NETWORK LESSON

OSPF Stub Area

Published by

Recto Orpia

on


In this lecture notes, I will be going through another important topics in OSPF which is Special Areas. We know we have Backbone Areas which is the default Area. Backbone Area is required for other Area routers to communicate to one another. Now, Let’s discuss more OSPF Special Areas including Stub Area, Totally Stub Area, NSSA, and Totally NSSA. My main focus for this post will be STUB AREA.

 Concepts:

 

 

Laboratory Objective:

  1. Redistributed the Loopback Interfaces on MANILA router so as they will become a Type 5 LSA.
     Verify this from the LSDB of SINGAPORE router.
 2. Summarized the 4 prefixes from MANILA router so as to received a single entry LSDB entry for
      the 10.x.x.x network  on SINGAPORE   router.
3.   To set up SINGAPORE Router to have the LSDB LINK ID of  0.0.0.0 as advertised by 
     MELBOURNE router. This can be accomplished by configuring the stub routers within the Stub
      Area. This can be verified when the default route is pointed to MELBOURNE router.

 4. To proved that Stub routers are not accepting Type 5 External LSA.
 5.  To verify that ASBR Summary LSA or Type 4 LSA are not present on Stub routers.
 

 Here is my topology for my laboratory:



Here are my router configurations:

 

 MANILA Router: 
 R1#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#hostname MANILA
MANILA(config)#int s2/0
MANILA(config-if)#ip address 192.168.12.1 255.255.255.0
MANILA(config-if)#no shut
MANILA(config-if)#exit
MANILA(config)#int lo0
MANILA(config-if)#ip add 10.10.10.1 255.255.255.0
MANILA(config-if)#exit
MANILA(config)#int lo1
MANILA(config-if)#ip add 10.10.11.1 255.255.255.0
MANILA(config-if)#exit
MANILA(config)#int lo2
MANILA(config-if)#ip address 10.10.12.1 255.255.255.0
MANILA(config-if)#exit
MANILA(config)#int lo3
MANILA(config-if)#ip add 10.10.13.1 255.255.255.0
MANILA(config-if)#exit

MANILA(config)#router ospf 1
MANILA(config-router)#network 192.168.12.0 0.0.0.255 area 0
MANILA(config-router)#redistribute connected subnets
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 s2/0
 MELBOURNE(config-if)#ip address 192.168.12.2 255.255.255.0 
MELBOURNE(config-if)#no shut 
MELBOURNE(config-if)#exit
MELBOURNE(config)#int s2/1
MELBOURNE(config-if)#ip add 192.168.23.2 255.255.255.0
 MELBOURNE(config-if)#exit
 MELBOURNE(config)#int lo2 
MELBOURNE(config-if)#ip add 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 0
 MELBOURNE(config-router)#network 20.20.20.0 0.0.0.255 area 0 
MELBOURNE(config-router)#network 192.168.23.0 0.0.0.255 area 1
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 s2/1
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)#router ospf 1
SINGAPORE(config-router)#network 30.30.30.0 0.0.0.255 area 1
SINGAPORE(config-router)#network 192.168.23.0 0.0.0.255 area 1

Verifications:


A. What I wanted to see is the LSDB of SINGAPORE router. What we can see below are the 4 prefixes which we have redistributed into OSPF.


SINGAPORE#show ip ospf database | begin Type-5

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.10.10.0      10.10.13.1      75          0x80000001 0x0072ED 0
10.10.11.0      10.10.13.1      75          0x80000001 0x0067F7 0
10.10.12.0      10.10.13.1      75          0x80000001 0x005C02 0
10.10.13.0      10.10.13.1      75          0x80000001 0x00510C 0


I should also be seeing them as OSPF external type 2 routes ( LSA Type 5 routes).

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

      10.0.0.0/24 is subnetted, 4 subnets
O E2     10.10.10.0 [110/20] via 192.168.23.2, 00:01:26, Serial2/1
O E2     10.10.11.0 [110/20] via 192.168.23.2, 00:01:26, Serial2/1
O E2     10.10.12.0 [110/20] via 192.168.23.2, 00:01:26, Serial2/1
O E2     10.10.13.0 [110/20] via 192.168.23.2, 00:01:26, Serial2/1
      20.0.0.0/32 is subnetted, 1 subnets
O IA     20.20.20.1 [110/65] via 192.168.23.2, 00:01:26, Serial2/1
O IA  192.168.12.0/24 [110/128] via 192.168.23.2, 00:01:26, Serial2/1
SINGAPORE#

 Now, lets apply what we have learned previously in summarizing Type 5 LSA routes by applying the command “summary-address”. I will do the summarization in ASBR or from the MANILA router.


MANILA(config)#router ospf 1
MANILA(config-router)#summary-address 10.0.0.0 255.0.0.0   <<< This is how we summarized routes that were redistributed into OSPF from the ASBR.
MANILA(config-router)#^Z
MANILA#

Let’s take a look on the SINGAPORE router LSDB. As noticed below, I can see both the Type 4 Summary ASBR LSA and the Type 5 AS External LSA. But as seen instead of 4 entries of LSA, we have managed to reduce into just single entry by applying the summarization of Type 5 LSA’s.
SINGAPORE#show ip ospf database

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

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
20.20.20.1      20.20.20.1      416         0x8000000D 0x00229C 2
30.30.30.1      30.30.30.1      411         0x80000008 0x006BCD 3

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
20.20.20.1      20.20.20.1      439         0x80000005 0x00C6F6
192.168.12.0    20.20.20.1      439         0x80000005 0x00E560

                Summary ASB Link States (Area 1)
Link ID         ADV Router      Age         Seq#       Checksum
10.10.13.1      20.20.20.1      439         0x80000001 0x00811B
                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.0.0.0        10.10.13.1      80          0x80000001 0x00591B 0
SINGAPORE#
 

If you wanted to see what is MELBOURNE router’s LSA, here it is. Interesting right, since both MELBOURNE & SINGAPORE routers have the Sequence Number and I’m pretty sure that they have the same LSDB’s.
 MELBOURNE#show ip ospf database | begin Summary ASB
 
                Summary ASB Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.13.1      20.20.20.1      632         0x80000001 0x00811B

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.0.0.0        10.10.13.1      271         0x80000001 0x00591B 0
MELBOURNE#

 But my main objective for this lecture notes is to observed the default route on SINGAPORE router. Take note we NEVER set up the “defaul-information originate always” on MANILA router. This is not the goal of this post. Instead we can accomplished this by enabling Stub Area on both MELBOURNE and SINGAPORE which belongs Area 1. 

So what should I expect if I have enabled STUB Area on both routers?  Well, as mentioned above, STUB routers hate to see Type 5 LSA’s  and also the ASBR. But as we know MANILA is our ASBR and the previous LSDB’s are showing Type 5 LSA’s on SINGAPORE router. Let’s modify the configurations of the router members of STUB AREA based on my topology.



MELBOURNE#sh run | sec ospf
router ospf 1
 network 20.20.20.0 0.0.0.255 area 0
 network 192.168.12.0 0.0.0.255 area 0
 network 192.168.23.0 0.0.0.255 area 1
MELBOURNE#config t
Enter configuration commands, one per line.  End with CNTL/Z.
MELBOURNE(config)#router ospf 1
MELBOURNE(config-router)#area 1 stub  <<< This is the COMMAND to enable STUB AREA on the routers.
MELBOURNE(config-router)#
*Jul 28 00:25:17.131: %OSPF-5-ADJCHG: Process 1, Nbr 30.30.30.1 on Serial2/1 from FULL to DOWN, Neighbor Down: Adjacency forced to reset
MELBOURNE(config-router)#
*Jul 28 00:25:42.255: %OSPF-5-ADJCHG: Process 1, Nbr 30.30.30.1 on Serial2/1 from LOADING to FULL, Loading Done
MELBOURNE(config-router)#

<<< As seen from MELBOURNE router, the moment I have added “area 1 stub” under OSPF process, it resets the connections and re-run the SPF and re-sent the LSU with the new LSDB entries.


SINGAPORE#sh run | sec ospf
router ospf 1
 network 30.30.30.0 0.0.0.255 area 1
 network 192.168.23.0 0.0.0.255 area 1
SINGAPORE#config t
Enter configuration commands, one per line.  End with CNTL/Z.
SINGAPORE(config)#router ospf 1
SINGAPORE(config-router)#area 1 stub
SINGAPORE(config-router)#^Z
SINGAPORE#
*Jul 28 00:25:23.123: %OSPF-5-ADJCHG: Process 1, Nbr 20.20.20.1 on Serial2/1 from FULL to DOWN, Neighbor Down: Adjacency forced to reset
*Jul 28 00:25:23.607: %SYS-5-CONFIG_I: Configured from console by console
SINGAPORE#
SINGAPORE#
*Jul 28 00:25:26.215: %OSPF-5-ADJCHG: Process 1, Nbr 20.20.20.1 on Serial2/1 from LOADING to FULL, Loading Done
SINGAPORE#


<<< The same happens in SINGAPORE router. OSPF adjacencies were force to reset and were formed again.
 Now, I’m interested to know what happens to the LSDB of both SINGAPORE & MELBOURNE router, Let’s check them out.


SINGAPORE#show ip ospf database

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

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
20.20.20.1      20.20.20.1      269         0x8000000F 0x003C82 2
30.30.30.1      30.30.30.1      266         0x8000000B 0x0083B4 3

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         20.20.20.1      294         0x80000001 0x00C937
20.20.20.1      20.20.20.1      294         0x80000007 0x00E0DC
192.168.12.0    20.20.20.1      294         0x80000007 0x00FF46
SINGAPORE#

<<< See what happens? The Type 4 LSA’s were removed . This is true as STUB AREA hates to see ASBR which generates this LSA. 
<< What else happens?  The previous Type 5 LSA with the Link ID of  10.0.0.0  ( which is created by the summarizing the 4 prefixes) were converted into a Type 3 Summary LSA which will appear as Inter-Area Routes on the routing table.
 << What else can we see?  The Router advertising the LSA is MELBOURNE router and a new Sequence Number were created for the new LSA entries.
Now, let’s check the LSDB of MELBOURNE router,
MELBOURNE#show ip ospf database

            OSPF Router with ID (20.20.20.1) (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.10.13.1      10.10.13.1      1646        0x80000005 0x00E24E 2
20.20.20.1      20.20.20.1      1623        0x80000008 0x0022A2 3

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
30.30.30.1      20.20.20.1      675         0x80000001 0x00E77B
192.168.23.0    20.20.20.1      1623        0x80000002 0x0072CB

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
20.20.20.1      20.20.20.1      679         0x8000000F 0x003C82 2
30.30.30.1      30.30.30.1      678         0x8000000B 0x0083B4 3

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
0.0.0.0         20.20.20.1      705         0x80000001 0x00C937
20.20.20.1      20.20.20.1      705         0x80000007 0x00E0DC
192.168.12.0    20.20.20.1      705         0x80000007 0x00FF46

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
10.0.0.0        10.10.13.1      1425        0x80000001 0x00591B 0
MELBOURNE#

<< Huh, it seems  I can see Type 5 LSA. This is normal because Area 0 or the Backbone Area could never be a STUB Area. Remember MELBOURNE router has one leg connected on Area 0. But if you notice, this LSA was not allowed to enter the STUB router's LSDB or routing information.
<< But as we can also notice, we have the Type 3 Summary LSA which have a LINK ID of 0.0.0.0 and is being advertise by MELBOURNE router itself. Let's see later which will be installed on MELBOURNE routing table.
<< If you notice, the Sequence number for this LSA is the same with the one we have with SINGAPORE.
That’s enough for the LSDB verifications. I wanted to show now the routes on the routers.
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 192.168.23.2 to network 0.0.0.0

O*IA  0.0.0.0/0 [110/65] via 192.168.23.2, 00:16:19, Serial2/1
      20.0.0.0/32 is subnetted, 1 subnets
O IA     20.20.20.1 [110/65] via 192.168.23.2, 00:16:19, Serial2/1
O IA  192.168.12.0/24 [110/128] via 192.168.23.2, 00:16:19, Serial2/1
 


<<< So we have injected a default route on SINGAPORE router by just configuring the " area 1 stub” under OSPF process on all the member routers of the STUB Area.

For MELBOURNE, we can see that we  have only the external summary routes ( Type 5 LSA).

 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

O E2  10.0.0.0/8 [110/20] via 192.168.12.1, 00:18:55, Serial2/0
      30.0.0.0/32 is subnetted, 1 subnets
O        30.30.30.1 [110/65] via 192.168.23.3, 00:18:25, Serial2/1
MELBOURNE#

 For MANILA, nothing much especial on the routing table. We have the summarized OSPF routes and the Inter-Area Routes from Area 1.

MANILA#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

      10.0.0.0/8 is variably subnetted, 9 subnets, 3 masks
O        10.0.0.0/8 is a summary, 00:32:46, Null0
      20.0.0.0/32 is subnetted, 1 subnets
O        20.20.20.1 [110/65] via 192.168.12.2, 01:08:11, Serial2/0
      30.0.0.0/32 is subnetted, 1 subnets
O IA     30.30.30.1 [110/129] via 192.168.12.2, 00:20:22, Serial2/0
O IA  192.168.23.0/24 [110/128] via 192.168.12.2, 01:06:03, Serial2/0
MANILA#



 Verification  for Network connectivity:

 

 My objective is just to ensure after all we have done, network connectivity is working. I will just do a simple ping and traceroutes from MANILA router sourcing its Loopback interface to all of the Loopback Interfaces of MANILA router.

 

 SINGAPORE#ping 10.10.10.1 source 30.30.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:
Packet sent with a source address of 30.30.30.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/60/84 ms
SINGAPORE#
SINGAPORE#
SINGAPORE#ping 10.10.11.1 source 30.30.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.11.1, timeout is 2 seconds:
Packet sent with a source address of 30.30.30.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/56/60 ms
SINGAPORE#
SINGAPORE#
SINGAPORE#ping 10.10.12.1 source 30.30.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.12.1, timeout is 2 seconds:
Packet sent with a source address of 30.30.30.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 44/50/56 ms
SINGAPORE#
SINGAPORE#ping 10.10.13.1 source 30.30.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.13.1, timeout is 2 seconds:
Packet sent with a source address of 30.30.30.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/62/92 ms
SINGAPORE#
SINGAPORE#
SINGAPORE#tra
SINGAPORE#traceroute 10.10.10.1
Type escape sequence to abort.
Tracing the route to 10.10.10.1
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.23.2 56 msec 56 msec 56 msec
  2 192.168.12.1 72 msec 60 msec 56 msec
SINGAPORE#
SINGAPORE#
SINGAPORE#traceroute 10.10.11.1
Type escape sequence to abort.
Tracing the route to 10.10.11.1
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.23.2 60 msec 52 msec 52 msec
  2 192.168.12.1 80 msec 80 msec 80 msec
SINGAPORE#
SINGAPORE#
SINGAPORE#traceroute 10.10.12.1
Type escape sequence to abort.
Tracing the route to 10.10.12.1
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.23.2 56 msec 36 msec 56 msec
  2 192.168.12.1 48 msec 56 msec 32 msec
SINGAPORE#
SINGAPORE#
SINGAPORE#traceroute 10.10.13.1
Type escape sequence to abort.
Tracing the route to 10.10.13.1
VRF info: (vrf in name/id, vrf out name/id)
  1 192.168.23.2 56 msec 52 msec 52 msec
  2 192.168.12.1 52 msec 56 msec 52 msec
SINGAPORE#

This ends my laboratory for this section of the Special OSPF Area called STUB Area.

Leave a comment

Previous Post
Next Post

Recent posts

Quote of the week

"People ask me what I do in the winter when there's no baseball. I'll tell you what I do. I stare out the window and wait for spring."

~ Rogers Hornsby

NETWORK LESSON

About

Passionate about simplifying the complex world of networking and automation for learners everywhere.”.

Topics

Follow Us

Create a website or blog at WordPress.com