Inter-area OSPF
Lab file
Startup configs
In this lab you will configure various features of inter-area OSPF routing. IP addressing, the underlying OSPF configuration, and BGP are all pre-configured. All transit links follow the forumla 10.X.Y.X/24. I’ve left them off the diagram to make it cleaner.
Configure XR10 and R11 so that they inject a default route into OSPF, but only under certain conditions:
R11 should only inject a default route if it knows a default route via its eBGP peer, ISP2
XR10 should only inject a default route if it knows 12.12.12.12/32 via eBGP
(I cannot find a good way to match the 0/0 from eBGP and not iBGP in XR10)
Configure area 3 so that the external routes can be injected, but summaries and externals are hidden. Then redistribute eBGP into OSPF on R3.
R1 cannot ping R4 at 4.100.0.1. Why not? Fix this so that any router can ping R4’s Lo1.
Configure area 1 so that R1 will not see the E2 route for 4.100.0.0/24
Configure area 2 so that you achieve the output below on R2. R2 will not need to redistrbute externals into OSPF.
Achieve the following output on R8. Only configure ABRs or ASBRs.
Answers
On R11, create a route-map that matches a prefix-list permitting only 0/0, and a next-hop of the eBGP peer (using an ACL). Under ospf, originate default-information refrencing this route-map. On XR10, I did the same procedure except I used a route-policy with if rib-has-route 12.12.12.12/32. You cannot use a route-policy matching on destination or next-hop and attach it to default-information originate under OSPF.
On R3 add area 3 nssa under OSPF. On the ABRs (R6 and XR7) add area 3 nssa no-summary. When using nssa, the no-summary keyword filters Type 3 LSAs, and also adds a default route automatically into the area. Without no-summary, you must also use area # nssa default-information originate if you wish for the ABRs to inject a default route. The no-summary keyword makes the area Totally NSSA. On R3, redistribute BGP into OSPF. This creates a Type 7 LSA. The ABRs translate this into a Type 5 LSA that is flooded throughout the entire OSPF domain. However, only the ABR with the heighest router ID injects the Type 5 into the backbone. In this case it is XR7. R6 will only inject a Type 5 if XR7 goes down.
You have multiple options for achieving this. You could create a static 0/0 route on R4 pointing to R3. You could inject a 0/0 into BGP on R3 and advertise the 0/0 to the neighbor, R4. You could also redistribute OSPF into BGP on R3. If you redistribute OSPF you still have to make sure you have default-information originate under BGP so that the 0/0 from OSPF will be injected into BGP.
Configure area 1 as stub. This hides externals but allows summaries into the area. The ABR automatically injects a default route into the stub area as an inter-area route. In our case, we previously had the default as an E2 route. This is changed to an IA route now.
Configure area 2 as totally stub. To do this, you configure R2 for area 2 stub and the ABR(s), in this case R6, as area 2 stub no-summary. The ABR will automatically inject an IA 0/0 route.
On R8, 1.100.0.0/24 and 1.100.3.0/24 are missing. On XR5, filter these two prefixes from injection into the backbone by using area 1 range 1.100.X.0/24 not-advertise. On R8, 2.100.0.0/23 is aggregated from the two prefixes on Lo1 of R2. On R6, summarize these using area 2 range 2.100.0.0 255.255.254.0. On R8, 4.100.0.0/24 has become aggregated to 4.100.0.0/22. This is an NSSA external so you have two options to achieve this. You can configure a summary-address on R3, or you can configure a summary-prefix on XR7. (If you configure the summary-address on R6, it isn’t advertised because XR7 has a higher RID). Both commands achieve the same functionality - they aggregate/summarize or filter External routes. The summary-prefix command syntax is used on IOS-XR.
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