LAB: OSPF Forwarding Address Supression

- Modify R5’s NSSA configuration so that devices outside of area 3 maintain connectivity to SW4’s redistributed Loopback 10.10.10.10/32.

for External routes in OSPF database, the Forward Address field determines who the next recursive lookup should be performed towards. for Type-5 external LSAs, such as RIP routes  R4 redistributes into OSPF, the forward address is normally set to 0.0.0.0. it means the next router lookup should be performed towards the Advertising router.

Examining the  normal Type-5 External LSA lookup, the output below shows the forward Address is 0.0.0.0 and the advertising router is 150.1.4.4

Rack1R1#show ip ospf database external 30.0.0.0

            OSPF Router with ID (150.1.1.1) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 2 (DoNotAge)
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 30.0.0.0 (External Network Number )
  Advertising Router: 150.1.4.4
  LS Seq Number: 80000001
  Checksum: 0xD80A
  Length: 36
  Network Mask: /16
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

R1 sees that the DR for VLAN 146 is R6 155.1.146.1

Rack1R1#show ip ospf database router 150.1.1.1 self-originate  | begi Area 1
                Router Link States (Area 1)

  LS age: 294
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 150.1.1.1
  Advertising Router: 150.1.1.1
  LS Seq Number: 80000002
  Checksum: 0xADEA
  Length: 36
  Area Border Router
  Number of Links: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 155.1.146.6
     (Link Data) Router Interface address: 155.1.146.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

Rack1R1#show ip ospf database network 155.1.146.6

            OSPF Router with ID (150.1.1.1) (Process ID 1)

                Net Link States (Area 1)

  Routing Bit Set on this LSA
  LS age: 406
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 155.1.146.6 (address of Designated Router)
  Advertising Router: 150.1.6.6
  LS Seq Number: 80000001
  Checksum: 0x324E
  Length: 36
  Network Mask: /24
        Attached Router: 150.1.6.6
        Attached Router: 150.1.1.1
        Attached Router: 150.1.4.4

the output below shows that the 30.0.0.1 route is reachable via R4 with metric of 20 (type-2 external) and forwarding  metric of 1.

Rack1R1#show ip route 30.0.0.1
Routing entry for 30.0.0.0/16
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 1
  Last update from 155.1.146.4 on FastEthernet0/0, 00:05:41 ago
  Routing Descriptor Blocks:
  * 155.1.146.4, from 150.1.4.4, 00:05:41 ago, via FastEthernet0/0
      Route metric is 20, traffic share count is 1


Examining the Type-5 External LSA that was translated from a type-7 NSSA External LSA.

in this scenario SW4 redistribute routes 10.10.10.10/32 into area 3 as a type-7 NSSA External LSA and R5 translate it into a Type-5 External LSA as it moves to area 0.


the output below shows a non-zero forwarding address, so the next recursive lookup is performed towards 150.10.10.10 , instead of the advertising router 150.1.5.5.

Rack1R3#show ip ospf database external 10.10.10.10

            OSPF Router with ID (150.1.3.3) (Process ID 1)

                Type-5 AS External Link States

  LS age: 547
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.10.10 (External Network Number )
  Advertising Router: 150.1.5.5
  LS Seq Number: 80000001
  Checksum: 0x1912
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 150.1.10.10
        External Route Tag: 0

Rack1R3#show ip route 150.1.10.10
% Subnet not in table

the problem here is that 150.1.10.0 LSAs was filtered out at ABR R5. so the recursion towards the forwarding address fails and it cannot be installed in the routing table.

the solution here is the supression of forwarding address in translated type-5 LSAs. which instructs ABRs to not preserve the value in the forward address field as a type-7 NSSA External LSA is traslated into a type-5 Extenal LSA.

Rack1R5(config)#router ospf 1
Rack1R5(config-router)#area 3 nssa no-redistribution no-summary translate type7 suppress-fa

verification: the result ow is that R3 now sees the forward address to be 0.0.0.0 which means a lookup must be performed on the advertising router 150.1.5.5

Rack1R3#show ip ospf database external 10.10.10.10            

            OSPF Router with ID (150.1.30.30) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 24
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 10.10.10.10 (External Network Number )
  Advertising Router: 150.1.5.5
  LS Seq Number: 80000002
  Checksum: 0x8352
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

Rack1R3#show ip ospf database router 150.1.30.30 self-originate

            OSPF Router with ID (150.1.30.30) (Process ID 1)

                Router Link States (Area 0)

  LS age: 54
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 150.1.30.30
  Advertising Router: 150.1.30.30
  LS Seq Number: 80000004
  Checksum: 0xEEB8
  Length: 60
  Area Border Router
  AS Boundary Router
  Number of Links: 3

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 150.1.5.5
     (Link Data) Router Interface address: 155.1.0.3
      Number of TOS metrics: 0
       TOS 0 Metrics: 64

final result is that the external route is installed with a metric of 20 which derive from SW4 default redistribution metric type-2 , plus the forward metric of 65.

Rack1R3#show ip route 10.10.10.10
Routing entry for 10.10.10.10/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 64
  Last update from 155.1.0.5 on Serial1/0, 00:00:53 ago
  Routing Descriptor Blocks:
  * 155.1.0.5, from 150.1.5.5, 00:00:53 ago, via Serial1/0
      Route metric is 20, traffic share count is 1



Suboptimal routing may be introduced when there are multiple exit points out of the NSSA.

for type7 to type5 translator the ABR with the highest  router-id wins. also the forward address calcution is kept separate which is base in metric.  in short, the control plane advertisement of the route does not need to do the following the traffic forwarding plane, but with the forwarding address supression the traffic will always flow through the type-7 to 5 translator.

in this scenario.  SW3's loopback int 9.9.9.9/32 that was redistributed to area 2.  R5 sees the external LSA 9.9.9.9/32 with a advertising router 150.1.30.30 R3 (type 7 to 5 translator)  and a forward address of  150.1.9.9

Rack1R5#show ip ospf database external 9.9.9.9

            OSPF Router with ID (150.1.5.5) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 112
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 9.9.9.9 (External Network Number )
  Advertising Router: 150.1.30.30
  LS Seq Number: 80000001
  Checksum: 0xE519
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 150.1.9.9
        External Route Tag: 0

the lookup towards 150.1.9.9 results of using R6 exit point in area 2 via R1.

Rack1R5#show ip route 150.1.9.9
Routing entry for 150.1.9.0/24
  Known via "ospf 1", distance 110, metric 68, type inter area
  Last update from 155.1.0.1 on Serial0/0, 00:02:11 ago
  Routing Descriptor Blocks:
  * 155.1.0.1, from 150.1.6.6, 00:02:11 ago, via Serial0/0
      Route metric is 68, traffic share count is 1

verified using traceroute

Rack1R5#traceroute 9.9.9.9

Type escape sequence to abort.
Tracing the route to 9.9.9.9

  1 155.1.0.1 16 msec 60 msec 12 msec
  2 155.1.146.6 56 msec 40 msec 12 msec
  3 155.1.67.7 12 msec 12 msec 8 msec
  4 155.1.79.9 8 msec *  44 msec

now adding the translate type7 supress-fa command

Rack1R3(config)#router ospf 1
Rack1R3(config-router)#area 2 nssa no-summary translate type7 suppress-fa

verification:

Rack1R5#show ip ospf database external 9.9.9.9

            OSPF Router with ID (150.1.5.5) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 112
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 9.9.9.9 (External Network Number )
  Advertising Router: 150.1.30.30
  LS Seq Number: 80000001
  Checksum: 0xE519
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

Rack1R5#traceroute 9.9.9.9

Type escape sequence to abort.
Tracing the route to 9.9.9.9

  1 155.1.0.1 16 msec 60 msec 12 msec
  2 155.1.146.6 56 msec 40 msec 12 msec
  3 155.1.67.7 12 msec 12 msec 8 msec
  4 155.1.79.9 8 msec *  44 msec

Rack1R5#show ip route 9.9.9.9
Routing entry for 9.9.9.9/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 64
  Last update from 155.1.0.3 on Serial0/0, 01:37:23 ago
  Routing Descriptor Blocks:
  * 155.1.0.3, from 150.1.30.30, 01:37:23 ago, via Serial0/0
      Route metric is 20, traffic share count is 1