A quick lab to to test how OSPF transit area capability works.

Topology:

R4 - area0
R2 and R3 are ABRs between area 0 and area 1
R1 is in area 1 with a loopback in area 2
a Virtual Link is created between R1 and R2

Addresses: link Rx --> Ry is 10.0.xy.0/24 with .x and .y being the two routers.

Configuration is basic, no need to write it down here. OSPF cost on R2 interface to R4 is set to 10 to make the path from R1 to R4 pass through R3.





1° test - trace from R1 Loopback0 to R4 Loopback0


R1#traceroute 4.4.4.4 so lo0

Type escape sequence to abort.
Tracing the route to 4.4.4.4

1 10.0.123.3 4 msec 8 msec 4 msec
2 10.0.34.4 8 msec * 8 msec

R1#sh ip route 4.4.4.4
Routing entry for 4.4.4.4/32
Known via "ospf 1", distance 110, metric 3, type intra area
Last update from 10.0.123.3 on FastEthernet0/0, 00:00:01 ago
Routing Descriptor Blocks:
* 10.0.123.3, from 4.4.4.4, 00:00:01 ago, via FastEthernet0/0
Route metric is 3, traffic share count is 1


Traffic path is R1-->R3-->R4


2° test - same trace disabling transit capability on R1


R1(config)#router ospf 1
R1(config-router)#no capability transit

R1#sh ip route 4.4.4.4
Routing entry for 4.4.4.4/32
Known via "ospf 1", distance 110, metric 12, type intra area
Last update from 10.0.123.2 on FastEthernet0/0, 00:00:03 ago
Routing Descriptor Blocks:
* 10.0.123.2, from 4.4.4.4, 00:00:03 ago, via FastEthernet0/0
Route metric is 12, traffic share count is 1

R1#traceroute 4.4.4.4 so lo0
1 10.0.123.2 4 msec 4 msec 4 msec
2 10.0.123.3 8 msec 4 msec 8 msec
3 10.0.34.4 8 msec * 8 msec

Traffic path no is R1-->R2-->R3-->R4 even if it’s not the lowest-cost.

On some config templates the transit capability is disabled on both sides of the virtual link. Let’s try to disable only on R2:


R1(config)#router ospf 1
R1(config-router)#capability transit

R1#traceroute 4.4.4.4 so lo0
1 10.0.123.3 8 msec 8 msec 4 msec
2 10.0.34.4 12 msec * 8 msec

R2(config)#router ospf 1
R2(config-router)#no capability transit

R1#traceroute 4.4.4.4 so lo0
1 10.0.123.3 4 msec 4 msec 0 msec
2 10.0.34.4 12 msec * 4 msec

As the traceroute output clearly shows, dsabling transit capability on R2 doesn’t affect R1.



3° test - what happens if Virtual-Link goes down?


Trace from R1 Lo0 can’t work without Virtual-link, trace from F0/0 has the same output:

R1#traceroute 4.4.4.4
1 10.0.123.2 4 msec 4 msec 16 msec
2 10.0.24.4 8 msec * 8 msec

Let’s get rid of the virtual link:

R1(config)#router ospf 1
R1(config-router)#no area 1 virtual-link 2.2.2.2

and trace again:

R1#traceroute 4.4.4.4
1 10.0.123.3 4 msec 4 msec 4 msec
2 10.0.34.4 8 msec * 8 msec

So R1 transit capability is affected only if the Virtual Tunnel stays up.

HTH




LINK

OSPF Area Transit Capability