2.3.23 Example for Configuring OSPF NSSAs

Latest reply: Nov 4, 2015 14:46:28 3629 2 0 0

2.3.23 Example for Configuring OSPF NSSAs-1322293-1


At first, I didn’t think of it carefully and used Ethernet ports to do my NSSA experiment. Unluckily, Ethernet port of AR1220 is only for layer 2. What am I going to do?

This lab needs three Gigabit Ethernet ports for some routers, so I add 4GEW-T module to AR1200 so that I can have 4 L3 Gigabit Ethernet ports for each router. Of course I need to turn off AR1200 first.

Use the function Delete All Lines and then connect as below:

2.3.23 Example for Configuring OSPF NSSAs-1322293-2

<The related eNSP config file is attached>


I configured OSPF well and found it wasn’t working. Jesus, I forgot to configure IP address for RouterB and RouterC, no wonder they didn’t give me face. <display ip interface brief> is useful.

After configuration, I only need to use display ospf routing to check the output of routing on RouterD to see whether my configuration is correct or not.

OSPF Process 1 with Router ID 4.4.4.4

         Routing Tables

 

 Routing for Network

 Destination        Cost  Type       NextHop         AdvRouter       Area

 192.168.3.0/24     1     Transit    192.168.3.2     4.4.4.4         0.0.0.1

 192.168.4.0/24     1     Transit    192.168.4.1     4.4.4.4         0.0.0.1

 192.168.0.0/24     2     Inter-area 192.168.3.1     192.168.0.1     0.0.0.1

 192.168.1.0/24     2     Stub       192.168.4.2     2.2.2.2         0.0.0.1

 192.168.2.0/24     2     Inter-area 192.168.4.2     2.2.2.2         0.0.0.1

 

 Total Nets: 5 

 Intra Area: 3  Inter Area: 2  ASE: 0  NSSA: 0

Since 192.168.0.0 and 192.168.2.0 belong to area 0(backbone area), so the routings are labeled as Inter-area.

The route 192.168.1.0 stub isn’t comfortable, so I check RouterA by using display current-configuration and find no OSPF is running for 192.168.1.0 and configure it. Now I get the following output of RouterD:

OSPF Process 1 with Router ID 4.4.4.4

         Routing Tables

 

 Routing for Network

 Destination        Cost  Type       NextHop         AdvRouter       Area

 192.168.3.0/24     1     Transit    192.168.3.2     4.4.4.4         0.0.0.1

 192.168.4.0/24     1     Transit    192.168.4.1     4.4.4.4         0.0.0.1

 192.168.0.0/24     2     Inter-area 192.168.3.1     192.168.0.1     0.0.0.1

 192.168.1.0/24     2     Transit    192.168.3.1     2.2.2.2         0.0.0.1

 192.168.1.0/24     2     Transit    192.168.4.2     2.2.2.2         0.0.0.1

 192.168.2.0/24     2     Inter-area 192.168.4.2     2.2.2.2         0.0.0.1

 

 Total Nets: 6 

 Intra Area: 4  Inter Area: 2  ASE: 0  NSSA: 0

From output as above, we can see there’re two routings to 192.168.1.0. Here we can also see routing 192.168.0.0 has AdvRouter 192.168.0.1 instead of 1.1.1.1. Probably this is the rule of OSPF, once DR and BDR are configured, nobody can replace it even the router has highest priority, which isn’t democracy. My ability is higher, why can't I replace you?

I’ll save settings of all routers and reboot them to see what happens to the routing table of RouterD.

OSPF Process 1 with Router ID 4.4.4.4

         Routing Tables

 

 Routing for Network

 Destination        Cost  Type       NextHop         AdvRouter       Area

 192.168.3.0/24     1     Transit    192.168.3.2     4.4.4.4         0.0.0.1

 192.168.4.0/24     1     Transit    192.168.4.1     4.4.4.4         0.0.0.1

 192.168.0.0/24     2     Inter-area 192.168.3.1     1.1.1.1         0.0.0.1

 192.168.1.0/24     2     Transit    192.168.4.2     2.2.2.2         0.0.0.1

 192.168.1.0/24     2     Transit    192.168.3.1     2.2.2.2         0.0.0.1

 192.168.2.0/24     2     Inter-area 192.168.4.2     2.2.2.2         0.0.0.1

 

 Total Nets: 6 

 Intra Area: 4  Inter Area: 2  ASE: 0  NSSA: 0

Until now, I get everything right. Now the AdvRouter is 1.1.1.1.

DR&BDR, I capture some packets from GE 2/0/3 of RouterA and can find that DR is 192.168.1.2(which is IP address of GE 2/0/1 of RouterB). The router priority of both routers is 1. The highest Router ID makes DR. RouterB’s router ID is 2.2.2.2 while RouterA’s router ID is 1.1.1.1. That’s why RouterB is chose to be DR(Don’t mess 2.2.2.2 and 192.168.1.2 up. One is router ID and the other is interface IP address, router ID decides DR and DBR, DR's role is like the head of a classroom(segment).)

2.3.23 Example for Configuring OSPF NSSAs-1322293-3


Probably I’m too wordy, but I hope I'm familiar with other topics while studying one topic. Now let’s learn NSSA, not so stubby area.

Before my configuration of NSSA, I have to understand stub area, totally stubby area and not so stubby area. A stub area does not accept information about routes external to the AS. A totally stubby area does not accept external AS routes and summary routes from other areas internal to the AS. With the introduction of the NSSA, there is another, easier option. An NSSA acts like a stub network in the sense that it does not allow Type 5 LSAs. It can also be configured to prevent floods of Type 3 and Type 4 summary LSAs, just as a totally stubby area would. However, an NSSA does allow Type 7 LSAs, which can carry external routing information and be flooded throughout the NSSA. 

The routing of RouterC is:  

Routing for ASEs

 Destination        Cost      Type       Tag         NextHop         AdvRouter

100.0.0.0/8        1         Type2      1           192.168.0.1     2.2.2.2

Though there’re two AdvRouter 1.1.1.1 and 2.2.2.2, OSPF selects ABR with larger router ID as translator(Transfer another route into OSPF).

We can make Router A translator.

Routing for ASEs

 Destination        Cost      Type       Tag         NextHop         AdvRouter

 100.0.0.0/8        1         Type2      1           192.168.0.1     1.1.1.1

If I want to understand NSSA, I can capture packets on RouterD’s GE 2/0/1. After several minutes, I capture one packet of LS Update from RouterD to RouterA.

2.3.23 Example for Configuring OSPF NSSAs-1322293-4

Finally the picture shows a clear NSSA.

2.3.23 Example for Configuring OSPF NSSAs-1322293-5



 

 

 

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Created Aug 5, 2014 21:35:28 Helpful(0) Helpful(0)

LOOK!
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Created Nov 4, 2015 14:46:28 Helpful(0) Helpful(0)

Even though there is no environment, I still learn network hard then. Good!
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