ospf external default route in database but not in routing table

nitinsidhu · ‎10-31-2014

Hello

I am experiencing an issue with the default route not being installed in the routing table though it is present in the ospf database. The scenario is as below:

R1 --(area 249)-- R2----(area 0)--- R3 --- (area 0)--- R4

All routers in under OSPF process 20. R2,R3 and R4 are in area 0 whereas R1 is in area 249. R2 is the ABR and R4 is the ASBR.

R4 has a 'default-information originate always metric-type 1' command under its ospf 20 process.

The default route is getting propagates through the network and R1 has a default ospf external type 1 default route in its ospf database.

---R1 output---

#sh ip os database

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
0.0.0.0 10.248.0.2 190 0x80000087 0x00D3C0 20

#sh ip os da external -------- 'WHEN ROUTE IS NOT INSTALLED IN THE ROUTING TABLE'

OSPF Router with ID (10.249.0.2) (Process ID 20)

Type-5 AS External Link States

LS age: 225
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.248.0.2
LS Seq Number: 80000087
Checksum: 0xD3C0
Length: 36
Network Mask: /0
Metric Type: 1 (Comparable directly to link state metric)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 20

#sh ip os da external----- 'WHEN ROUTE IS INSTALLED IN THE ROUTING TABLE'

OSPF Router with ID (10.249.0.2) (Process ID 20)

Type-5 AS External Link States

Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 287
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 10.248.0.2
LS Seq Number: 80000087
Checksum: 0xD3C0
Length: 36
Network Mask: /0
Metric Type: 1 (Comparable directly to link state metric)
MTID: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 20

#sh ip ro--- 'WHEN ROUTE IS NOT INSTALLED IN THE ROUTING TABLE'

10.0.0.0/8 is variably subnetted, 14 subnets, 7 masks
O IA 10.0.6.0/24 [110/11] via 10.249.0.1, 00:34:43, GigabitEthernet0/0
O IA 10.248.0.0/16 [110/22] via 10.249.0.1, 00:00:49, GigabitEthernet0/0

C 10.249.0.0/30 is directly connected, GigabitEthernet0/0

#sh ip ro-- 'WHEN ROUTE IS INSTALLED IN THE ROUTING TABLE'

O*E1 0.0.0.0/0 [110/22] via 10.249.0.1, 00:00:52, GigabitEthernet0/0
10.0.0.0/8 is variably subnetted, 14 subnets, 7 masks
O IA 10.0.6.0/24 [110/11] via 10.249.0.1, 00:35:51, GigabitEthernet0/0
O IA 10.248.0.0/16 [110/21] via 10.249.0.1, 00:00:01, GigabitEthernet0/0

C 10.249.0.0/30 is directly connected, GigabitEthernet0/0

The issue I am facing is that R1 despite having the default route in its ospf database it installs the route in the routing table for one minute and then removes it from the routing table for one minute and then again installs for one min. The default route stays in the ospf database though, it does not get removed from the database.

The output in the red is when the route in installed in the routing table. the black output is when it is not installed in the routing.

Also I have other routes being propagated by R4 when reach R1 and they are installed in the routing table and they are NOT flapping like default route.

I have tried changing the metric-type to 2, restarting ospf process and restarting router but none of them worked

ios version is: c2900-universalk9-mz.SPA.154-2.T1.bin

Please if anyone could help regarding this, its urgent.

Best regard

Nitin sidhu

John Blakley · ‎10-31-2014

Is this only happening on R1? Can you see the default on R2? Can you post the config of R1 and R2's ospf config?

O IA 10.248.0.0/16 [110/22] via 10.249.0.1, 00:00:49, GigabitEthernet0/0

From this, the 10.248.0.0/16 has only been in the routing table for 50 sec. Are you losing this route for some reason?

HTH,
John

HTH, John *** Please rate all useful posts ***

Rolf Fischer · ‎11-01-2014

Hi,

as John has already pointed out, there has been some change in Area 0:

(1) O IA 10.248.0.0/16 [110/22] via 10.249.0.1, 00:00:49, GigabitEthernet0/0
(2) O IA 10.248.0.0/16 [110/21] via 10.249.0.1, 00:00:01, GigabitEthernet0/0

=> The newer summary route for 10.248.0.0 has been installed with a lower cost for some reason whereas the 10.0.6.0 summary is unchanged.

The Type-5 LSA is always flooded from Area 0 to Area 249 unchanged and we can see that even the Sequence Number is the same in both cases.

The more interesting component in this case is the Type-4 LSA, which provides the required information about how to reach the ASBR in Area 0. So most likely, depending on what's going on in Area 0, the ABR does not originate that Type-4 LSA. The Type-5 LSA then is not flagged with the (internal) Routing Bit, which means it's not a candidate to be installed in the routing table.

The relevant parts of the configurations would be helpful, as requested by John.

HTH
Rolf

P.S.: The show-command for the Type-4 LSA is "show ip ospf database asbr-summary"