OSPF inter area routes are not installed

[sachin30720041]

Hello All,

R1(vrf1)--(area1)--(vrf1)R2(vrf1)----(area 0)---(vrf1)R3(vrf1)--(area2)--(vrf1)R4

R1 is in area 1.

R2 is an ABR between area 1 and area 0.

R3 is an ABR between area 2 and area 0.

R4 is in area 2.

R1, R2, R3, and R4's interfaces are under vrf1.

I see R1's loopback ip in R2's and R3's vrf1 routing table.

But I do not see R2's and R3's loopback in R1's vrf1 routing table.

I see R'2 loopback in ospf database of R1. And I do not see a set down bit on the 2.2.2.2 summary route. then why do not I see 2.2.2.2 as an installed route on R1? 

R1#show ip ospf database summary 2.2.2.2

OSPF Router with ID (1.1.1.1) (Process ID 1)

Summary Net Link States (Area 1)

Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 2.2.2.2 (summary Network Number)

vios1

--------------------

vrf def vrf1

address-family ipv4 unicast

exit

int lo0

vrf forwarding vrf1

ip address 1.1.1.1 255.255.255.255

exit

int gi0/0

vrf forwarding vrf1

ip address 100.10.20.1 255.255.255.0

ip ospf 1 area 1

router ospf 1 vrf vrf1

router-id 1.1.1.1

vios2

--------------------

vrf def vrf1

address-family ipv4 unicast

exit

int lo0

vrf forwarding vrf1

ip address 2.2.2.2 255.255.255.255

exit

int gi0/0

vrf forwarding vrf1

ip address 100.10.20.2 255.255.255.0

ip ospf 1 area 1

no sh

int gi0/3

vrf forwarding vrf1

ip address 100.20.30.2 255.255.255.0

ip ospf 1 area 0

no sh

router ospf 1 vrf vrf1

router-id 1.1.1.1

vios3

-----------------------------

vrf def vrf1

address-family ipv4 unicast

exit

int lo0

vrf forwarding vrf1

ip address 3.3.3.3 255.255.255.255

exit

int gi0/3

vrf forwarding vrf1

ip address 100.20.30.3 255.255.255.0

ip ospf 1 area 0

no sh

int gi0/0

vrf forwarding vrf1

ip address 100.30.40.3 255.255.255.0

ip ospf 1 area 2

no sh

router ospf 1 vrf vrf1

router-id 3.3.3.3

vios4

------------------

vrf def vrf1

address-family ipv4 unicast

exit

int lo0

vrf forwarding vrf1

ip ospf 1 area 2

ip address 4.4.4.4 255.255.255.255

exit

int gi0/0

vrf forwarding vrf1

ip address 100.30.40.4 255.255.255.0

ip ospf 1 area 2

router ospf 1 vrf vrf1

router-id 4.4.4.4

[johnd2310]

Hi,

Are you advertising the loopback interfaces in ospf?

Thanks

John

[sachin30720041]

I forgot to add the command here. Yes, I am advertising the loopbacks. After advertising the loopbacks Area 0 has routes from area 1 and area 2. But Area 1 and Area 2 do not have inter-area routes.

[MHM Cisco World]

...

[MHM Cisco World]

...

[Harold Ritter · Accepted solution]

Hi @sachin30720041 ,

This is due to the routing loop detection mechanism. Make sure you configure "capability vrf lite" under router ospf on the CEs to fix this issue.

vios1:

router ospf 1 vrf vrf1

capability vrf lite

vios4:

router ospf 1 vrf vrf1

capability vrf lite

Regards,

[sachin30720041]

routes got exchanged with 'capability vrf lite' but I do not see set down bit on inter-area routes.

[AnthonySylvester]

This is because the DN bit is set by MP-BGP (during redistribution), which you are not using. So, you will not see an LSA with DN bit set in the current setup you have.

[Harold Ritter]

Hi @sachin30720041 ,

The "capability vrf-lite" is required for the ospf summary links to be installed in the RIB whenever VRF lite is used on the CE and the summary is received via a non backbone area. This is regardless whether the DN bit is set or not.

The reason is that without the "capability vrf-lite", the router will consider itself connected to the super backbone. It will therefore also consider itself as an ABR and as such will not use a summary link received from a non backbone area, which is an expected behavior in ospf.

Regards,

[MHM Cisco World]

..

[Harold Ritter]

HI @MHM Cisco World ,

This is not a bug. It is rather expected behavior. Please refer to the explanation I gave in this thread and let me know if you have questions.

https://community.cisco.com/t5/mpls/ospf-inter-area-routes-are-not-installed/m-p/4579914/highlight/true#M23783

Regards,

[MHM Cisco World]

...

[Harold Ritter]

Hi @MHM Cisco World ,

> If I am understand that right DN bit set only when redistribute the MP-BGP to OSPF, other router check this bit and do filter.

That is correct. In addition to that, it is not because the DN bit is not set that the LSA is necessarily going to be used. Please  read the explanation I provided in this thread on why the summary link coming from R2 is not used in spite of the fact that the DN bit is not set.

Regards,