OE1 vs OE2 from different OSPF processes

[rwu]

Hi network experts,

    I have a router which receives 10.21.32.10/24 as OE2 from ospf process 1 and the same route as OE1 from ospf process 2. The OE2 route has the metric 65 and OE1 route has the metric 21. Currently, the router prefers the OE1 route. 

    I'm under the impression that for different OSPF processes, first compare AD and then metric. That's it. If they're the same, there will be load balancing. The router/switch won't care if it's OE1 or OE2 or any other type of LSA. 

    I've done the following tests:

1. Increase the AD for the OE1 route to 115 and then the router prefers OE2 with default AD 110.

2. Recover OE1 route AD to 110. The router prefers OE1 due to its lower metric 21. 

3. Increase the metric for the OE1 route 70. HOWEVER, the router STILL prefers OE1. I shutdown the transit vlan interface in ospf process 2 and then as expected, the router prefers OE2. Then I no shut the transit vlan interface in process 2, the router prefers OE1 again even if OE1 has higher metric 70 over OE2 65. Don't know why. 

4. The question of the above item3 happens as well in the scenario that OE1 route metric is increased to 65 (64 + 1), 1 is the default transit vlan interface OSPF cost. Now AD's and metrics are the same for OE1 and OE2. I shutdown the transit vlan interface in process 2 and no shut it. The results are the same that OE1 is always preferred. Don't know why. 

    Could anyone please explain the theory behind it? I thought about doing some virtual tests but it was confirmed before that the IOL virtual node doesn't work the same as the real switches, so it makes no sense to do the virtual test. 

    Appreciate it!

================

The following is one item of my test:

switch# show ip route 10.21.32.10
Routing entry for 10.21.32.0/24
Known via "ospf 2", distance 110, metric 70               ------->>> ospf 2, metric increased to 70
Tag 666, type extern 1                                               ------->>> OE1
Last update from 10.21.48.254 on Vlan226, 00:00:04 ago
Routing Descriptor Blocks:
* 10.21.48.254, from 172.18.67.120, 00:00:04 ago, via Vlan226         ------>>> it's preferred, why????? 
Route metric is 70, traffic share count is 1
Route tag 666

switch#show ip ospf database external 10.21.32.0 adv-router 192.168.99.2           ------->>> process id1 neighbor

OSPF Router with ID (192.168.97.3) (Process ID 2)

OSPF Router with ID (192.168.99.3) (Process ID 1)

Type-5 AS External Link States

LS age: 644
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 10.21.32.0 (External Network Number )
Advertising Router: 192.168.99.2
LS Seq Number: 8000015E
Checksum: 0x3E2C
Length: 36
Network Mask: /24
Metric Type: 2 (Larger than any link state path)                   ---------->>> OE2
MTID: 0
Metric: 65                                                                           

                                            ------>>> metric is 65 and smaller than 70
Forward Address: 0.0.0.0
External Route Tag: 666

##########When I shutdown the ospf process 2 transit vlan interface

switch# show ip route 10.21.32.10
Routing entry for 10.21.32.0/24
Known via "ospf 1", distance 110, metric 65
Tag 666, type extern 2, forward metric 1
Redistributing via ospf 2
Last update from 192.168.97.2 on Vlan3501, 00:00:04 ago
Routing Descriptor Blocks:
* 192.168.97.2, from 192.168.99.2, 00:00:04 ago, via Vlan3501           

                                                            ---->>> choose process 1 OE2 as expected with metric 65
Route metric is 65, traffic share count is 1

######### Bring the process 2 transit interface vlan back

switch#show ip route 10.21.32.10
Routing entry for 10.21.32.0/24
Known via "ospf 2", distance 110, metric 70
Tag 666, type extern 1
Last update from 10.21.48.254 on Vlan226, 00:00:01 ago
Routing Descriptor Blocks:
* 10.21.48.254, from 172.18.67.120, 00:00:01 ago, via Vlan226           

                                                                               ------->>> OE1 is preferred again even if it has higher metric 70
Route metric is 70, traffic share count is 1
Route tag 666

[paul driver]

Hello

---

@rwu wrote:

Hi network experts,

    I have a router which receives 10.21.32.10/24 as OE2 from ospf process 1 and the same route as OE1 from ospf process 2. The OE2 route has the metric 65 and OE1 route has the metric 21. Currently, the router prefers the OE1 route. -- Due to E1 LSA is preferred over E2 LSA

    I'm under the impression that for different OSPF processes, first compare AD and then metric. That's it. If they're the same, there will be load balancing. The router/switch won't care if it's OE1 or OE2 or any other type of LSA. 

    I've done the following tests:

1. Increase the AD for the OE1 route to 115 and then the router prefers OE2 with default AD 110. -  --correct

2. Recover OE1 route AD to 110. The router prefers OE1 due to its lower metric 21.  LSA type 

3. Increase the metric for the OE1 route 70. HOWEVER, the router STILL prefers OE1. - due to E1 is preferred over E2 LSA

---

Route path selection-
Intra-Area (O)
Inter-Area (O IA)
NSSA Type 1 (N1)
External Type 1 (E1)
NSSA Type 2 (N2)
External Type 2 (E2)

[rwu]

Hi Paul

    Thanks for the reply. I know these rules inside the "same" process. But I was assuming if there're two processes, the rule will be to compare AD first and then metric. Don't care about LSA type. The reference comes from: https://community.cisco.com/t5/routing/route-selection-criteria-in-a-multi-process-ospf-environment/td-p/3858868

    But my test results in the real environment as shown above are contradictory to the reference link. 

    The rules for two processes should be:

1. Compare AD first. Lower AD wins. 

2. If AD is equal, follow the route selection rules based on LSA type, say, OE1 over OE2 in any circumstance, the one with lower metric of two OE1 routes wins, etc. 

    I was under the impression that two OSPF processes are like two different routing protocols which is why it should be AD first and then metric. But it's actually not  

[paul driver · Accepted solution]

Hello
AD rule will take precedence first, just as if you have differing routing protocols active, Then as these are two ospf process thus two separate ospf domains then my understanding is LSA type will take precedence for the same route.