OSPF forward address

[gongya001]

Anyone can tell me why I added R3 e0/0 to ospf, R4 could not see 6.6.6.6 any more ?

thanks !!

R4#show ip ospf database external 6.6.6.6

OSPF Router with ID (4.4.4.4) (Process ID 1)

Type-5 AS External Link States

LS age: 1154
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 6.6.6.6 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000008
Checksum: 0x2829
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 10.10.10.6
External Route Tag: 0

R4#show ip route 6.6.6.6
% Network not in table
R4#show ip cef 6.6.6.6
0.0.0.0/0

[Giuseppe Larosa]

Hello Gongya001,

R4 needs to see the forwarding address 10.10.10.6 as reachable via an internal OSPF route in order to install the external route to 6.6.6.6

if you have put r3's eth0/0 in an area different from area 10 you are in trouble as that area would be not correct in OSPF design

Hope to help

Giuseppe Larosa

[gongya001]

I did add R3 e0/0 to area 10

R3#show run int e0/0
Building configuration...

Current configuration : 85 bytes
!
interface Ethernet0/0
 ip address 10.10.10.3 255.255.255.0
 ip ospf 1 area 10
end

From R4, I can see 6.6.6.6 in the OSPF database

R4#show ip ospf data external 6.6.6.6

            OSPF Router with ID (4.4.4.4) (Process ID 1)

                Type-5 AS External Link States

  LS age: 24
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 6.6.6.6 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x3622
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 10.10.10.6
        External Route Tag: 0

but not in the routing and cef table

R4#show ip route 6.6.6.6
% Network not in table
R4#show ip cef 6.6.6.6
0.0.0.0/0
  no route

If I take R3 e0/0 out of OSPF, 

R4 will install a route to 6.6.6.6

thanks for help !!

[Harold Ritter]

Giuseppe is right. This is indeed the source of the problem. You could fix the issue by configuring R3 e0/0 as a passive ospf interface. This would have the effect of removing the fowarding address from the external LSA.

Regards,

[gongya001]

I did want to see the non-zero forward address and also see the route to 6.6.6.6, Is it possible in the scenario ?

thanks !!

[Harold Ritter]

According to the diagram, the L3 link between R4 and R6 is also on that same subnet (10.10.10.0/24). You could activate ospf on R4 e0/1, which should be sufficient to pass the forwarding address check.

Regards,

[gongya001]

The reason to replace zero with non-zero forward address is to resolve non-optimal path. right ?
If I enabled R4 e0/1 for OSPF, no non-optimal path to 6.6.6.6. If not, R4 goes to R3 --> R6. I just want to verify this. Not sure this topology is appropriate or not.
thanks !!

[Harold Ritter]

It does sound like the perfect scenario indeed and the concept of forwarding address was introduced specifically for this type of scenarios.

Regards,

[gongya001]

unfortunately I could not get it working. After I replaced Forward address with non-zero FA, R4 could not reach 6.6.6.6 any more.

[Harold Ritter]

Could you please provide a "show ip ospf database external 6.6.6.6 internal" and "show ip ospf int e0/1" from R4.

Regards,

[gongya001]

R4#show ip ospf int e0/1
%OSPF: OSPF not enabled on Ethernet0/1
R4#
R4#
R4#show ip ospf data external 6.6.6.6 internal

OSPF Router with ID (4.4.4.4) (Process ID 1)

Type-5 AS External Link States

Changed flag set for this LSA
LSA prefix priority: High
Distance: 16777215
Now in min table
Table index: 11 min 45 sec
LS age: 34
Options: (No TOS-capability, DC, Upward)
LS Type: AS External Link
Link State ID: 6.6.6.6 (External Network Number )
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0x3622
Length: 36
Network Mask: /32
Metric Type: 2 (Larger than any link state path)
MTID: 0
Metric: 20
Forward Address: 10.10.10.6
External Route Tag: 0

R4#show ip route 6.6.6.6
% Network not in table
R4#show ip cef 6.6.6.6
0.0.0.0/0
no route

thanks so much !!

[Harold Ritter · Accepted solution]

As per one of my previous posts, you need to activate ospf on R4 e0/1 for it to work.

Regards,

[gongya001]

thanks !!

[gongya001]

thanks so much. It works.