I have a OSPF Path selection question, I understand that OSPF has some native path selection rules that cannot be overridden by metric or AD;
Such as O is preferred over OIA, is preferred over E1, over E2, over N1, over N2.
This makes sense to me at a highlevel, but after lab’ing OSPF up and starting to play around with this I have a question.
Using INE’s Topology, with R5 being the Frame Relay Hub (for R1, R2, R3, R4) and using OSPF network type of point-to-multipoint non-broadcast..all in Area0. As well as activating P2P between R1 and R3 (Area 4), and the P2P between R2 and R3 (Area 5). Everything else behind R3 (Sw1 and Sw3) are in Area 2.
If we look at this from R2 perspective of 155.1.9.0/24. R2 has an interface in Area 0 and Area 5. It learns the route from both R3 by way of the P2P across area 5, as well as from R3 by way of Area0 / R5.
The Route originates in Area2, making this a OIA routes from both perspectives. If the frame-relay interfaces is up and the OSPF adjacency up the preferred path is via Area 0 by way of R5 at a cost of 131.
*IF* the FR is down, the new path that gets installed is over the P2P link from R3 directly by way of Area 5, with a cost of 67.
My thinking on this is: Even though in both cases these are OIA routes; OSPF preferred to transit Area0 if the choice is there? Is that a correct assumption? Could someone provide some insight on this?
Thanks.
Rack1SW3#show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Vl79 1 2 155.1.79.9/24 1 BDR 1/1
Vl9 1 2 155.1.9.9/24 1 DR 0/0
Rack1SW1#show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Lo0 1 2 150.1.7.7/24 1 LOOP 0/0
Fa0/3 1 2 155.1.37.7/24 1 DR 1/1
Vl79 1 2 155.1.79.7/24 1 DR 1/1
Vl67 1 2 155.1.67.7/24 1 DR 1/1
Vl7 1 2 155.1.7.7/24 1 DR 0/0
Rack1R3#show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Se1/0 1 0 155.1.0.3/24 781 P2MP 1/1
Fa0/0 1 2 155.1.37.3/24 1 BDR 1/1
Se1/2 1 4 155.1.13.3/24 781 P2P 1/1
Se1/3 1 5 155.1.23.3/24 781 P2P 1/1
Rack1R3#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
150.1.5.5 0 FULL/ - 00:01:56 155.1.0.5 Serial1/0
150.1.7.7 1 FULL/DR 00:00:30 155.1.37.7 FastEthernet0/0
150.1.1.1 0 FULL/ - 00:00:31 155.1.13.1 Serial1/2
150.1.2.2 0 FULL/ - 00:00:31 155.1.23.2 Serial1/3
\
Rack1R2#show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Se0/0/0 1 0 155.1.0.2/24 64 P2MP 0/0
Se0/1/0 1 5 155.1.23.2/24 64 P2P 1/1
Fa0/0 1 51 192.10.1.2/24 1 BDR 1/1
Rack1R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
150.1.5.5 0 FULL/ - 00:01:58 155.1.0.5 Serial0/0/0
150.1.3.3 0 FULL/ - 00:00:35 155.1.23.3 Serial0/1/0
192.10.1.254 1 FULL/DR 00:00:37 192.10.1.254 FastEthernet0/0
Rack1R5#show ip ospf interface brief
Interface PID Area IP Address/Mask Cost State Nbrs F/C
Se0/1/0 1 0 155.1.45.5/24 64 P2P 1/1
Se0/0/0 1 0 155.1.0.5/24 64 P2MP 3/3
Fa0/1 1 3 155.1.5.5/24 1 DR 0/0
Fa0/0 1 3 155.1.58.5/24 1 BDR 1/1
Rack1R5#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
223.255.255.255 0 FULL/ - 00:00:30 155.1.45.4 Serial0/1/0
223.255.255.255 0 FULL/ - 00:01:40 155.1.0.4 Serial0/0/0
150.1.3.3 0 FULL/ - 00:01:40 155.1.0.3 Serial0/0/0
150.1.2.2 0 FULL/ - 00:00:43 155.1.0.2 Serial0/0/0
150.1.1.1 0 FULL/ - 00:01:40 155.1.0.1 Serial0/0/0
150.1.8.8 1 FULL/DR 00:00:34 155.1.58.8 FastEthernet0/0
Rack1R2#show ip route 155.1.9.0
Routing entry for 155.1.9.0/24
Known via "ospf 1", distance 110, metric 131, type inter area
Last update from 155.1.0.5 on Serial0/0/0, 00:00:34 ago
Routing Descriptor Blocks:
* 155.1.0.5, from 150.1.3.3, 00:00:34 ago, via Serial0/0/0
Route metric is 131, traffic share count is 1
Rack1R2#ping 155.1.9.9
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 155.1.9.9, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 112/116/124 ms
Rack1R2#
Rack1R2#
Rack1R2#config t
Enter configuration commands, one per line. End with CNTL/Z.
Rack1R2(config)#int s0/0/0
Rack1R2(config-if)#shut
Rack1R2(config-if)#end
Rack1R2#
*Dec 4 18:54:42.639: %OSPF-5-ADJCHG: Process 1, Nbr 150.1.5.5 on Serial0/0/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Dec 4 18:54:43.175: %SYS-5-CONFIG_I: Configured from console by console
Rack1R2#
*Dec 4 18:54:44.639: %LINK-5-CHANGED: Interface Serial0/0/0, changed state to administratively down
*Dec 4 18:54:45.639: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/0, changed state to down
Rack1R2#show ip route 155.1.9.0
Routing entry for 155.1.9.0/24
Known via "ospf 1", distance 110, metric 67, type inter area
Last update from 155.1.23.3 on Serial0/1/0, 00:00:00 ago
Routing Descriptor Blocks:
* 155.1.23.3, from 150.1.3.3, 00:00:00 ago, via Serial0/1/0
Route metric is 67, traffic share count is 1
Rack1R2#
Rack1R2#ping 155.1.9.9
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 155.1.9.9, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/29/32 ms
Rack1R2#