LAB: OSPF NSSA Type-7 to Type-5 Translator Election

- Create a new Loopback9 interface on SW3 with the IP address 9.9.9.9/32, and redistribute it into OSPF.
- Modify the OSPF domain so that only R3 advertises this route into area 0, but traffic from R5 going to this destination transits R6.
- If R6 is down this traffic should be rerouted to R3.

initial command sets:

SW3:
interface Loopback9
ip address 9.9.9.9 255.255.255.255
!
router ospf 1
redistribute connected subnets


for Normal External routes, only Type-5 is originated by the router performing the redistribution.  When this routes moves between areas each ABR originates a Type-4 ASBR summary LSA advertising their reachability to the ASBR.

Fort Type-7 LSAs, since this information needed to be re-originated at the ABR level as the route moves into area 0.  if there are 2 or more ABR in the NSSA area, only one ABR will be doing the Type-7 to Type-5 translation, but maintains the forward address field. essentially separating the relationship between the routing advertisement and the traffic flow.  This means it doesn't necessarily that the ABR performing the translation will be the gateway or the router to reach that external network in the NSSA area, metric still is the basis for reaching that NSSA external network.

note:  ABR who has the highest router-id wins the election for the type-7 to type-5 translation.

in our example here R5 performs a lookup on the Type-5 LSA for 9.9.9.9 that was translated from Type-7.  at this point R3 has an OSPF RID 150.1.3.3 and R6 150.1.6.6. So R6 won the translator election due to higher RID.  the forward address is set to 150.1.9.9 (SW3). this means that R5 needs to figure out how to route towards 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: 278
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 9.9.9.9 (External Network Number )
  Advertising Router: 150.1.6.6
  LS Seq Number: 80000002
  Checksum: 0x1D11
  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

since 150.1.9.9 doesnt belong to the same area as R5 inter-area lookup is performed on the type-3 summary.  R5 finds 2 ABRs wit the same metric.

Rack1R5#show ip ospf database summary 150.1.9.0

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

                Summary Net Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 53
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 150.1.9.0 (summary Network Number)
  Advertising Router: 150.1.3.3
  LS Seq Number: 80000004
  Checksum: 0x12E6
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 3

  LS age: 2 (DoNotAge)
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 150.1.9.0 (summary Network Number)
  Advertising Router: 150.1.6.6
  LS Seq Number: 80000004
  Checksum: 0xEA08
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 3


from R5, finding the metrics reaching those ABRs

Rack1R5#show ip ospf database router 150.1.5.5 self-originate

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

                Router Link States (Area 0)

  LS age: 233
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 150.1.5.5
  Advertising Router: 150.1.5.5
  LS Seq Number: 80000008
  Checksum: 0xE755
  Length: 108
  Area Border Router
  Number of Links: 7
<output omitted>

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 150.1.1.1
     (Link Data) Router Interface address: 155.1.0.5
      Number of TOS metrics: 0
       TOS 0 Metrics: 64

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 150.1.3.3
     (Link Data) Router Interface address: 155.1.0.5
      Number of TOS metrics: 0
       TOS 0 Metrics: 64

for R6 since it has a VL with R1.

Rack1R5#sho ip ospf database router 150.1.1.1

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

                Router Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 200
  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: 80000009
  Checksum: 0x96DB
  Length: 72
  Area Border Router
  Number of Links: 4

    Link connected to: a Virtual Link
     (Link ID) Neighboring Router ID: 150.1.6.6
     (Link Data) Router Interface address: 155.1.146.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

total metrics via R6 = 3 + 1 + 64 = 68
total metrics vai R3 = 3 + 64 = 67
verification via show ip route command:

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 67
  Last update from 155.1.0.3 on Serial0/0, 00:02:53 ago
  Routing Descriptor Blocks:
  * 155.1.0.3, from 150.1.6.6, 00:02:53 ago, via Serial0/0
      Route metric is 20, traffic share count is 1

to modify the type-7 to type-5 translator  we can increase the RID of R3.

Rack1R3(config-router)#router ospf 1
Rack1R3(config-router)#rout
Rack1R3(config-router)#router-id 150.1.30.30
Reload or use "clear ip ospf process" command, for this to take effect

Rack1R3#clear ip ospf 1 process
Reset OSPF process? [no]: yes

@ R5 after changing the R3 RID. R5 now sees that R3 is the advertising router.

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: 44
  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

although the advertising router has change, the forward address is still the same meaning it will still choose the route with smaller metric which in this case is also R3

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 67
  Last update from 155.1.0.3 on Serial0/0, 00:00:12 ago
  Routing Descriptor Blocks:
  * 155.1.0.3, from 150.1.30.30, 00:00:12 ago, via Serial0/0
      Route metric is 20, traffic share count is 1

only when R3's forward metric is higher than R6 forward metric the path selection will change.  we can test it by increasing the ospf cost of R3's fa0/0 interface.

Rack1R3(config)#int fa0/0
Rack1R3(config-if)#ip ospf cost 1000 

verifying the metric for R3.

Rack1R5#show ip ospf database summary 150.1.9.0 adv-router 150.1.30.30

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

                Summary Net Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 2
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 150.1.9.0 (summary Network Number)
  Advertising Router: 150.1.30.30
  LS Seq Number: 80000002
  Checksum: 0xDFF9
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 1002


resulting route is via R6 which is using the VL link to 155.1.0.1 (R1)

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 68
  Last update from 155.1.0.1 on Serial0/0, 00:00:17 ago
  Routing Descriptor Blocks:
  * 155.1.0.1, from 150.1.30.30, 00:00:17 ago, via Serial0/0
      Route metric is 20, traffic share count is 1

traceroute verification

Rack1R5#traceroute 9.9.9.9

Type escape sequence to abort.
Tracing the route to 9.9.9.9

  1 155.1.0.1 24 msec 76 msec 4 msec
  2 155.1.146.6 16 msec 20 msec 12 msec
  3 155.1.67.7 8 msec 28 msec 12 msec
  4 155.1.79.9 12 msec *  24 msec

Note:  if R6 loses connectivity to Area0, traffic will be re-route to R3.