OSPF Forwarding Address (FA) Part 2 - How FA Interacts With NSSA

Let's continue the series by looking at FA and it's interaction with NSSA (Not So Stubby Area).

Below is our topology for the day:

Assuming everything is working fine. let's do a quick verification:

R1#sh ip route ospf | b ^G
Gateway of last resort is not set

      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 123.0.0.2, 00:31:19, GigabitEthernet1/0
      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/2] via 123.0.0.3, 00:26:44, GigabitEthernet1/0
      4.0.0.0/32 is subnetted, 1 subnets
O IA     4.4.4.4 [110/3] via 123.0.0.2, 00:20:26, GigabitEthernet1/0
      5.0.0.0/32 is subnetted, 1 subnets
O E2     5.5.5.5 [110/20] via 123.0.0.2, 00:02:07, GigabitEthernet1/0
      24.0.0.0/24 is subnetted, 1 subnets
O IA     24.0.0.0 [110/2] via 123.0.0.2, 00:31:09, GigabitEthernet1/0
      34.0.0.0/24 is subnetted, 1 subnets
O IA     34.0.0.0 [110/65] via 123.0.0.3, 00:20:21, GigabitEthernet1/0
      45.0.0.0/24 is subnetted, 1 subnets

O E2     45.0.0.0 [110/20] via 123.0.0.2, 00:20:02, GigabitEthernet1/0

So control plane seems to be working fine since 5.5.5.5/32 network is present in routing table.

Now let's verify the data plane :

R1#ping 5.5.5.5 source loopback 0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 120/170/236 ms

So far so good...

Now let's review LSA-5 information from R1's prospective:

R1#sh ip ospf database external

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

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 284
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 5.5.5.5 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x6F01
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 4.4.4.4
        External Route Tag: 0

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 1355
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 45.0.0.0 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0xB4C
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 4.4.4.4

        External Route Tag: 0

Now here are couple of interesting things to take a look at.

The "Advertising Router" is marked as 3.3.3.3, Which means between R2 & R3, it's only R3 doing the translation of LSA-7 into LSA-5. Now to make it simple, when we have two or more ABRs between NSSA & Backbone Area. Only the router with higher Router ID (RID) is responsible for Type-7 to Type-5 translation. But on the flip side this translator router need not be in Data Plane path technically to reach the external prefixes. Let's verify this:

R1#traceroute 5.5.5.5 source loopback 0
Type escape sequence to abort.
Tracing the route to 5.5.5.5
VRF info: (vrf in name/id, vrf out name/id)
  1 123.0.0.2 116 msec *  64 msec
  2 24.0.0.4 188 msec 184 msec 136 msec

  3 45.0.0.5 164 msec 92 msec 220 msec

So basically election of path to reach External Prefixes completely depends upon cost to ASBR.

Also the FA is set to 4.4.4.4 instead of 45.0.0.5. This is because at the moment OSPF in not enabled on 45.0.0.X/24 Network. Also FA is set automatically instead of being 0.0.0.0 like in previous case. Which is also interesting.

Now let's try to filter Forwarding-Address (FA) and see what happens:

R1(config)#ip prefix-list FILTER-FA deny 4.4.4.4/32
R1(config)#ip prefix-list FILTER-FA permit 0.0.0.0/0 le 32

R1(config)#router ospf 1
R1(config-router)#distribute-list prefix FILTER-FA in

R1(config-router)#exit

Now Let's verify the Data Plane again:

R1#ping 5.5.5.5 source loopback 0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
.....

Success rate is 0 percent (0/5)

Oops....Seems to be broken.

Let's try traceroute:

R1#traceroute 5.5.5.5 source loopback 0
Type escape sequence to abort.
Tracing the route to 5.5.5.5
VRF info: (vrf in name/id, vrf out name/id)
  1  *  *  *
  2  *  *  *

  3  *  *  *

No Luck....

Let's verify the control plane by looking at Routing Table:

R1#sh ip route ospf | b ^G
Gateway of last resort is not set

      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 123.0.0.2, 00:02:15, GigabitEthernet1/0
      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/2] via 123.0.0.3, 00:02:15, GigabitEthernet1/0
      24.0.0.0/24 is subnetted, 1 subnets
O IA     24.0.0.0 [110/2] via 123.0.0.2, 00:02:15, GigabitEthernet1/0
      34.0.0.0/24 is subnetted, 1 subnets

O IA     34.0.0.0 [110/65] via 123.0.0.3, 00:02:15, GigabitEthernet1/0

Hmmm...here seems to be the problem since 5.5.5.5/32 is not in RIB. 

Let's review LSA-5 and see if the information is in OSPF LSDB atleast.

R1#sh ip ospf database external

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

                Type-5 AS External Link States

  LS age: 780
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 5.5.5.5 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x6F01
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 4.4.4.4
        External Route Tag: 0

  LS age: 1851
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 45.0.0.0 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0xB4C
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 4.4.4.4

        External Route Tag: 0

So We have LSA-5 for this external prefix in database but the entry is not getting installed into the Routing Table.

So this is basically one of the reasons (Filtered FA) because of which OSPF Route is in database but not in Routing table. So add this to your ospf troubleshooting checklist :)

But the next question is - How to we fix this ?

Obviously Removing Distribute List on R1 is simple options :)

Now let's solve it the other way which is known as "Forwarding Address Suppression". Which basically means let's filter the FA this time from LSA-5 itself. And since now FA will be set to 0.0.0.0 with this, it means filtering FA using Distribute List won't play any role.

R2(config)#router ospf 1
R2(config-router)#area 1 nssa translate type7 suppress-fa

R2(config-router)#end

Let's verify Control Plane:

R1#sh ip route ospf | b ^G
Gateway of last resort is not set

      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 123.0.0.2, 00:09:33, GigabitEthernet1/0
      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/2] via 123.0.0.3, 00:09:33, GigabitEthernet1/0
      24.0.0.0/24 is subnetted, 1 subnets
O IA     24.0.0.0 [110/2] via 123.0.0.2, 00:09:33, GigabitEthernet1/0
      34.0.0.0/24 is subnetted, 1 subnets

O IA     34.0.0.0 [110/65] via 123.0.0.3, 00:09:33, GigabitEthernet1/0

Hmmm...Doesn't work right ?

This is because as I described above, it's only R3 that doing the translation to LSA-7 to LSA-5

So let's put same configuration on R3:

R3(config)#router ospf 1
R3(config-router)#area 1 nssa translate type7 suppress-fa

R3(config-router)#end

Let's verify the Control Plane again:

R1#sh ip route ospf | b ^G
Gateway of last resort is not set

      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 123.0.0.2, 00:10:34, GigabitEthernet1/0
      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/2] via 123.0.0.3, 00:10:34, GigabitEthernet1/0
      5.0.0.0/32 is subnetted, 1 subnets
O E2     5.5.5.5 [110/20] via 123.0.0.3, 00:00:19, GigabitEthernet1/0
      24.0.0.0/24 is subnetted, 1 subnets
O IA     24.0.0.0 [110/2] via 123.0.0.2, 00:10:34, GigabitEthernet1/0
      34.0.0.0/24 is subnetted, 1 subnets
O IA     34.0.0.0 [110/65] via 123.0.0.3, 00:10:34, GigabitEthernet1/0
      45.0.0.0/24 is subnetted, 1 subnets

O E2     45.0.0.0 [110/20] via 123.0.0.3, 00:00:19, GigabitEthernet1/0

Now this looks far better. So let's review LSA-5 again:

R1#sh ip ospf database external

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

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 914
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 5.5.5.5 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0xA4DA
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0

        External Route Tag: 0

So as you can see the FA is now set to 0.0.0.0, And since it all Zeros now, filtering FA won't have any affect.

Let's review Data Plane one more time.

R1#ping 5.5.5.5 source loopback 0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 88/163/240 ms


R1#traceroute 5.5.5.5 source loopback 0
Type escape sequence to abort.
Tracing the route to 5.5.5.5
VRF info: (vrf in name/id, vrf out name/id)
  1 123.0.0.3 148 msec 156 msec 32 msec
  2 34.0.0.4 208 msec 100 msec 72 msec

  3 45.0.0.5 100 msec 328 msec *

Gr8....

Now for one little last testing remove the Distribute List we applied on R1 and Remove FA suppression command from R2 & R3. Also we will enable OSPF on 45.0.0.X/24 to see if anything changes from FA prospective:

R1#sh ip ospf database external

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

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 33
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 5.5.5.5 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000003
  Checksum: 0xD972
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 45.0.0.5

        External Route Tag: 0

And surely it does :)

HTH...
Deepak Arora
Evil CCIE

Okay let's not forget the initials if you wan't to test this by your own:

R1
++

!
en
!
conf t
!
ho R1
!
no ip do lo
!
no cdp run
!
line con 0
 logging syn
 no exec-time
 exit
!
int lo0
 ip add 1.1.1.1 255.255.255.255
 exit
!
int gi1/0
 ip add 123.0.0.1 255.255.255.0
 no sh
 exit
!
router ospf 1
 net 1.1.1.1 0.0.0.0 area 0
 net 123.0.0.1 0.0.0.0 area 0
 exit
!
end
!

===============================

R2
++

!
en
!
conf t
!
ho R2
!
no ip do lo
!
no cdp run
!
line con 0
 logging syn
 no exec-time
 exit
!
int lo0
 ip add 2.2.2.2 255.255.255.255
 exit
!
int gi1/0
 ip add 123.0.0.2 255.255.255.0
 no sh
 exit
!
int gi2/0
 ip add 24.0.0.2 255.255.255.0
 no sh
 exit
!
router ospf 1
 net 2.2.2.2 0.0.0.0 area 0
 net 123.0.0.2 0.0.0.0 area 0
 net 24.0.0.2 0.0.0.0 area 1 
 area 1 nssa
 area 1 nssa default-information-originate
 exit
!
end
!
==========================================

R3
++

!
en
!
conf t
!
ho R3
!
no ip do lo
!
no cdp run
!
line con 0
 logging syn
 no exec-time
 exit
!
int lo0
 ip add 3.3.3.3 255.255.255.255
 exit
!
int gi1/0
 ip add 123.0.0.3 255.255.255.0
 no sh
 exit
!
int s2/0
 ip add 34.0.0.3 255.255.255.0
 no sh
 exit
!
router ospf 1
 net 3.3.3.3 0.0.0.0 area 0
 net 123.0.0.3 0.0.0.0 area 0
 net 34.0.0.3 0.0.0.0 area 1 
 area 1 nssa
 area 1 nssa default-information-originate
 exit
!
end
!

==========================================

R4
++

!
en
!
conf t
!
ho R4
!
no ip do lo
!
no cdp run
!
line con 0
 logging syn
 no exec-time
 exit
!
int lo0
 ip add 4.4.4.4 255.255.255.255
 exit
!
int gi1/0
 ip add 24.0.0.4 255.255.255.0
 no sh
 exit
!
int s3/0
 ip add 34.0.0.4 255.255.255.0
 no sh
 exit
!
int gi2/0
 ip add 45.0.0.4 255.255.255.0
 no sh
 exit
!
router ospf 1
 net 4.4.4.4 0.0.0.0 area 1
 net 24.0.0.4 0.0.0.0 area 1
 net 34.0.0.4 0.0.0.0 area 1 
 area 1 nssa
 redistribute eigrp 100 subnet
 exit
!
router eigrp 100
 no auto
 net 45.0.0.4 0.0.0.0
 redistribute ospf 1 metric 1 1 1 1 1
 exit
!
end
!

==========================================

R5
++

!
en
!
conf t
!
ho R5
!
no ip do lo
!
no cdp run
!
line con 0
 logging syn
 no exec-time
 exit
!
int lo0
 ip add 5.5.5.5 255.255.255.255
 exit
!
int gi1/0
 ip add 45.0.0.5 255.255.255.0
 no sh
 exit
!
router eigrp 100
 no auto
 net 5.5.5.5 0.0.0.0
 net 45.0.0.5 0.0.0.0
 exit
!
end
!