ECMP in case of redistribution of OSPF

[sachin30720041]

Hello All,

I have configured following lab setup. 18.18.18.18 is advertised by XR18. and XR16 and XR17 redistribute(18.18.18.18) towards XR15.

On XR17,

RP/0/0/CPU0:XR17#show route 18.18.18.18
Routing entry for 18.18.18.18/32
Known via "ospf 1", distance 110, metric 2, type extern 2
Installed Apr 29 17:15:46.645 for 00:03:02
Routing Descriptor Blocks
100.150.170.15, from 100.150.160.16, via GigabitEthernet0/0/0/1
Route metric is 2

XR17 is installing route with metric 2, sourced from 100.150.160.16 in its routing table.

And if I shutdown gi0/0/0/1(on XR17) then XR17 installs 18.18.18.18 with metric 2, sourced from 18.18.18.18

RP/0/0/CPU0:ios#show route 18.18.18.18

Routing entry for 18.18.18.18/32
Known via "ospf 2", distance 110, metric 2, type intra area
Installed Apr 29 17:26:19.431 for 00:00:03
Routing Descriptor Blocks
100.170.180.18, from 18.18.18.18, via GigabitEthernet0/0/0/0
Route metric is 2

Now I did 'no shut' on Gi0/0/0/1(on XR17).

If both intra-area and external routes have metric of 2 then why there is no ECMP for 18.18.18.18 on XR17 ? Why XR17 is only installing external route sourced from XR16.

                           R15 (gi0/0/0/0)------(ospf 1)-------(gi0/0/0/0) R16

                    gi0/0/0/0                                                          gi0/0/0/1

                            |                                                                  |

                        ospf 1                                                            ospf 2

                           |                                                                    |

                    gi0/0/0/1                                                         gi0/0/0/1

                          R17(gi0/0/0/0)-------(ospf 2)------- (gi0/0/0/0) R18

xrv15
-----------------
int gi0/0/0/0
ip address 100.150.160.15 255.255.255.0
no sh

int gi0/0/0/1
ip address 100.150.170.15 255.255.255.0
no sh

int lo0
ip address 15.15.15.15 255.255.255.255

router ospf 1
area 0
int gi0/0/0/0
int gi0/0/0/1
int lo0
exit

xrv16
------------------
int gi0/0/0/0
ip address 100.150.160.16 255.255.255.0
no sh

int gi0/0/0/1
ip address 100.160.180.16 255.255.255.0
no sh

int lo0
ip address 16.16.16.16 255.255.255.255
exit

router ospf 1
area 0
int gi0/0/0/0
int lo0
exit

router ospf 2
area 0
int gi0/0/0/1
exit

xr17
--------------------------
int gi0/0/0/1
ip address 100.150.170.17 255.255.255.0
no sh

int gi0/0/0/0
ip address 100.170.180.17 255.255.255.0
no sh

int lo0
ip address 17.17.17.17 255.255.255.255
exit

router ospf 2
redistribute ospf 1
area 0
int gi0/0/0/0
int lo0
exit

router ospf 1
area 0
int gi0/0/0/1

xr18
--------------------------
int gi0/0/0/1
ip address 100.160.180.18 255.255.255.0
no sh

int gi0/0/0/0
ip address 100.170.180.18 255.255.255.0
no sh

int lo0
ip address 18.18.18.18 255.255.255.255
exit

router ospf 1
area 0
int gi0/0/0/1
int lo0
int gi0/0/0/0
exit

Thanks,

Sachin

[MHM Cisco World]

Need to config 

Ospf max path 

To make opsf use both path 

[sachin30720041]

By default, ospf is configured with 'maximum path' value of 4.

https://www.techrepublic.com/article/how-to-configure-equal-cost-multi-path-in-ospf/

[Harold Ritter · Accepted solution]

Hi @sachin30720041 ,

ECMP will not work for routes learnt from different routing processes.

Regards,

[sachin30720041]

From R17 point of view, Is there any reason to prefer R17->R15->R16->R18 over R17->R18. Both routes have metric of 2.

[MHM Cisco World · Accepted solution]

I follow your two post for multi Process 
please check this link 

https://blog.ipspace.net/2021/06/ospf-inter-process-route-selection.html

with multiprocess OSPF behavior is unpredictable.

[Harold Ritter · Accepted solution]

Hi @sachin30720041 ,

As the two routes are considered equals from a RIB point of view, the first route getting installed will remain in the RIB. If you want to make it more deterministic, you can always change the distance on one of the two ospf processes to make it preferred over the other.

Regards,