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