Hello,

Ed Ravin wrote:

     d1   d2   d3                          d4   d5   d6
      \    |   /                            \    |   /
       \   |  /                              \   |  /
        switch---------leased line------------switch
           |                                     |
       Router A                               Router B
           |                                     |
           +-- -- -- -- backup tunnel -- -- -- --+


I think the answer to the diagram above is "don't do that", or at
least "don't do that unless all the devices speak OSPF, and you've
made sure that none of your important traffic uses the IP addresses
in the broadcast domain that could be unreachable if the Ethernet gets
partitioned".

The network works fine as long as every device in the broadcast
domain speaks OSPF and can follow the announced routes whether they
come from the broadcast domain or from elsewhere.  But for the
devices that don't speak OSPF, there's no way to reach them from
the "other side" of the leased line when it is down unless I play
tricks with /32 routes.

the point is that even if all your devices speak OSPF, they will suffer from this issue as well. d4 speaking OSPF doesn't help Router A not to use it's connected interface to try to reach the network, and d4 also(and all the possible networks behind d4), still creating the blackhole, as far as our tests shows.

Gabor
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