OSPF Routing Issue

Answered Question
Jul 5th, 2007

Hello together,

we have 2 adjacent router in area 0 on a LAN.

On the first router we configured static-routes with redistribution into the OSPF.

On second router we can see the network in the ospf external database but not in the routing table. see commands:

R01#sh ip route 192.168.8.0

Routing entry for 192.168.8.0/24

Known via "static", distance 1, metric 0

Redistributing via ospf 100

Advertised by ospf 100 metric-type 1 subnets

Routing Descriptor Blocks:

* 172.21.1.174

Route metric is 0, traffic share count is 1

R01#sh ip ospf database external 192.168.8.0

OSPF Router with ID (192.168.30.48) (Process ID 100)

Type-5 AS External Link States

LS age: 248

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 192.168.8.0 (External Network Number )

Advertising Router: 192.168.30.48

LS Seq Number: 80000001

Checksum: 0x5D29

Length: 36

Network Mask: /24

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 172.21.1.174

External Route Tag: 0

R02#sh ip route 192.168.8.0

% Network not in table

R02#sh ip ospf database external 192.168.8.0

OSPF Router with ID (192.168.30.49) (Process ID 100)

Type-5 AS External Link States

LS age: 281

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 192.168.8.0 (External Network Number )

Advertising Router: 192.168.30.48

LS Seq Number: 80000001

Checksum: 0x5D29

Length: 36

Network Mask: /24

Metric Type: 1 (Comparable directly to link state metric)

TOS: 0

Metric: 20

Forward Address: 172.21.1.174

External Route Tag: 0

What could be the reason ?

I have this problem too.
0 votes
Correct Answer by Harold Ritter about 9 years 6 months ago

There seems to be several issues here. R02 sees the route to 172.21.1.174 as local via a /30 while another router uses 172.21.1.174 as its router id and also advertises it as a /32. I'm not sure why this is but you should definitely check into it.

Why is the static route configured with a non-connected NH (i.e. 172.21.1.174)? Can that be changed?

Regards,

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Harold Ritter Thu, 07/05/2007 - 04:53

The reason is that the "forward address", which is set to 172.21.1.174 is probably reachable via a protocol other than ospf.

When an external LSA has a "forward address", this address has to be reachable via ospf for the external route to be considered as valid.

One way to get rid of the issue is to make sure 172.21.1.174 is reachable via ospf on R02. The other way is to cause the external route to be generated without a "forward address". This can be achieved by configuring the interface to which address 172.21.1.174 belong, as a passive interface.

Hope this helps,

walu Thu, 07/05/2007 - 05:39

Thanks for your fast response.

I have check the configuration of both routers.

It seems to be that the forward address is reachable via ospf from R01. see commands:

R01#sh ip route 172.21.1.174

Routing entry for 172.21.1.174/32

Known via "ospf 100", distance 110, metric 131, type intra area

Last update from 172.21.1.174 on ATM2/0.246, 03:13:31 ago

Routing Descriptor Blocks:

* 172.21.1.174, from 192.168.30.6, 03:13:31 ago, via ATM2/0.246

Route metric is 131, traffic share count is 1

R01#sh ip ospf int AT2/0.246

ATM2/0.246 is up, line protocol is up

Internet Address 0.0.0.0/0, Area 4

Process ID 100, Router ID 192.168.30.48, Network Type POINT_TO_POINT, Cost: 130

Transmit Delay is 1 sec, State POINT_TO_POINT

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

oob-resync timeout 40

Hello due in 00:00:03

Supports Link-local Signaling (LLS)

Cisco NSF helper support enabled

IETF NSF helper support enabled

Index 36/37, flood queue length 0

Next 0x0(0)/0x0(0)

Last flood scan length is 1, maximum is 51

Last flood scan time is 0 msec, maximum is 4 msec

Neighbor Count is 1, Adjacent neighbor count is 1

Adjacent with neighbor 192.168.30.6

Suppress hello for 0 neighbor(s)

But from Router R02 the route is not learned via ospf:

R02#sh ip route 172.21.1.174

Routing entry for 172.21.1.172/30

Known via "connected", distance 0, metric 0 (connected, via interface)

Redistributing via ospf 100

Routing Descriptor Blocks:

* directly connected, via Loopback246

Route metric is 0, traffic share count is 1

Could at be a problem that on R02 the 172.21.1.172/30 is configured for backup purpose ?

If the primary atm pvc on R01 fails the atm backbone switches the pvc to R02.

The atm subinterfaces on both routers use the same network.

Thanks for further hints.

Correct Answer
Harold Ritter Thu, 07/05/2007 - 06:17

There seems to be several issues here. R02 sees the route to 172.21.1.174 as local via a /30 while another router uses 172.21.1.174 as its router id and also advertises it as a /32. I'm not sure why this is but you should definitely check into it.

Why is the static route configured with a non-connected NH (i.e. 172.21.1.174)? Can that be changed?

Regards,

walu Thu, 07/05/2007 - 22:41

Hello

Thanks for all responses. This conversation hepls us to fix this issue.

We have change the static route entries from next hop to interface statement. And now the route is redistributed without a forwarded address in the LSA and therefor the route is taken to the RIB !

royalblues Thu, 07/05/2007 - 04:55

i see that the redistributed route has a Forward Address: 172.21.1.174

The issue could be due to the route to the forward address is learnt via a static route. The route to the forward address should always be learnt via OSPF. Otherwise, the external route will never be installed in the RIB as you are now experiencing.

Do a sh ip route 172.21.1.174 and check whether it is being learned by ospf or via any static route

HTH

Narayan

mohammedmahmoud Thu, 07/05/2007 - 05:20

Hi,

I think that it is related to the forward address of the redistributed route it should always be learned via OSPF, please check and feed back.

[edit] Sorry guys for the replicated reply :)

HTH,

Mohammed Mahmoud.

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