Refer to the exhibit. After troubleshooting an OSPF adjacency issue, routers 1, 2, and 3 have formed OSPF neighbor relationships. Which statement about the configuration is true?
A.
Router 2 receives a Type 5 LSAs from router 1 for its connected subnets
B.
Router 2 uses router 3 as the next hop for 192.168.0.0/24
C.
Router 2 uses router 1 as the next hop for 192.168.0.0/24
D.
Router 2 receives a Type 7 LSAs from router 3 for its connected subnets
The proposed answer "Router 2 receives a Type 5 LSAs from router 1 for its connected subnets " is wrong. R1 is an "NSSA ASBR" router and it generates a type 7 LSA.
"Router 2 receives a Type 7 LSAs from router 3 for its connected subnets" is also wrong. Area 3 is not a NSSA area so redistribution does not produce type 7 LSA, it produces type 5 LSA.
I am voting answer C, tested on lab with exact configuration and default RFC3010.
(ios xe default running on rfc3010, LSA type 7 is preferable over LSA type 5 for same network)
R2(config-router)#do show ip ospf | i RFC
Supports NSSA (compatible with RFC 3101)
R2(config-router)#do show ip route | i N2
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
O N2 192.168.0.0/24 [110/20] via 192.168.12.1, 00:00:31, GigabitEthernet0/0
=====================================================
but when I did below, rfc1587 :
R2(config-router)#compatible rfc1587
The answer would be B. because LSA type 5 is preferable over LSA type 7 on rfc 1587.
R2(config-router)#do show ip ospf | i RFC
Supports NSSA (compatible with RFC 1587)
R2(config-router)#do show ip route | i E2
E1 - OSPF external type 1, E2 - OSPF external type 2
O E2 192.168.0.0/24 [110/20] via 192.168.23.3, 00:00:00, GigabitEthernet0/1
I have spent a few hours working this out, and here is what I've found.
1. RFC 3101 states the following preference: N1 > E1 > N2 > E2
2. RFC 1587 states the following preference: E1 > N1 > E2 > N2
---Basically, you can think of it like this: The old way was to prefer E routes, and the new way is to prefer N routes. Type 1 is always preferred over type 2. (Type 2 is the default if the metric-type isn't specified when doing redistribution).
3. By default ALL IOS-XE versions will use RFC 3101 and allow you to configure it to use RFC 1587 if you'd like, with the command "compatible rfc1587"
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/212608-ospf-external-path-selection-external-t.html#anc8
- See the section "Background Information" and read the rest of the document to understand the way RFC 3101 chooses N over E routes.
4. IOS-XR will use ONLY RFC 1587. There is no option to enable RFC 3101. The documentation that Gi0v4ne linked only mentions that RFC 1587 is supported. That's it. It says nothing about RFC 3101. WHY does Cisco not use RFC 3101 in IOS-XR? Who knows! Seriously, please tell me if you know.
https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r6-3/routing/configuration/guide/b-routing-cg-asr9000-63x/b-routing-cg-asr9000-63x_chapter_0111.html
So, to answer this question, we need to know whether the routers are running IOS-XR or IOS-XE. This is actually pretty easy to know, because all three routers show OSPF configurations using the "network" command. This is only used in IOS-XE. IOS-XR enables interfaces under the area configuration in OSPF. So this configuration is NOT IOS-XR like Gi0v4ne said. It is IOS-XE
Conclusion: The routers are running IOS-XE where RFC 3101 is used by default. RFC 3101 says to prefer N2 routes over E2 routes, therefore the answer is C. Router 2 will use the N2 route (which has a next hop of Router 1).
If you don't believe me, PLEASE prove me wrong. I spent a few hours labbing this up. When IOS-XR was the ABR, it preferred the E2 route over the N2 route. This lines up with RFC 1587. When I used IOSv as the ABR, it preferred the N2 route over the E2 route (which lines up with RFC 3101).
By the way, in IOS-XE you can clearly see which RFC version is supported by using "show ip ospf | in RFC" - there is no such output for IOS-XR that I've found. Leading me to further believe that IOS-XR supports RFC 1587 ONLY.
I found on XRv 9000 (6.5.3, from Cisco Digital Learning) this command:
router ospf 1
capability type7 prefer
I think you can now enable the RFC 3101 mode
under "show ospf | Capability"
"Capability: Prefer Type 7 LSAs over Type 5"
No more cisco documentation found.
The configuration is in IOS XR, where RFC 3101 is applies, and RFC 1587 is supported.
https://www.cisco.com/c/en/us/td/docs/routers/asr9000/software/asr9k-r6-3/routing/configuration/guide/b-routing-cg-asr9000-63x/b-routing-cg-asr9000-63x_chapter_0111.html
I think you are incorrect twice, which leads to the correct answer.
The config is not IOS-XR it is IOS-XE. Look at the "network" statements. Show me where IOS-XR uses that configuration?
Also IOS-XR uses RFC 1587 not 3101. Show me where in that document that 3101 applies?
IOS-XE uses RFC 3101.
So you arrived at the correct answer but by using completely wrong steps.
Since Cisco IOS release 15.1(2)S, Cisco uses the path selection order from RFC 3101 which obsoletes RFC 1587. What this means is that it prefers N1 routes before E1 and N2 over E2 routes. In other words, the prefered path list is O > O IA > N1 > E1 > N2 > E2.
i down`t know with RFC need learn that answer this question
he will not have route exchange with R1 by OSPF. (on router 2 we dont have any configuration for are1 nssa, also by default nssa dont propagate external routes to area 0)
The correct answer is C. Router 2 uses router 1 as the next hop for 192.168.0.0/24
From IOS 15.1(2)S onwards, Cisco uses the OSPF path selection order is O > O IA > N1 > E1 > N2 > E2
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/212608-ospf-external-path-selection-external-t.html
I think correct answer: B. Router 2 uses router 3 as the next hop for 192.168.0.0/2
upvoted 3 times
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