OSPF Forwarding Address (FA) Part 1 - Is It All About Taking A Shortcut ?

OSPF Forwarding Address or FA is relatively less known as feature among network engineers. Okay I might be wrong... But as far I remember this feature was never been on CCNP Route/BSCI blueprint. So unless people are working on CCIE or reading through OSPF RFC, chances are quite slim they ever heard of it.

Anyways, lets start this series with Introduction of FA and later we see how it turns into good option or bad option to be enabled on network.

Now let's review our topology to begin with:

In this network we are running OSPF Area 0 between R2, R3 & R4 and their respective Loopback interfaces are advertised into OSPF. Now consider that it's a transition phase for this network where you are adding BGP IN but at the moment only R2 is capable of running BGP where as R3 requires some RAM upgrade or IOS Image upgrade to run BGP. So we have enabled EBGP between R1 & R2 at the moment which means R2 is currently Acting as OSPF ASBR. Also there is no OSPF running on 123.0.0.X/24 network at the moment.

Now lets review what CLI has to tell us:

Starting with R4's Routing Table:

R4#sh ip route ospf | begin ^Gateway
Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets
O E2     1.1.1.1 [110/1] via 24.0.0.2, 00:00:48, Serial2/0
      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/65] via 24.0.0.2, 00:02:49, Serial2/0
      3.0.0.0/32 is subnetted, 1 subnets
O        3.3.3.3 [110/2] via 34.0.0.3, 00:02:49, GigabitEthernet1/0

So as we can see, R4 is pointing Route towards R2 (ASBR). Which of course seems to be okay as the only way to get out of this network to reach prefix 1.1.1.1/32 is that way only. But on the flip it doesn't seem to be most optimal path since link between R2 - R4 is a T1 connection Vs R3 - R4 as GigE connection. 

Let's verify the Data Plane from R4 prospective:

R4#traceroute 1.1.1.1 source loopback 0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
  1 24.0.0.2 88 msec 80 msec 36 msec
  2 123.0.0.1 140 msec 120 msec 168 msec

Now let's take a quick look what network view R3 has:

R3#sh ip route ospf | begin ^G
Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets
O E2     1.1.1.1 [110/1] via 34.0.0.4, 00:04:55, GigabitEthernet2/0
      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/66] via 34.0.0.4, 00:06:56, GigabitEthernet2/0
      4.0.0.0/32 is subnetted, 1 subnets
O        4.4.4.4 [110/2] via 34.0.0.4, 00:06:56, GigabitEthernet2/0
      24.0.0.0/24 is subnetted, 1 subnets
O        24.0.0.0 [110/65] via 34.0.0.4, 00:06:56, GigabitEthernet2/0

R3#traceroute 1.1.1.1 source loopback 0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
  1 34.0.0.4 84 msec 76 msec 164 msec
  2 24.0.0.2 116 msec 64 msec 216 msec
  3 123.0.0.1 156 msec 140 msec 120 msec

Now this definitely doesn't look good even. Since we are following a longer path which also has a Poor Man's T1 connection in between.

Can we fix this design even without Running BGP on R3 ?

Of course....with FA :)

FA or Forwarding Address is a field we find under OSPF LSA Type-5. In most cases the FA is 0.0.0.0 which means "No" FA. In such case the traffic will be forwarded out to ASBR itself.

To include FA in OSPF LSA - 5, there are couple of conditions that needs to meet as follows:

"These conditions set the forwarding address field to a non-zero address:

OSPF is enabled on the ASBR's next hop interface AND

ASBR's next hop interface is non-passive under OSPF AND

ASBR's next hop interface is not point-to-point AND

ASBR's next hop interface is not point-to-multipoint AND

ASBR's next hop interface address falls under the network range specified in the router ospf command."

Okay let's cut the crap out spelled above and make it simple " The OSPF Network Type on ASBR External interface should be Broadcast while enabling OSPF and The Interface shouldn't be passive under OSPF Process"

Now let first review R4 and R3's LSA-5 before making any changes:

R4#sh ip ospf database external

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

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 143
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0xCD75
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 100

R3#sh ip ospf database external
            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 367
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0xCD75
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 100

Now let's first enable OSPF on R2 (ASBR) only on interface facing R1 to see if that helps with anything.

R2(config)#router ospf 1
R2(config-router)#network 123.0.0.2 0.0.0.0 area 0
R2(config-router)#end

Now let's review back the LSA-5 again on R3 & R4

R4#sh ip ospf database external

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

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 183
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000002
  Checksum: 0x289D
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 123.0.0.1
        External Route Tag: 100

Let's verify the Data Plane:

R4#traceroute 1.1.1.1 source loopback 0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
  1 24.0.0.2 56 msec 96 msec 36 msec
  2 123.0.0.1 160 msec 116 msec 156 msec


Seems like though R4 knows about FA now, it is not changing anything in Data Plane.

Now we need to take a Look at R3's LSA-5 :

R3#sh ip ospf database external

            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Type-5 AS External Link States

  LS age: 49
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000002
  Checksum: 0x289D
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 123.0.0.1
        External Route Tag: 100

Again FA is set but let's verify Data Plane to see if it has some affect on traffic forwarding:

R3#traceroute 1.1.1.1 source loopback 0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
  1  *  *  *
  2  *  *  *
  3  *  *  *
  4  *  *  *
  5  *  *  *
  6

R3#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 [110/66] via 34.0.0.4, 00:01:23, GigabitEthernet2/0
      4.0.0.0/32 is subnetted, 1 subnets
O        4.4.4.4 [110/2] via 34.0.0.4, 00:35:47, GigabitEthernet2/0
      24.0.0.0/24 is subnetted, 1 subnets
O        24.0.0.0 [110/65] via 34.0.0.4, 00:35:47, GigabitEthernet2/0

R3#sh ip route 123.0.0.1
Routing entry for 123.0.0.0/24
  Known via "connected", distance 0, metric 0 (connected, via interface)
  Routing Descriptor Blocks:
  * directly connected, via GigabitEthernet1/0
      Route metric is 0, traffic share count is 1

That's probably you don't want to see on a production network :)

We will return to details in next post talking about why R3 isn't able to reach 1.1.1.1/32 at the moment.

Let's Enable OSPF on R3's interface also facing 123.0.0.X/24 network.

R3(config)#router ospf 1
R3(config-router)#network 123.0.0.3 0.0.0.0 area 0
R3(config-router)#end

*Nov  5 14:27:11.195: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on GigabitEthernet1/0 from LOADING to FULL, Loading Done

Now Let's check our poor fellow R3 again to see if this helped:

R3#sh ip ospf database external

            OSPF Router with ID (3.3.3.3) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 314
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 1.1.1.1 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000002
  Checksum: 0x289D
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 1
        Forward Address: 123.0.0.1
        External Route Tag: 100


R3#sh ip ospf border-routers

            OSPF Router with ID (3.3.3.3) (Process ID 1)


                Base Topology (MTID 0)

Internal Router Routing Table
Codes: i - Intra-area route, I - Inter-area route

i 2.2.2.2 [1] via 123.0.0.2, GigabitEthernet1/0, ASBR, Area 0, SPF 5



R3#traceroute 1.1.1.1 source loopback 0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
  1 123.0.0.1 64 msec 80 msec 108 msec

R3#sh ip route ospf | begin ^G
Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets
O E2     1.1.1.1 [110/1] via 123.0.0.1, 00:01:50, GigabitEthernet1/0
      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 123.0.0.2, 00:01:50, GigabitEthernet1/0
      4.0.0.0/32 is subnetted, 1 subnets
O        4.4.4.4 [110/2] via 34.0.0.4, 00:17:15, GigabitEthernet2/0
      24.0.0.0/24 is subnetted, 1 subnets
O        24.0.0.0 [110/65] via 123.0.0.2, 00:01:50, GigabitEthernet1/0
                  [110/65] via 34.0.0.4, 00:17:15, GigabitEthernet2/0

Now it looks much better.

But anything changed from R4 prospective ? ... Remember that Poor Man's T1 Link ? :)

R4#traceroute 1.1.1.1 source loopback 0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
  1 34.0.0.3 52 msec 76 msec 48 msec
  2 123.0.0.1 120 msec 124 msec 168 msec

Now wait till 2nd Part to unfold some more details and Caveats of FA.

HTH...
Deepak Arora
Evil CCIE

In case you want to lab it up yourself, below are the initials:

Initials:

R1
++

!
en
!
conf t
!
ho R1
!
no ip do lo
!
line con 0
 no exec-time
 exit
!
int loop0
 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 bgp 100
 no auto
 no sync
 nei 123.0.0.2 remote 200
 net 1.1.1.1 mask 255.255.255.255
 exit
!
end
!

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

R2
++

!
en
!
conf t
!
ho R2
!
no ip do lo
!
line con 0
 no exec-time
 exit
!
int loop0
 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 s2/0
 ip add 24.0.0.2 255.255.255.0
 no sh
 exit
!
router bgp 200
 no auto
 no sync
 nei 123.0.0.1 remote 100
 redistribute ospf 1 match internal external 1 external 2
 exit
!
router ospf 1
 net 2.2.2.2 0.0.0.0 area 0
 net 24.0.0.2 0.0.0.0 area 0
 redistribute bgp 200 subnets
 exit
!
end
!

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

R3
++

!
en
!
conf t
!
ho R3
!
no ip do lo
!
line con 0
 no exec-time
 exit
!
int loop0
 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 gi2/0
 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 34.0.0.3 0.0.0.0 area 0
 exit
!
end
!

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

R4
++

!
en
!
conf t
!
ho R4
!
no ip do lo
!
line con 0
 no exec-time
 exit
!
int loop0
 ip add 4.4.4.4 255.255.255.255
 exit
!
int s2/0
 ip add 24.0.0.4 255.255.255.0
 no sh
 exit
!
int gi1/0
 ip add 34.0.0.4 255.255.255.0
 no sh
 exit
!
router ospf 1
 net 4.4.4.4 0.0.0.0 area 0
 net 34.0.0.4 0.0.0.0 area 0
 net 24.0.0.4 0.0.0.0 area 0
 exit
!
end
!