inter-area routing trough nonbackbone via virtual link

 

ospf

 

Hi all,
I can't figure out why are some routing choices made inside this topology.
Topology is simple, several areas, ABRs, ethernet segment in Area0, and direct serial link between R1 and R2 in Area 0.

Loopback of R2 is in Area 55 and is being advertised.
Also, there is a virtual link configured between R1 and R6 passing trough Area 1, so that R6 looks like legitimate ABR connected to backbone area.

What I wanted to test is how will R3 reach loopback address of R2. I specifically configured minimal ospf cost on interfaces going form R1, R6, R7 to R3. I imagined R3 would use path going to Area 2, Area 1 (or rather Area0 virtual link) and then go into backbone area and reach R2.

And indeed R7 uses that path:

R7#trace 192.10.1.2
Type escape sequence to abort.
Tracing the route to 192.10.1.2
  1 155.1.67.6 60 msec 48 msec 28 msec
  2 155.1.16.1 60 msec 56 msec 64 msec
  3 155.12.12.2 88 msec * 92 msec

R7#sh ip route 192.10.1.2
Routing entry for 192.10.1.2/32
  Known via "ospf 1", distance 110, metric 602, type inter area
  Last update from 155.1.67.6 on FastEthernet0/1, 00:33:14 ago
  Routing Descriptor Blocks:
  * 155.1.67.6, from 150.1.6.6, 00:33:14 ago, via FastEthernet0/1
  Route metric is 602, traffic share count is 1

But R3 chooses to go directly into Area 0 with much higher metric:

R3#trace 192.10.1.2
Type escape sequence to abort.
Tracing the route to 192.10.1.2

 1 155.1.123.2 52 msec * 52 msec

R3#sh ip route 192.10.1.2
Routing entry for 192.10.1.2/32
  Known via "ospf 1", distance 110, metric 50001, type inter area
  Last update from 155.1.123.2 on FastEthernet0/0, 00:05:03 ago
  Routing Descriptor Blocks:
  * 155.1.123.2, from 150.1.2.2, 00:05:03 ago, via FastEthernet0/0
  Route metric is 50001, traffic share count is 1

That's the part i don't get. I haven't played around with virtual links, but i think that RFC says R3 should be able to go into nonbackbone area first and then into backbone to reach R2's loopback if the distance if the metric cost is smaller then going directly into area 0.

IF i shut ethernet link in area0, R3 chooses Serial link between R2 (how can it not go into Area 0?).
Only when i shut that serial as well, i force R3 to go via virtual link to R2 (notice smallest metric).

R3#trace 192.10.1.2

Type escape sequence to abort.
Tracing the route to 192.10.1.2

  1 155.1.37.7 80 msec 28 msec 32 msec
  2 155.1.67.6 60 msec 72 msec 80 msec
  3 155.1.16.1 88 msec 88 msec 92 msec
  4 155.12.12.2 120 msec * 128 msec

R3#sh ip route 192.10.1.2
Routing entry for 192.10.1.2/32
  Known via "ospf 1", distance 110, metric 603, type inter area
  Last update from 155.1.37.7 on FastEthernet0/1, 00:01:04 ago
  Routing Descriptor Blocks:
  * 155.1.37.7, from 150.1.6.6, 00:01:04 ago, via FastEthernet0/1
  Route metric is 603, traffic share count is 1


Can someone explain this a bit please?



Comments

  • Inter-Area Routing is always done through Area 0.

    The Only case where Inter-Area routing may go through another area will be when the area is a Transit Area. And as per the RFC a Transit area is one which has one or more fully adjacent virtual-links.

    Thus R7 can use to route via Area 1 ( a transit Area ) but R3 cannot because Area 2 is not a Transit Area. When R3 loses its connection to Area 0. It becomes a part of Area 2 and can route via R7 which is itself is using Area 1 as a transit Area to reach the Loopback.

     

    HTH,

     

    Zeeshan Sanaullah , CCIE #25196

  • Ok that makes sense i figured that router only checks area 0 for lsa3... what throw me off was that there is something like this in workbook, and i'm positive r3 went trough area 2. Ill have to check that again. Also have to check why R3 is taking that serial link in area 5 when its cost is higher than taking a route in area 2 (when i shut link to area0).

    thx

     

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