Subject: [OSPF] ospf backbone area

From: Sinbad <[email-masked]>
Date: Tue, 6 Oct 2009 15:50:23 +0530

...........................
             .   +                     .
             .   | 3+---+              .      N12      N14
             . N1|--|RT1|\ 1           .        \ N13 /
             .   |  +---+ \            .        8\ |8/8
             .   +         \ ____      .          \|/
             .              /    \   1+---+8    8+---+6
             .             *  N3  *---|RT4|------|RT5|--------+
             .              \____/    +---+      +---+        |
             .    +         /      \   .           |7         |
             .    | 3+---+ /        \  .           |          |
             .  N2|--|RT2|/1        1\ .           |6         |
             .    |  +---+            +---+8    6+---+        |
             .    +                   |RT3|------|RT6|        |
             .                        +---+      +---+        |
             .                      2/ .         Ia|7         |
             .                      /  .           |          |
             .             +---------+ .           |          |
             .Area 1           N4      .           |          |
             ...........................           |          |
          ..........................               |          |
          .            N11         .               |          |
          .        +---------+     .               |          |
          .             |          .               |          |    N12
          .             |3         .             Ib|5         |6 2/
          .           +---+        .             +----+     +---+/
          .           |RT9|        .    .........|RT10|.....|RT7|---N15.
          .           +---+        .    .        +----+     +---+ 9    .
          .             |1         .    .    +  /3    1\      |1       .
          .            _|__        .    .    | /        \   __|_       .
          .           /    \      1+----+2   |/          \ /    \      .
          .          *  N9  *------|RT11|----|            *  N6  *     .
          .           \____/       +----+    |             \____/      .
          .             |          .    .    |                |        .
          .             |1         .    .    +                |1       .
          .  +--+   10+----+       .    .   N8              +---+      .
          .  |H1|-----|RT12|       .    .                   |RT8|      .
          .  +--+SLIP +----+       .    .                   +---+      .
          .             |2         .    .                     |4       .
          .             |          .    .                     |        .
          .        +---------+     .    .                 +--------+   .
          .            N10         .    .                     N7       .
          .                        .    .Area 2                        .
          .Area 3                  .    ................................
          ..........................

1) In the above ospf topology according to ospf rfc 2328 in section 3.4, it
states that

   "Note that a failure of the line between Routers RT6 and RT10
   will cause the backbone to become disconnected."

Though there is router RT7 in area 2 still connected to the backbone.It
confused my understanding of
disconnected backbone area. Can someone shed some light on this.

2) why should routers connected by a point to point link having interface
addresses assigned,
  should advertise a stub link a stub connection to the other router's
interface address.

thanks

---

From: Acee Lindem <[email-masked]>
Date: Tue, 6 Oct 2009 11:10:32 -0400

Hi Sinbad,
See inline.

On Oct 6, 2009, at 6:20 AM, Sinbad wrote:

>           .             |1         .    .    +  /3    1\      | 
> 1       .
>           .            _|__        .    .    | /        \   __| 
> _       .
>           .           /    \      1+----+2   |/          \ /     
> \      .
>           .             |          .    .    |                 
> |        .
>           .             |1         .    .    +                | 
> 1       .
>           .             |2         .    .                     | 
> 4       .
>           .             |          .    .                      
> |        .
>           .            N10         .    .                      
> N7       .
>           .                        .    .Area  
> 2                        .
> 1) In the above ospf topology according to ospf rfc 2328 in section  
> 3.4, it states that
>
> Though there is router RT7 in area 2 still connected to the  
> backbone.It confused my understanding of

I believe this issue has been raised before although I couldn't find  
an errata. The figure is incorrect. Network N8 should be in the  
backbone area rather than Area 2.

>
>
> 2) why should routers connected by a point to point link having  
> interface addresses assigned,
>   should advertise a stub link a stub connection to the other  
> router's interface address.

This dates back to the fact that P2P links were represented as a host  
route to the destination network on BSD based systems. Those of you  
who are familiar with GateD will remember this. Note that there is  
also an option to represent/advertise P2P links as a subnets. Most if  
not all routers vendors advertise the subnet.

Hope this helps,
Acee

>
>
> thanks
>
> _______________________________________________
> OSPF mailing list
> [email-masked]
> https://www.ietf.org/mailman/listinfo/ospf

---

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 6 Oct 2009 17:22:04 +0200

> not all routers vendors advertise the subnet.

Do you have a choice here? I read the RFC that if the P2P link has
a host route (mask = 255.255.255.255) you have to advertise a stub
link to the other router's interface address.

---

From: Acee Lindem <[email-masked]>
Date: Tue, 6 Oct 2009 12:07:42 -0400

Hi Joakim,

On Oct 6, 2009, at 11:22 AM, Joakim Tjernlund wrote:

>>>           .             |          .               |           
>>> |    N12

 From RFC 2328:

             12.4.1.1.  Describing point-to-point interfaces

                 For point-to-point interfaces, one or more link
                 descriptions are added to the router-LSA as follows:

                 o   If the neighboring router is fully adjacent, add a
                     Type 1 link (point-to-point). The Link ID should be
                     set to the Router ID of the neighboring router. For
                     numbered point-to-point networks, the Link Data
                     should specify the IP interface address. For
                     unnumbered point-to-point networks, the Link Data
                     field should specify the interface's MIB-II [Ref8]
                     ifIndex value. The cost should be set to the output
                     cost of the point-to-point interface.

                 o   In addition, as long as the state of the interface
                     is "Point-to-Point" (and regardless of the
                     neighboring router state), a Type 3 link (stub
                     network) should be added. There are two forms that
                     this stub link can take:

                     Option 1
                         Assuming that the neighboring router's IP
                         address is known, set the Link ID of the Type 3
                         link to the neighbor's IP address, the Link  
Data
                         to the mask 0xffffffff (indicating a host
                         route), and the cost to the interface's
                         configured output cost.[15]

                     Option 2
                         If a subnet has been assigned to the point-to-
                         point link, set the Link ID of the Type 3 link
                         to the subnet's IP address, the Link Data to  
the
                         subnet's mask, and the cost to the interface's
                         configured output cost.[16]

Hope this helps,
Acee

---

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 6 Oct 2009 19:25:36 +0200

>                         link to the neighbor's IP address, the Link Data
>                         to the subnet's IP address, the Link Data to the

but mask 255.255.255.255 does not qualify as a subnet? So
that leaves only Option 1

   Jocke

---

From: Acee Lindem <[email-masked]>
Date: Tue, 6 Oct 2009 14:19:25 -0400

On Oct 6, 2009, at 1:25 PM, Joakim Tjernlund wrote:

<pruned>

> that leaves only Option 1

Yes - if you use 255.255.255.255, you will need to support option 1.  
We already have 2 options, we don't need a third.

Acee

---

From: Joakim Tjernlund <[email-masked]>
Date: Tue, 6 Oct 2009 20:35:58 +0200

Acee Lindem <[email-masked]> wrote on 06/10/2009 20:19:25:

> From:
>
> Acee Lindem <[email-masked]>
>
> To:
>
> Joakim Tjernlund <[email-masked]>
>
> Cc:
>
> [email-masked], [email-masked]
>
> Date:
>
> 06/10/2009 20:20
>
> Subject:
>
> Re: [OSPF] ospf backbone area
>
>

Maybe I read too much into this statement:
 "This dates back to the fact that P2P links were represented as a host
  route to the destination network on BSD based systems. Those of you
  who are familiar with GateD will remember this. Note that there is
  also an option to represent/advertise P2P links as a subnets. Most if
  not all routers vendors advertise the subnet."
I got the impression that you meant that you could choose to advertise P2P/32
with Option 2 instead of Option 1. I guess I read too much into it :)