Subject: Re: Summary's from backbone

From: Don Goodspeed <[email-masked]>
Date: Thu, 11 Apr 2002 15:28:54 -0400

Sandor,

Looping of packets can still occur in your second scenario
in circumstances where costing has been modified.

Let's assume router 6 sends the packet to router 3 due to
costing that makes router 3 the closest exit point in area 1.

The packet is destined for router 1, and router 3's best path
to router 1 is via the virtual-link back thru area 1 via router
6.  Oops, router 6 has router 3 as it's next-hop, and the
loop occurs.  If the virtual-link does not exist, router 3
has to send the packet to routers 7, then 5, then 1 (no loop).

With no summarization, router 6 always learns the best path
out of the area for each route in Area 0.

-don

--- On Wed 04/10, Sandor Ecker  wrote:
> Hi!
>
> Can anybody show me an example why is at the end of 12.4.3 this:
>
>         "If an area is capable of carrying transit traffic, routing
>         information concerning backbone networks should not be
>         condensed before being summarysed into the area.  ... In other
>         words, the backbone's ranges should be ignored..."
>
> I have one example, but i need an other one...
>
> My example:     Here when a range were configured for A0, then router's
> in
> A1 could not decide the rigth way. (i'm right)
>
>                       +---+
>                       | 1 |
>                       +---+
>                         | ttyS0
>                         |
>                         |
>                         |
>                         | ttyS0         A0
>                       +---+
>         --------------| 2 |-----------------
>                       +---+
>                   eth2 /|\ eth1
>                       / | \
>                      /  |  \
>                     /   |   \
>                    /eth2|    \ eth2
>                  +---+  |   +---+       ttyS0 +---+
>                  | 6 | VL0  | 4 |-------------| 5 |
>                  +---+  |   +---+ ttyS0       +---+
>                    \ eth1    / eth1
>                     \   |   /
>                      \  |  /
>                       \ | /
>                   eth2 \|/ eth1         A1
>                       +---+
>         --------------| 3 |-----------------
>                       +---+
>                         | ttyS0
>                         |
>                         |
>                         |
>                         | ttyS0         A0
>                       +---+
>                       | 7 |
>                       +---+
>
>
> Other example for the next topology: The R1-R5, and the R5-R7 links
>         are in A0 also, but the R4-R5 in A1.
>
> Here can anybody show me what were wrong, when the backbone would make
>         range summary to A1?
>
>
>                       +---+ eth2
>                       | 1 |----------------------------------+
>                       +---+                                  |
>                         | ttyS0                              |
>                         |                                    |
>                         |                                    |
>                         |                                    |
>                         | ttyS0         A0                   |
>                       +---+                                  |
>         --------------| 2 |-------------------------------   |
>                       +---+                                  |
>                   eth2 /|\ eth1                              |
>                       / | \                                  |
>                      /  |  \                                 |
>                     /   |   \                                |eth1
>                    /eth2|    \ eth2                          |
>                  +---+  |   +---+       ttyS0              +---+
>                  | 6 | VL0  | 4 |--------------------------| 5 |
>                  +---+  |   +---+ ttyS0                    +---+
>                    \ eth1    / eth1                          |
>                     \   |   /                                |eth2
>                      \  |  /                                 |
>                       \ | /                                  |
>                   eth2 \|/ eth1         A1                   |
>                       +---+                                  |
>         --------------| 3 |-------------------------------   |
>                       +---+                                  |
>                         | ttyS0                              |
>                         |                                    |
>                         |                                    |
>                         |                                    |
>                         | ttyS0         A0                   |
>                       +---+                                  |
>                       | 7 |----------------------------------+
>                       +---+ eth1
>
> Why don't need to make so restriction for an non backbone area, just for
>         backbone?
>
>
> Regards,
> Sandor Ecker
>

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