface used to reach the mcast source?

> From: Alexander Gall [mailto:[email-masked]]
> Sent: Tuesday, January 10, 2006 12:01 PM
> To: Jonathan Natale
> Cc: Dev; [email-masked]
> Subject: RE: [pim] PIM-SM - IP Address of Register messages
>
>

---

From: "Jonathan Natale" <[email-masked]>
Date: Tue, 10 Jan 2006 12:42:26 -0500

it may matter if any of the routers along the path of the register do a uRPF
(which may be done for security purposes).

---

From: Alexander Gall <[email-masked]>
Date: Tue, 10 Jan 2006 19:30:04 +0100

> why default to the address of the interface used to reach the mcast source?

You seem to have a different interpretation of what the OP meant by
"correct".  I read the question to ask whether PIM-SM makes any
statement about which source address to chose.  I think it doesn't and
the choice is a matter of local policy.  Your first reply suggested
something different to me, hence my statement.

--
Alex