OSPF External Type-5 vs Static route

[AllezLom63194]

Hi the community,

I would like to have your point of view and suggestion about my problem.

As you can see on the draw attached, I have static routing and dynamic routing for the same remote sites LANs.

My goal is to replace the static routing by the OSPF routing.

I have made a GNS3 lab besause I can't make tests on the production environment.

The remote site LAN is 10.45.50.0/24

Actually I have a static route on the OLV S1 5K (Lo 11.11.11.11) to join this network

"ip route 10.45.50.0 255.255.255.0 10.45.237.254 115"

The router is configured on area 0 on the 10.45.208.0/20

I have added a new router (ASR Lo 1.2.3.4) on the 10.45.208.0/20 LAN on the area 0

This router is an ABR and is connected to the area 5 NSSA too.

router ospf 1
router-id 1.2.3.4
log-adjacency-changes
area 5 nssa no-redistribution default-information-originate metric 4
network 1.2.3.4 0.0.0.0 area 0
network 10.45.208.0 0.0.15.255 area 0
network 10.45.237.0 0.0.0.255 area 5

My problem is that the router OLV S1 5K (Lo 11.11.11.11) prefer the static route to the remote site LAN 10.45.50.0 instead of the OSPF route annonced by the ASR router. The OSPF route is not inserted on the routing table.

As you can see, I have modified the AD of the static route to 115 to make OSPF more interesting (AD 110).

The 10.45.50.0/24 is on the OSPF database of the OLV S1 5K (Lo 11.11.11.11) as 

Type-5 AS External Link States

Link ID                ADV Router                 Age               Seq#                     Checksum               Tag
10.45.50.0         1.2.3.4                        628               0x8000000A         0x00EB45                0

When I shut the interface of the OLV S1 5K to the network 10.45.237.0/24 connected to the provider router, the static route disapear and the OSPF route is correctly inserted to the routing table of the OLV S1 5K.

I think that my problem comes from that the 5K has a direct connection to the 10.45.237.0/24 and the router that redistribute the remote site LAN is on that network too, as you can see below but I can't explain why becaus OSPF AD is lower.

OLV_S1_5K#sh ip ospf database external 10.45.50.0

OSPF Router with ID (11.11.11.11) (Process ID 1)

Type-5 AS External Link States

LS age: 771
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 10.45.50.0 (External Network Number )
Advertising Router: 1.2.3.4
LS Seq Number: 8000000A
Checksum: 0xEB45
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 100
Forward Address: 10.45.237.253
External Route Tag: 0

Thank for your ideas or suggestions.

Regards.

[Giuseppe Larosa]

Hello AllezLomb63194,

an OSPF external route can be installed only if the forwarding address is reachable via an internal route ( O or O IA).

you have the following forwarding address:

>> Forward Address: 10.45.237.253

if this address is for OSPF known via another external route the LSA is not installed.

To fix this try the following

add a statement for network 10.45.237.0 0.0.0.255 area 5

make passive the corresponding interface.

This way you should be fine.

Edit:

I see that you have already this line on the ABR

network 10.45.237.0 0.0.0.255 area 5

on the other router check how it sees the forwarding address with

show ip route 10.45.237.253

Hope to help

Giuseppe

[AllezLom63194]

Hi Giuseppe,

Thank you for your response.

As I mentionned on my post, the 10.45.237.253 is directly connected to the OLV S1 5K. I think that the problem comes from that point. Because the OSPF route is installed on  the routing table when I shut this interface.

But I can't explain why and I can't shut this interface on the production router because this interface is used during the migration.

[AllezLom63194 · Accepted solution]

Hi the community,

I have tested to create the area 5 on a different network than 10.45.237.0/24 and all works as I wanted. So this is the solution but I still can't explain the problem.

I mark this post as solved. It may help someone.