OSPF LSA Type 7 ( Not-So-Stubby-Area LSA)

This post is another notes which I am compiling for the final series of OSPF LSA types which is the NSSA LSA type or LSA TYPE 7 . It was hard for me before to comprehend the OSPF LSA Types but when I do it in laboratory and verify the results, learning is more fun and interesting.

Laboratory Objective:

1. To set up a simple scenarios for NSSA topology.
2. Understand the contents of the Link State Database (LSDB) and verify the various LSA types and for this post we focus on the LSA TYPE 7.
3. To understand that the OSPF routers in a Not-So-Stubby-Area will store a copy of the LSA TYPE 7 on its LSDB but the routers on other areas will not see LSA TYPE 7 but rather the Area Border Router (ABR) on the Not-So-Stubby-Area will convert it into a Type 5 LSA and keep it to it’s LSDB and send a Link State Update (LSU) to the rest of the OSPF routers within the Area and other Areas.
3. Verify the connectivity of the whole network.

So again here is my topology. I have three OSPF Areas, Area 1, the Backbone Area (Area 0) and Area 2. I have set up Area 2 as the NSSA.

Here are the configurations of my routers:

MANILA ROUTER:

R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#hostname MANILA
MANILA(config)#int fa0/0
MANILA(config-if)#ip add 192.168.12.1 255.255.255.0
MANILA(config-if)#no shut
MANILA(config-if)#
MANILA(config-if)#exit
MANILA(config)#int lo1
MANILA(config-if)#ip add 10.10.10.10.1 255.255.255.0
MANILA(config-if)#exit

MANILA(config)#router ospf 1
MANILA(config-router)#network 192.168.12.0 0.0.0.255 area 1
MANILA(config-router)#network 10.10.10.0 0.0.0.255 area 1
MANILA(config-router)#^Z

MELBOURNE ROUTER:

R2#config t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#hostname MELBOURNE
MELBOURNE(config)#int fa0/0
MELBOURNE(config-if)#ip add 192.168.12.2 255.255.255.0
MELBOURNE(config-if)#no shut
MELBOURNE(config-if)#exit

MELBOURNE(config)#int s3/0
MELBOURNE(config-if)#ip add 192.168.23.2 255.255.255.0
MELBOURNE(config-if)#no shut

MELBOURNE(config-if)#exit

MELBOURNE(config)#int lo2
MELBOURNE(config-if)#ip address 20.20.20.1 255.255.255.0
MELBOURNE(config-if)#exit

MELBOURNE(config)#router ospf 1
MELBOURNE(config-router)#network 192.168.12.0 0.0.0.255 area 1
MELBOURNE(config-router)#network 192.168.23.0 0.0.0.255 area 0
MELBOURNE(config-router)#network 20.20.20.0 0.0.0.255 area 0
MELBOURNE(config-router)#^Z

SINGAPORE ROUTER:

R3#config t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#hostname SINGAPORE
SINGAPORE(config)#int s3/0
SINGAPORE(config-if)#ip add 192.168.23.3 255.255.255.0
SINGAPORE(config-if)#no shut
SINGAPORE(config-if)#exit
SINGAPORE(config)#int lo3
SINGAPORE(config-if)#ip add 30.30.30.1 255.255.255.0
SINGAPORE(config-if)#exit
SINGAPORE(config)#int s3/1
SINGAPORE(config-if)#ip add 192.168.34.3 255.255.255.0
SINGAPORE(config-if)#no shut
SINGAPORE(config-if)#exit

SINGAPORE(config)#router ospf 1
SINGAPORE(config-router)#network 192.168.23.0 0.0.0.255 area 0
SINGAPORE(config-router)#network 30.30.30.0 0.0.0.255 area 0
SINGAPORE(config-router)#network 192.168.34.0 0.0.0.255 area 2
SINGAPORE(config-router)#area 2 nssa <<<< I am configuring SINGAPORE Router as an NSSA router
SINGAPORE(config-router)#^Z

HONGKONG ROUTER:

HONGKONG#config t
Enter configuration commands, one per line. End with CNTL/Z.
HONGKONG(config)#hostname HONGKONG
HONGKONG(config)#int lo4
HONGKONG(config-if)#ip add 40.40.40.1 255.255.255.0
HONGKONG(config-if)#exit
HONGKONG(config)#int s3/1
HONGKONG(config-if)#ip add 192.168.34.4 255.255.255.0
HONGKONG(config-if)#no shut
HONGKONG(config-if)#exit

HONGKONG(config)#router ospf 1
HONGKONG(config-router)#network 192.168.34.0 0.0.0.255 area 2
HONGKONG(config-router)#area 2 nssa <<<< I'm configuring HONGKONG router as NSSA router.
HONGKONG(config-router)#redistributed connected subnets <<< I am redistributing the LOOPBACK interface of this router.
HONGKONG(config-router)#
HONGKONG(config-router)#^Z

VERIFICATIONS:

A. Now that I have the above configurations, let’s check first whether adjacency were formed. Actually when I’m doing my configurations, the moment that I have enabled the OSPF Process and add the OSPF network interface IP subnets, the OSPF nieghbors were already established. For the sake of verifications, let’s check it once again.

MANILA#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
20.20.20.1 1 FULL/DR 00:00:39 192.168.12.2 FastEthernet0/0

MELBOURNE#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
30.30.30.1        0   FULL/ –        00:00:31    192.168.23.3    Serial3/0
10.10.10.1        1   FULL/BDR        00:00:31    192.168.12.1    FastEthernet0/0

>> As we can notice above, MELBOURNE router (20.20.20.1) were elected as DR because it has the highest active IP address as compared to the other OSPF router (MANILA) on the same multi-access network in AREA 1.

>> Remember the default priority is 1 and I’d never modify it so it has to look for the router with the highest router ID to be the DR. Consequently, we have MANILA router as the BDR.

>> Another Observations on MELBOURNE Router, the adjacency were established to SINGAPORE router with the state of FULL as it is a Serial link (point-to-point).

SINGAPORE#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
20.20.20.1        0   FULL/ –        00:00:36    192.168.23.2    Serial3/0
40.40.40.1        0   FULL/ –        00:00:35    192.168.34.4    Serial3/1

HONGKONG#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
30.30.30.1 0 FULL/ – 00:00:33 192.168.34.3 Serial3/1

B. Now, let’s check the most important parts of this post which is verifications of the LSDB among the 4 routers.

Let’s start with HONGKONG Router.

HONGKONG#show ip ospf database

OSPF Router with ID (40.40.40.1) (Process ID 1)

Router Link States (Area 2)

Link ID ADV Router Age Seq# Checksum Link count

30.30.30.1 30.30.30.1 1411 0x80000007 0x00F555 2

40.40.40.1 40.40.40.1 1281 0x80000007 0x00B577 2

Summary Net Link States (Area 2)

Link ID ADV Router Age Seq# Checksum

10.10.10.1 30.30.30.1 1411 0x80000003 0x007503

20.20.20.1 30.30.30.1 1411 0x80000003 0x000259

30.30.30.1 30.30.30.1 1411 0x80000003 0x001667

192.168.12.0 30.30.30.1 1411 0x80000003 0x00A87A

192.168.23.0 30.30.30.1 1411 0x80000003 0x0025F3

Type-7 AS External Link States (Area 2)

Link ID ADV Router Age Seq# Checksum Tag

40.40.40.0 40.40.40.1 1281 0x80000002 0x00060E 0

>>> As seen above, we have a TYPE-7 LSA with the LINK ID of the network that is redistributed into OSPF and the Advertising router (40.40.40.1).

SINGAPORE#show ip ospf database

            OSPF Router with ID (30.30.30.1) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
20.20.20.1      20.20.20.1      521         0x80000009 0x00165E 3
30.30.30.1      30.30.30.1      147         0x80000006 0x0078BF 3

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.1      20.20.20.1      521         0x80000003 0x003E9E
192.168.12.0    20.20.20.1      773         0x80000003 0x007116
192.168.34.0    30.30.30.1      389         0x80000003 0x00060E

                Router Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum Link count
30.30.30.1      30.30.30.1      1918        0x80000007 0x00F555 2
40.40.40.1      40.40.40.1      1791        0x80000007 0x00B577 2

                Summary Net Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.1      30.30.30.1      1918        0x80000003 0x007503
20.20.20.1      30.30.30.1      1918        0x80000003 0x000259
30.30.30.1      30.30.30.1      1918        0x80000003 0x001667
192.168.12.0    30.30.30.1      1918        0x80000003 0x00A87A
192.168.23.0    30.30.30.1      1918        0x80000003 0x0025F3

                Type-7 AS External Link States (Area 2)

Link ID         ADV Router      Age         Seq#       Checksum Tag
40.40.40.0      40.40.40.1      1791        0x80000002 0x00060E 0

                Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
40.40.40.0 30.30.30.1 1918 0x80000002 0x008BB0 0

>>> Now, what we can see is a TYPE 7 External LSA ( NSSA) and Type 5 External LSA but it has the same Link ID. So what does this means? As we know that the SINGAPORE is the ABR (Area Border Router) between Area 0 and Area 2 and at the same time, it’s a NSSA Router. So what it does is to convert the TYPE 7 LSA ( NSSA LSA) into a TYPE 5 LSA ( AS External LSA) and it does stored into it’s LSDB and only the converted TYPE 5 LSA will be propagated to the rest of the routers in different areas.

>> We can also notice that the new Type 5 AS External LSA is now advertised by SINGAPORE (30.30.30.1) router. We should expect that this same LSA TYPE 5 is propagated to both the MELBOURNE & MANILA router.

Let’s check the LSDB of MELBOURNE

MELBOURNE#show ip ospf database | begin Type-5

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
40.40.40.0 30.30.30.1 848 0x80000004 0x0087B2 0

>> As observed above, we can see that the TYPE 5 AS External LSA sent by SINGAPORE is on MELBOURNE LSDB.

But for curiosity sake, does MELBOURNE receiving the TYPE 7 LSA?

MELBOURNE#show ip ospf database | begin Type-7

>> There was no LSA TYPE 7 that were propagated to other routers as this was already been converted by SINGAPORE being the ABR to the NSSA network.

And finally, what does the MANILA router LSDB tell us?

MANILA#show ip ospf database

            OSPF Router with ID (10.10.10.1) (Process ID 1)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.10.10.1      10.10.10.1      1737        0x80000005 0x0003CF 2
20.20.20.1      20.20.20.1      1746        0x80000005 0x0016AE 1

                Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.12.2    20.20.20.1      1746        0x80000004 0x00CA5B

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
20.20.20.1      20.20.20.1      1987        0x80000004 0x00C8F5
30.30.30.1      20.20.20.1      1746        0x80000004 0x00E17E
192.168.23.0    20.20.20.1      1746        0x80000004 0x006ECD
192.168.34.0    20.20.20.1      1486        0x80000004 0x007779

                Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
30.30.30.1      20.20.20.1      1486        0x80000004 0x00C996

                Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag
40.40.40.0 30.30.30.1 1129 0x80000004 0x0087B2 0
MANILA#

>>> As we can see from the results above, we have seen LSA TYPE 4 (Summary ASBR LSA) that provides details how we can reach the ASBR (30.30.30.1 or the SINGAPORE router) which is via MELBOURNE Router.

Summary ASB Link States (Area 1)

Link ID ADV Router Age Seq# Checksum
30.30.30.1 20.20.20.1 1486 0x80000004 0x00C996

>>> We can also see the TYPE 7 LSA providing the details such as who is advertising the subnets and the LINK ID information.

C. Now, let’s have a look into the routing table.

I want to focus on SINGAPORE Router being the ABR routing informations.

SINGAPORE#show ip route ospf
Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
       E1 – OSPF external type 1, E2 – OSPF external type 2
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      10.0.0.0/32 is subnetted, 1 subnets
O IA     10.10.10.1 [110/66] via 192.168.23.2, 02:10:32, Serial3/0
      20.0.0.0/32 is subnetted, 1 subnets
O        20.20.20.1 [110/65] via 192.168.23.2, 02:10:32, Serial3/0
      40.0.0.0/24 is subnetted, 1 subnets
O N2     40.40.40.0 [110/20] via 192.168.34.4, 02:07:05, Serial3/1
O IA 192.168.12.0/24 [110/65] via 192.168.23.2, 02:10:32, Serial3/0

>> As we can see, I have the N2 or NSSA External Type 2 routes for the subnets which I have redistributed from the HONGKONG router. We should notice that we can only see N2 or N1 routes on the ABR (SINGAPORE) only. We should not be able to see this as N2/N1 routes except on the ABR.

Let’s check the HONGKONG routing table,

HONGKONG#show ip route ospf

Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
       E1 – OSPF external type 1, E2 – OSPF external type 2
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      10.0.0.0/32 is subnetted, 1 subnets
O IA     10.10.10.1 [110/130] via 192.168.34.3, 02:21:51, Serial3/1
      20.0.0.0/32 is subnetted, 1 subnets
O IA     20.20.20.1 [110/129] via 192.168.34.3, 02:21:51, Serial3/1
      30.0.0.0/32 is subnetted, 1 subnets
O IA     30.30.30.1 [110/65] via 192.168.34.3, 02:21:51, Serial3/1
O IA 192.168.12.0/24 [110/129] via 192.168.34.3, 02:21:51, Serial3/1
O IA 192.168.23.0/24 [110/128] via 192.168.34.3, 02:21:51, Serial3/1

>>> As seen above, we only have the Inter-Area Routes ( or the Type 3 LSA or the Summary LSA) which is generated by SINGAPORE router. This routes were all from Area 1 and Area 0.

NOW, Let’s see the MELBOURNE routing information,

MELBOURNE#show ip route ospf
Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
       E1 – OSPF external type 1, E2 – OSPF external type 2
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      10.0.0.0/32 is subnetted, 1 subnets
O        10.10.10.1 [110/2] via 192.168.12.1, 02:17:53, FastEthernet0/0
      30.0.0.0/32 is subnetted, 1 subnets
O        30.30.30.1 [110/65] via 192.168.23.3, 02:15:42, Serial3/0
      40.0.0.0/24 is subnetted, 1 subnets
O E2     40.40.40.0 [110/20] via 192.168.23.3, 02:04:47, Serial3/0
O IA 192.168.34.0/24 [110/128] via 192.168.23.3, 02:13:47, Serial3/0

>> As seen, I have the Inter-Area Routes coming from OSPF AREA 2 via SINGAPORE router.

>> Similarly, I can also see the TYPE 5 LSA routes which is advertised by SINGAPORE also.

>> We also have the normal OSPF routes for the loopback interfaces of SINGAPORE and MANILA router.

FINALLY, let’s check the MANILA ROUTER routing information,

MANILA#sh ip route ospf

Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP

D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area

N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2

E1 – OSPF external type 1, E2 – OSPF external type 2

i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2

ia – IS-IS inter area, * – candidate default, U – per-user static route

o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP

+ – replicated route, % – next hop override

Gateway of last resort is not set

20.0.0.0/32 is subnetted, 1 subnets

O IA 20.20.20.1 [110/2] via 192.168.12.2, 02:52:46, FastEthernet0/0

30.0.0.0/32 is subnetted, 1 subnets

O IA 30.30.30.1 [110/66] via 192.168.12.2, 02:50:25, FastEthernet0/0

40.0.0.0/24 is subnetted, 1 subnets

O E2 40.40.40.0 [110/20] via 192.168.12.2, 02:39:30, FastEthernet0/0

O IA 192.168.23.0/24 [110/65] via 192.168.12.2, 02:51:25, FastEthernet0/0

O IA 192.168.34.0/24 [110/129] via 192.168.12.2, 02:48:30, FastEthernet0/0

>>> As observed, we have the 40.40.40.0 routes coming from HONGKONG router advertise as TYPE 5 LSA (AS External LSA) by SINGAPORE router ( ABR).

D. Let’s test the connectivity.

I just need to do a ping between the Loopback interface of HONGKONG router from the MANILA router. As seeb below, connectivity is working fine.

MANILA#ping 40.40.40.1 source 10.10.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 40.40.40.1, timeout is 2 seconds:
Packet sent with a source address of 10.10.10.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 108/158/236 ms

MANILA#traceroute 40.40.40.1
Type escape sequence to abort.
Tracing the route to 40.40.40.1
VRF info: (vrf in name/id, vrf out name/id)
1 192.168.12.2 120 msec 100 msec 60 msec
2 192.168.23.3 92 msec 136 msec 72 msec
3 192.168.34.4 168 msec 136 msec 160 msec
MANILA#

NETWORK LESSON