Daniels networking blog

In OSPF and other routing protocols we have something called forwarding address.
This can be used to route traffic in another direction than to the router that
originated the LSA. We start with the following topology.

It’s a basic OSPF setup where area 1 is a NSSA area. As you can see we have
two ABRs. Remember that in NSSA area, redistributed routes will be seen as N
internally but as E outside the area. To make this happen the ABR must translate
the type 7 LSA to type 5 LSA. If we have multiple ABRs, which one is responsible
for this task? The ABR with the highest RID will do the translation.

If we look at the LSA at R1, this is what it looks like.

R1#sh ip ospf data ex 10.10.4.0

            OSPF Router with ID (10.10.13.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 1373
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.4.0 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x7306
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 10.10.234.4
        External Route Tag: 0

So R3 is the ABR doing the translation but the forward address is set to
10.10.234.4 which is the address of R4. This means that traffic doesn’t need
to pass through R3 to reach the R4 network. The router will lookup the
10.10.234.0/24 prefix and use the routing information to reach the
10.10.4.0 network. This is proven by a traceroute.

R1#traceroute 10.10.4.4

Type escape sequence to abort.
Tracing the route to 10.10.4.4

  1 10.10.12.2 44 msec 44 msec 20 msec
  2 10.10.234.4 60 msec *  72 msec

What happens if the forwarding address network is not advertised? We will
do some filtering on R2.

R2(config-router)#area 1 range 10.10.234.0 255.255.255.0 not-advertise
R3(config-router)#area 1 range 10.10.234.0 255.255.255.0 not-advertise

R1#sh ip route 10.10.4.0
% Subnet not in table

There is no reachability for the network any longer? How can we resolve
this without removing the filtering?

We can tell R3 to suppress the FA in the LSA.

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

The network is back and we have reachability but now traffic must pass
through R3 since the FA is not set.

R1#sh ip route 10.10.4.0
Routing entry for 10.10.4.0/24
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.10.12.2 on FastEthernet0/0, 00:00:07 ago
  Routing Descriptor Blocks:
  * 10.10.12.2, from 3.3.3.3, 00:00:07 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1

R1#traceroute 10.10.4.4

Type escape sequence to abort.
Tracing the route to 10.10.4.4

  1 10.10.12.2 52 msec 76 msec 48 msec
  2 10.10.23.3 36 msec 48 msec 40 msec
  3 10.10.234.4 72 msec *  72 msec

So by setting the FA we achieve more effecient routing. The reason to have
a forwarding address is to reduce the number of LSAs needed. If all ABRs were
doing type 7 to type 5 translation then there would be more LSAs than what is
optimal.

Lets take a look at the LSA now. Note that the FA will be set to 0.0.0.0.

R1#sh ip ospf data ex 10.10.4.0

            OSPF Router with ID (10.10.13.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 212
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.4.0 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000003
  Checksum: 0x6218
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

By default the FA is always set when using NSSA areas. Now we take a look
at another use case where we have another routing protocol involved and
redistribution is done between the routing domains.

This is our example topology. Very similar to before. We just changed from
OSPF to RIP on the lefthand side.

R3 will be the router doing mutual redistribution between RIP and OSPF.
We will see that the FA will be set to 0.0.0.0. We check the route on R1.

R1#sh ip route 10.10.4.0
Routing entry for 10.10.4.0/24
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.10.12.2 on FastEthernet0/0, 00:01:07 ago
  Routing Descriptor Blocks:
  * 10.10.12.2, from 3.3.3.3, 00:01:07 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1

R1#sh ip ospf data ex 10.10.4.0

            OSPF Router with ID (10.10.13.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 79
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.4.0 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000001
  Checksum: 0x6616
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

As expected the FA is set to 0.0.0.0. This means that traffic must traverse
R3. We confirm with a traceroute.

R1#traceroute 10.10.4.4

Type escape sequence to abort.
Tracing the route to 10.10.4.4

  1 10.10.12.2 64 msec 28 msec 24 msec
  2 10.10.23.3 68 msec 40 msec 40 msec
  3 10.10.234.4 96 msec *  76 msec

Now what happens if we enable OSPF on R3 interface towards R4?

R3(config-if)#ip ospf 1 area 0

R1#traceroute 10.10.4.4

Type escape sequence to abort.
Tracing the route to 10.10.4.4

  1 10.10.12.2 56 msec 32 msec 24 msec
  2 10.10.234.4 60 msec *  72 msec

Traceroute is now takinig the shorter path. How did this happen? Take a
look at the LSA on R1.

R1#sh ip ospf data ex 10.10.4.0

            OSPF Router with ID (10.10.13.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 59
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.4.0 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0x7107
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 10.10.234.4
        External Route Tag: 0

The FA has now been set. How did this happen? The FA will be set for
external routes if we meet the following conditions.

• 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.

 

So we have met all the conditions needed to set the FA. I hope that
you know have a better understanding of the forwarding address and
as usual always poste questions/feedback in the comments field.