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Re: [OSPF] ospf backbone area



Hi Joakim,



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


Hi Sinbad,
See inline.

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

             ...........................
             .   +                     .
             .   | 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.

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.

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 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







Hope this helps,
Acee