Scenario:
You run a company that sells Virtual Worlds for gamers. The network you are using is running on OSPF and one of the areas is a NSSA. At the moment you are not sure which router is performing the translation from LSA Type 7 to Type 5 into your backbone router. Up to you to check it out!
Goal:
- All IP addresses have been preconfigured for you.
- Configure OSPF Area 0.
- Configure OSPF Area 1 as NSSA.
- Redistribute the loopback0 interface on router Sam into OSPF Area 1.
- Ensure router Tron is the router performing the translation from LSA Type 7 to Type 5 into area 0.
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IOS:
c3640-jk9s-mz.124-16.bin
Topology:
Video Solution:
Configuration Files
You need to register to download the GNS3 topology file. (Registration is free!)Once you are logged in you will find the configuration files right here.
The How to Master series helps you to understand complex topics like spanning-tree, VLANs, trunks, OSPF, EIGRP, BGP and more.
Written by René Molenaar - CCIE #41726
I believe for this example I need to use the forced translator command, correct? Also do we need to use the compatible rfc1587 command?
My IOS command doesnt have the type7 always option, so I cant actually test this. so would like to know if im write so I can make a note and move on.
Thanks
Hi,
It’s not a complex lab. When you configure one area to be a NSSA you will have only one router that will do the translation from LSA type 7’s in the NSSA to the backbone area. With two routers like in this topology there will be an election to determine which router will do the type 7 –> type 5 translation.
The question is who will win the election and why, so you can change it.
Good luck!
Rene
highest RID 😉 what’s interesting is that though the advertising router maybe router A, the path selection may use router B. The IETF doco was an interesting read to suss this one.
Good job. This is one of those labs that isn’t that hard but it helps to look at how OSPF really works.
I’m interested as to why the route from Kevin to the External Route does not go by way of the advertising router? Is there a short answer!
Hi George,
What exactly do you mean? We have two area border routers and only one of them will do the translation of the external type 7 LSA to the type 5 LSA. Depending on the router ID either router Tron or Quorra will do this 🙂
Rene
Increment the router id on each ABR’s , reload the ospf process and you will see in the sho ip ospf command that the Perform type-7/type-5 LSA Translation will move to the device with the highest ID.
Hi guys can someone just post short explanation if possible or link where i can read about this topic. As im confused with this one.
i have set Tron as the one who is doing the translation but the routing table on Kevin pick other router as transit. ( i have cleared all ip ospf processes but still the same)
THX guys!!!!
Here’s a good article to read more about this feature:
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9ef.shtml
Nice one, always good to learn about the weird things. I guess this is still a little off – why wouldn’t both routers perform the translation. Seems like if you have two routers you want them both working for you.
Hi guys i have changed the router id of tron to 222.222.222.222 and cleared ospf process, but Quorra is the router which is translating type 7 to type 5
Could someone tell me why in the nssa area we can see either lsa type 5 and lsa 7 in ospf database. I have red that in nssa area there is no any LSA about outside AS.
Sorry for the question above. I have seen LSA type 5 in ABR from other area then nssa.
Does someone find a reason that changing RID do not change router witch translate LSA type 7 to 5.
Is understand that router wich is doing that is router of next hop to packets going outside the as sending from Kevin
Hi Rene,
First of all thanks for the great work!
I’ve read some resources regarding this topic, and it’s now quite clear to me how this work! Nevertheless, one question subsists and I can’t find an answer to it (or I’m missing something). In the NSSA area, no type 5 LSA should exist. But here, when you check in the ospf database of Quorra or Tron, you still see a type 5 LSA (for 4.4.4.0). Router Tron even chooses the E2 route over the N2 one.
Could you tell me how this works actually,
LSA7s are area specific. Look closely at the database and youll see they are associated only with the NSSA. LSA5s are not associated with any area as they are domain wide (minus NSSAs).
OSPF path selection states that E2 and N2 may have their metrics compared directly, and if its a tie, E2 wins. The same is true for E1/N1. type-1 externals always beat type-2 externals. examples:
N1 always beats E2
E2 beats N2 if metrics tie
N2 can beat E2 if metric is better
N1 can beat E1 is metric is better