Information Technology: PART 7: OSPF Stub Areas, Totally Stub Areas

PART 7: OSPF Stub Areas, Totally Stub Areas

Figure 1 - Network Scenario

Well first we changed our configuration based on the network scenario provided up above the only fact is that we yet have not defined the area 1.1.1.1 as a stub network so before we do that lets just take a look out our databases on r2 and r5.

On R5:

vyatta@v5:~$ show ip ospf database
OSPF Router with ID (50.50.50.5)
Router Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum Link count
50.50.50.5 50.50.50.5 553 0x80000003 0x7db6 2
127.2.2.2 127.2.2.2 559 0x80000004 0xff0a 1
Net Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum
50.50.50.2 127.2.2.2 559 0x80000002 0xbe5b
Summary Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum Route
10.10.10.0 127.2.2.2 243 0x80000002 0xbeef 10.10.10.0/24
127.1.1.1 127.2.2.2 423 0x80000002 0xf14e 127.1.1.1/32
127.3.3.3 127.2.2.2 263 0x80000002 0xaf8a 127.3.3.3/32
ASBR-Summary Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum
127.3.3.3 127.2.2.2 623 0x80000002 0x3d06
AS External Link States
Link ID ADV Router Age Seq# CkSum Route
20.20.20.0 20.20.20.3 3296 0x80000001 0xe45e E2 20.20.20.0/24 [0x0]
20.20.20.0 127.3.3.3 1359 0x80000004 0x18de E2 20.20.20.0/24 [0x0]
60.60.60.0 127.3.3.3 1407 0x80000004 0x720c E2 60.60.60.0/24 [0x0]

This is the router which is going to be acting as the interior router in the stub area, based on the changes and configurations we make to this area the ospf database belonging to this router would change relevantly. In the following lines you see the current ospf routes including the network inter-area routes advertised by r2 which namely is the ABR between the 0.0.0.0 and 1.1.1.1 areas.

vyatta@v5:~$ show ip ospf route
============ OSPF network routing table ============
N IA 10.10.10.0/24 [20] area: 1.1.1.1
via 50.50.50.2, eth0
N 50.50.50.0/24 [10] area: 1.1.1.1
directly attached to eth0
N IA 127.1.1.1/32 [30] area: 1.1.1.1
via 50.50.50.2, eth0
N IA 127.3.3.3/32 [30] area: 1.1.1.1
via 50.50.50.2, eth0
============ OSPF router routing table =============
R 127.2.2.2 [10] area: 1.1.1.1, ABR
via 50.50.50.2, eth0
R 127.3.3.3 IA [20] area: 1.1.1.1, ASBR
via 50.50.50.2, eth0
============ OSPF external routing table ===========
N E2 20.20.20.0/24 [20/20] tag: 0
via 50.50.50.2, eth0
N E2 60.60.60.0/24 [20/20] tag: 0
via 50.50.50.2, eth0

vyatta@v5:~$ show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF,
I - ISIS, B - BGP, > - selected route, * - FIB route

O>* 10.10.10.0/24 [110/20] via 50.50.50.2, eth0, 00:40:00
O>* 20.20.20.0/24 [110/20] via 50.50.50.2, eth0, 00:39:59
O 50.50.50.0/24 [110/10] is directly connected, eth0, 00:40:05
C>* 50.50.50.0/24 is directly connected, eth0
O>* 60.60.60.0/24 [110/20] via 50.50.50.2, eth0, 00:39:59
O>* 127.1.1.1/32 [110/30] via 50.50.50.2, eth0, 00:40:00
O>* 127.3.3.3/32 [110/30] via 50.50.50.2, eth0, 00:40:00
C>* 127.5.5.0/24 is directly connected, lo

vyatta@r2:~$ show ip ospf database
OSPF Router with ID (127.2.2.2)
Router Link States (Area 0.0.0.0)
Link ID ADV Router Age Seq# CkSum Link count
127.1.1.1 127.1.1.1 1561 0x80000005 0x164e 2
127.2.2.2 127.2.2.2 786 0x80000007 0xec6a 2
127.3.3.3 127.3.3.3 1559 0x80000004 0xf45a 2
Net Link States (Area 0.0.0.0)
Link ID ADV Router Age Seq# CkSum
10.10.10.3 127.3.3.3 1559 0x80000003 0xb564
Summary Link States (Area 0.0.0.0)
Link ID ADV Router Age Seq# CkSum Route
50.50.50.0 127.2.2.2 804 0x80000002 0x191d 50.50.50.0/24
127.5.5.5 127.2.2.2 1385 0x80000002 0x6dc6 127.5.5.5/32
Router Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum Link count
50.50.50.5 50.50.50.5 667 0x80000003 0x7db6 2
127.2.2.2 127.2.2.2 671 0x80000004 0xff0a 1
Net Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum
50.50.50.2 127.2.2.2 671 0x80000002 0xbe5b
Summary Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum Route
10.10.10.0 127.2.2.2 354 0x80000002 0xbeef 10.10.10.0/24
127.1.1.1 127.2.2.2 534 0x80000002 0xf14e 127.1.1.1/32
127.3.3.3 127.2.2.2 374 0x80000002 0xaf8a 127.3.3.3/32
ASBR-Summary Link States (Area 1.1.1.1)
Link ID ADV Router Age Seq# CkSum
127.3.3.3 127.2.2.2 734 0x80000002 0x3d06
AS External Link States
Link ID ADV Router Age Seq# CkSum Route
20.20.20.0 20.20.20.3 3408 0x80000001 0xe45e E2 20.20.20.0/24 [0x0]
20.20.20.0 127.3.3.3 1470 0x80000004 0x18de E2 20.20.20.0/24 [0x0]
60.60.60.0 127.3.3.3 1518 0x80000004 0x720c E2 60.60.60.0/24 [0x0]
vyatta@r2:~$ show ip ospf route
============ OSPF network routing table ============
N 10.10.10.0/24 [10] area: 0.0.0.0
directly attached to eth0
N 50.50.50.0/24 [10] area: 1.1.1.1
directly attached to eth1
N 127.1.1.1/32 [20] area: 0.0.0.0
via 10.10.10.1, eth0
N 127.3.3.3/32 [20] area: 0.0.0.0
via 10.10.10.3, eth0
N 127.5.5.5/32 [20] area: 1.1.1.1
via 50.50.50.5, eth1

============ OSPF router routing table =============
R 127.3.3.3 [10] area: 0.0.0.0, ASBR
via 10.10.10.3, eth0

============ OSPF external routing table ===========
N E2 20.20.20.0/24 [10/20] tag: 0
via 10.10.10.3, eth0
N E2 60.60.60.0/24 [10/20] tag: 0
via 10.10.10.3, eth0

vyatta@r2:~$ show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF,
I - ISIS, B - BGP, > - selected route, * - FIB route

O 10.10.10.0/24 [110/10] is directly connected, eth0, 00:58:23
C>* 10.10.10.0/24 is directly connected, eth0
O>* 20.20.20.0/24 [110/20] via 10.10.10.3, eth0, 00:57:33
O 50.50.50.0/24 [110/10] is directly connected, eth1, 00:44:41
C>* 50.50.50.0/24 is directly connected, eth1
O>* 60.60.60.0/24 [110/20] via 10.10.10.3, eth0, 00:57:33
O>* 127.1.1.1/32 [110/20] via 10.10.10.1, eth0, 00:57:34
C>* 127.2.2.0/24 is directly connected, lo
O>* 127.3.3.3/32 [110/20] via 10.10.10.3, eth0, 00:57:34
O>* 127.5.5.5/32 [110/20] via 50.50.50.5, eth1, 00:42:39

In the previous scenario where we defined the asbr connecting to different autonomous systems together

We had connectivity from the ospf network to all interfaces of the rip network only because we redistributed rip routes to our ospf network on the r3 router

But here is the tricky part: the only reason we had connectivity from the rip network to the ospf network (although we didn’t redistribute ospf routes into the rip network)is that while we configured r3 to be the ASBR the only and only ospf network which was the 10.10.10.0 was attached directly to r3 so when we turned on rip on its other interface the route to that network was distributed as a rip route on the rip network whereas if we had another network such as the 60.60.60.0 network like the scenario we have right now connectivity back and forth to the rip network wouldn’t have been possible, therefore in order to establish the connectivity between the new network and the rip network we have to redistributed the ospf routes to the rip network as well.

vyatta@r3# set protocols rip redistribute ospf

Let’s check for connectivity form R5 to the RIP network.

vyatta@v5:~$ ping 60.60.60.4
PING 60.60.60.4 (60.60.60.4) 56(84) bytes of data.
64 bytes from 60.60.60.4: icmp_seq=1 ttl=62 time=10.1 ms
64 bytes from 60.60.60.4: icmp_seq=2 ttl=62 time=5.05 ms
64 bytes from 60.60.60.4: icmp_seq=3 ttl=62 time=4.61 ms
64 bytes from 60.60.60.4: icmp_seq=4 ttl=62 time=4.23 ms
64 bytes from 60.60.60.4: icmp_seq=5 ttl=62 time=4.30 ms
--- 60.60.60.4 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4000ms
rtt min/avg/max/mdev = 4.233/5.680/10.192/2.275 ms

Now we have connectivity! Keep in mind that we yet have not configured the stub network were just preparing for complete connectivity over the network.

Let’s configure the area 1.1.1.1 as a stub area both on r2 and r5, make sure that you make the configurations on both of the routers so that when they want to exchange routes they would be able to realize each other that they reside in a stub area , when they exchange routes there’s a certain flag which shows that you’re residing in a stub area , so in order for them to become stub neighbors they have to be both configured as stub areas in the ospf configuration.

vyatta@v5# set protocols ospf area 1.1.1.1 area-type stub
vyatta@r2# set protocols ospf area 1.1.1.1 area-type stub

Now let’s take a look at our information on r2 and r5 after configuring the area as a stub area

vyatta@v5:~$ show ip ospf database
OSPF Router with ID (50.50.50.5)
Router Link States (Area 1.1.1.1 [Stub])
Link ID ADV Router Age Seq# CkSum Link count
50.50.50.5 50.50.50.5 1745 0x80000013 0x7baa 2
127.2.2.2 127.2.2.2 1751 0x80000013 0xfffc 1
Net Link States (Area 1.1.1.1 [Stub])
Link ID ADV Router Age Seq# CkSum
50.50.50.2 127.2.2.2 1746 0x8000000f 0xc24c
Summary Link States (Area 1.1.1.1 [Stub])
Link ID ADV Router Age Seq# CkSum Route
0.0.0.0 127.2.2.2 1083 0x80000002 0xebeb 0.0.0.0/0
10.10.10.0 127.2.2.2 953 0x8000000f 0xc2e0 10.10.10.0/24
127.1.1.1 127.2.2.2 1033 0x8000000f 0xf53f 127.1.1.1/32
127.3.3.3 127.2.2.2 633 0x8000000f 0xb37b 127.3.3.3/32

As you see the AS routes which used to be in our ospf database are no longer there and instead of all those routes the default 0.0.0.0 route has been advertised by R2 which would forward packets which don’t match other routes with a lower priority to the 50.50.50.2 interface of R2. The following lines show that we only have a couple of inter-area routes advertised by the area border router in our ospf routing table.

vyatta@v5:~$ show ip ospf route
============ OSPF network routing table ============
N IA 0.0.0.0/0 [11] area: 1.1.1.1
via 50.50.50.2, eth0
N IA 10.10.10.0/24 [20] area: 1.1.1.1
via 50.50.50.2, eth0
N 50.50.50.0/24 [10] area: 1.1.1.1
directly attached to eth0
N IA 127.1.1.1/32 [30] area: 1.1.1.1
via 50.50.50.2, eth0
N IA 127.3.3.3/32 [30] area: 1.1.1.1
via 50.50.50.2, eth0
============ OSPF router routing table =============
R 127.2.2.2 [10] area: 1.1.1.1, ABR
via 50.50.50.2, eth0
============ OSPF external routing table ===========

Most of the changes are done on the R5 router and R2 still remains as it used to be.

LET’S JUST CHECK WE HAVE CONNECTIVITY FROM R5 TO 60.60.60.0/24

vyatta@v5:~$ ping 60.60.60.4
PING 60.60.60.4 (60.60.60.4) 56(84) bytes of data.
64 bytes from 60.60.60.4: icmp_seq=1 ttl=62 time=20.3 ms
64 bytes from 60.60.60.4: icmp_seq=2 ttl=62 time=4.09 ms
64 bytes from 60.60.60.4: icmp_seq=3 ttl=62 time=3.59 ms
64 bytes from 60.60.60.4: icmp_seq=4 ttl=62 time=3.26 ms
64 bytes from 60.60.60.4: icmp_seq=5 ttl=62 time=3.57 ms
--- 60.60.60.4 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4002ms
rtt min/avg/max/mdev = 3.266/6.977/20.355/6.694 ms

\\*\\ NOW LET’S CONFIGURE THE AREA AS A TOTALLY STUBBY AREA

To be a little bit more granular about the routes which are injected in our ospf database in the stub area we could further more configure the area as a totally stub area, in this case summary routes which used to be in our ospf database would be removed and the default route of 0.0.0.0 via 50.50.50.2 would be used for packets which do not match the existing lower priority routes.

Totally stub areas do not allow routes other than intra-area and the default routes to be propagated within the area. The ABR injects a default route into the area and all the routers belonging to this area use the default route to send any traffic outside the area.

vyatta@r2# set protocols ospf area 1.1.1.1 area-type stub no-summary

This additional no-summary parameter informs the ABR not to send summary updates from other areas into the totally stub area this parameter needs to be configured only on the area border router where its the only router responsible for summary advertisement link state flooding.

Let’s take a look at the changes made upon the configurations we made:

vyatta@v5:~$ show ip route
Codes: K - kernel route, C - connected, S - static, R - RIP, O - OSPF,
I - ISIS, B - BGP, > - selected route, * - FIB route
O>* 0.0.0.0/0 [110/11] via 50.50.50.2, eth0, 00:37:38
O 50.50.50.0/24 [110/10] is directly connected, eth0, 00:38:15
C>* 50.50.50.0/24 is directly connected, eth0
C>* 127.5.5.0/24 is directly connected, lo

vyatta@v5:~$ show ip ospf database
OSPF Router with ID (50.50.50.5)
Router Link States (Area 1.1.1.1 [Stub])
Link ID ADV Router Age Seq# CkSum Link count
50.50.50.5 50.50.50.5 502 0x80000017 0x73ae 2
127.2.2.2 127.2.2.2 478 0x80000018 0xf502 1
Net Link States (Area 1.1.1.1 [Stub])
Link ID ADV Router Age Seq# CkSum
50.50.50.2 127.2.2.2 478 0x80000002 0xdc3f
Summary Link States (Area 1.1.1.1 [Stub])
Link ID ADV Router Age Seq# CkSum Route
0.0.0.0 127.2.2.2 1091 0x80000005 0xe5ee 0.0.0.0/0

vyatta@v5:~$ show ip ospf route
============ OSPF network routing table ============
N IA 0.0.0.0/0 [11] area: 1.1.1.1
via 50.50.50.2, eth0
N 50.50.50.0/24 [10] area: 1.1.1.1
directly attached to eth0
============ OSPF router routing table =============
R 127.2.2.2 [10] area: 1.1.1.1, ABR
via 50.50.50.2, eth0
============ OSPF external routing table ===========

vyatta@v5:~$ ping 60.60.60.4
PING 60.60.60.4 (60.60.60.4) 56(84) bytes of data.
64 bytes from 60.60.60.4: icmp_seq=1 ttl=62 time=32.3 ms
64 bytes from 60.60.60.4: icmp_seq=2 ttl=62 time=4.88 ms
64 bytes from 60.60.60.4: icmp_seq=3 ttl=62 time=5.33 ms
64 bytes from 60.60.60.4: icmp_seq=4 ttl=62 time=3.84 ms
64 bytes from 60.60.60.4: icmp_seq=5 ttl=62 time=5.56 ms
--- 60.60.60.4 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4005ms
rtt min/avg/max/mdev = 3.843/10.397/32.358/10.996 ms

vyatta@v5:~$ show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
RXmtL RqstL DBsmL
127.2.2.2 1 Full/DR

CONTINUE TO PART 8 - NOT SO STUBBY AREAS