Subject: [OSPF] OSPFv3: Multiple Interfacee to Single Link .

From: "Keshava.Ayanur" <[email-masked]>
Date: Thu, 09 Jul 2009 17:32:09 +0530

Under the Section 3.9 in OSPFv3 RFC 5340
 
"Each of the multiple interfaces are assigned different Interface
IDs.  A router will automatically detect that multiple interfaces
are attached to the same link when a Hello packet is received with
one of the router's link-local addresses as the source address and
an Interface ID other than the Interface ID of the receiving interface."

If the 'Link Local address' is same for different interfaces with in the
same Router, then how to detect that the multiple interfaces are attached to
same link ?

Regards,
keshava

---

From: Michael Barnes <[email-masked]>
Date: Thu, 09 Jul 2009 15:25:15 -0700

Hello keshava,

Keshava.Ayanur wrote:
> “Each of the multiple interfaces are assigned different Interface
> an Interface ID other than the Interface ID of the receiving interface.”
> 
> If the ‘Link Local address’ is same for different interfaces with in the same Router, then how to detect that the multiple interfaces are attached to same link ?

That doesn't change the fact that the packet is received with one of the 
router's own link-local addresses as the source address. This text 
doesn't say that the source address must be different than the one for 
the interface on which it is received. The differentiator is the 
Interface ID, which must be unique even if the source address is not.

However, IMHO there is little advantage to having multiple interfaces to 
a link unless the interfaces have different link-local addresses. If the 
automatically generated link-local addresses happen to be the same, the 
user must manually configure unique addresses on those interfaces which 
will be connected to the same link. Otherwise any packet sent to the 
router will be received on all of the interfaces, which seems highly 
undesirable.

Regards,
Michael

---

From: "Keshava.Ayanur" <[email-masked]>
Date: Fri, 10 Jul 2009 12:06:51 +0530

Hi,

Consider the following scenario.

    E0  |N1
  R1----|				
	  | E1   E2  |N2
	  |---R2-----|

R1 E0: Link Local Address = X, Interface-Id = a.
R2 E1: Link Local Address = Y, Interface-Id = b.
R2 E2: Link Local Address = X, Interface-Id = a.

According to [OSPFv3], if R2 Receives Hello from R1,

Check if one of the router's link-local addresses as the source address and
Interface ID other than the Interface ID of the receiving interface.

So we find interface R2 E2.BUT, the hello packet received was not sent by R2
on E2.

How to handle this issue?

Regards,
Keshava.

-----Original Message-----
From: Michael Barnes [mailto:[email-masked]] 
Sent: Friday, July 10, 2009 3:55 AM
To: Keshava.Ayanur
Cc: [email-masked]
Subject: Re: [OSPF] OSPFv3: Multiple Interfacee to Single Link .

Hello keshava,

Regards,
Michael

---

From: Michael Barnes <[email-masked]>
Date: Thu, 09 Jul 2009 23:48:36 -0700

Hi Keshava,

It is strictly not allowed for R1 and R2 to use the same link-local 
address on the same link. Neighboring routers MUST use different 
link-local addresses, regardless of multiple interfaces to a link.

Regards,
Michael

---

From: ravi <[email-masked]>
Date: Fri, 10 Jul 2009 19:50:14 +0530

I feel couple of points are not clear in the "Multiple Interface to Single
Link" section in RFC-5340.

1. Detection of multiple interfaces to same link: (Same point as mentioned
by Keshav)

Below check should be done only for SELF-ORIGINATED HELLO packets. This is
not explicitly mentioned in the RFC(infact it is removed during transition
from 2740 to 5340). Related RFC extract is given below.

                ---RFC 5340 (4.9) ---
Each of the multiple interfaces are assigned different Interface
ID. A router will automatically detect that multiple interfaces
are attached to the same link when a Hello packet is received with
one of the router's link-local addresses as the source address and
an Interface ID other than the Interface ID of the receiving 
interface.
                ---RFC 5340 (4.9) ---

2. Intra-area-prefix LSA generation for prefixes on standby interface:

If router 'A' has multiple interfaces on the link and router 'B' is DR for
the link, then according to RFC, prefixes in link-LSA associated with
standby interface will not be included in intra-area-prefix LSA generated by
DR 'B' for the link, because Link state ID of standby interface's Link-LSA
will not match neighbor's(A) interface ID.

                   ---RFC 5340 (4.4.3.9) ---
Each link-LSA associated with Link L is examined (these are in the
Designated Router's interface structure for Link L).  If the link-
LSA's Advertising Router is fully adjacent to the Designated
Router and the Link State ID matches the neighbor's interface ID,
the list of prefixes in the link-LSA is copied into the intra-
area-prefix-LSA that is being built.
                   ---RFC 5340 (4.4.3.9) ---

                   ---RFC 5340 (4.9) ---
Prefixes on Standby Interfaces will be processed the same way as
prefixes on the Active Interface.  For example, if the router is
the DR for the link, the Active Interface's prefixes are included
in an intra-area-prefix-LSA which is associated with the Active
Interface's network-LSA, prefixes from Standby Interfaces on the
link will also be included in that intra-area-prefix LSA.
                   ---RFC 5340 (4.9) ---

Regards,
Ravi

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-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Michael Barnes
Sent: Friday, July 10, 2009 12:19 PM
To: Keshava.Ayanur
Cc: [email-masked]
Subject: Re: [OSPF] OSPFv3: Multiple Interfacee to Single Link .

Hi Keshava,

Regards,
Michael

> How to handle this issue?
_______________________________________________
OSPF mailing list
[email-masked]
https://www.ietf.org/mailman/listinfo/ospf

---

From: Paul Wells <[email-masked]>
Date: Fri, 10 Jul 2009 11:08:23 -0500

Hi Keshava,

Since the Hello packets sent by R1 and R2 will have different 
router IDs there should be no confusion in this case.

This isn't explicitly mentioned in section 4.9 of 5340. Perhaps 
because it was considered too obvious?

Regards,
Paul

---

From: Michael Barnes <[email-masked]>
Date: Fri, 10 Jul 2009 09:22:31 -0700

Hello again Keshava,

I misread your diagram. I initially thought that both of R2's interfaces 
were on the same link.

It is implicitly understood that you will also check the Router-ID in 
the packet. So packets received by R2 from R1 on N1 will not be 
considered in the multiple interfaces to a link check.

Regards,
Michael

---

From: Michael Barnes <[email-masked]>
Date: Fri, 10 Jul 2009 09:36:32 -0700

Paul Wells wrote:
> Hi Keshava,
> 
> Since the Hello packets sent by R1 and R2 will have different router IDs 
> there should be no confusion in this case.
> 
> This isn't explicitly mentioned in section 4.9 of 5340. Perhaps because 
> it was considered too obvious?

I guess all of the reviewers thought so. ;-) However, I agree it would 
be good to have it stated explicitly.

Cheers,
Michael