An engineer configured routing between multiple OSPF domains and introduced a routing loop that caused network instability. Which action resolves the problem?
A.
Set a tag using the redistribute command toward a domain and deny inbound in the other domain by a matching tag.
B.
Set a tag using the redistribute command toward a different domain and deny the matching tag when exiting from that domain.
C.
Set a tag using the network command in a domain and use the route-map command to deny the matching tag when exiting toward a different domain.
D.
Set a tag using the network command in a domain and use the route-map command to deny the matching tag when entering into a different domain.
https://networklessons.com/cisco/ccie-routing-switching/troubleshooting-metric-redistribution
Redistribution Rule: Never advertise prefixes from routing protocol X into Y and then back into X.
Redistribution goes from one protocol (or routing domain) into the other protocol (or routing domain) using the redistribute command.
It's easier to picture redistribution between different protocols. Answer "A" means something like this:
router ospf 1
redistribute eigrp 100 subnets tag 20
route-map FILTER deny
match tag 20
route-map FILTER permit
router eigrp 100
redistribute ospf 1 metric 100000 1 255 1 1500 route-map FILTER
1)redistribute into ospf and set the tag
2)redistribute into eigrp and match the tag (with action deny)
Set a tag so, routes originating from a routing domain will not be redistributed back into that domain.
The key word in A is "inbound". This accomplishes bad mutual redistribution on multiple routers. The other answers don't specify specifically what are the exit domains. This may or may not redice instability.
For example it can't be B:
Set a tag using the redistribute command toward a different domain and deny the matching tag when exiting from that domain.
We Set tag A-->B and exit B-->C won't prevent loop
D is also wrong:
Set a tag using the network command in a domain and use the route-map command to deny the matching tag when entering into a different domain
I set tag as 20 in domain A and redistribute into domain B from domain A-->B then I deny all tagged routes and they never get redistributed to begin with.
Remember we can set tags with route-maps (distribute-lists etc) so if I set the tag in a distribute-list route-map in domain A, and deny the tag when entering from A-->B I never redistribute the route to begin with. So the answer is not D. It is A.
D. in this case could be referring to a sequential domain.
area A --> area B --> area C
Set tage area A, deny into area C. We typically won't solve suboptimal routing.
The kyeword in option A is the "inbound" keyword.
area A --> area B --> inbound area A (the other domain)
we stop area A routes from getting redistributed into itself avoiding suboptimal routing
OSPF Redistribution Among Different OSPF Processes - Filter Routes Based on Tags
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/4170-ospfprocesses.html#anc16:~:text=be%20very%20difficult.-,Filter%20Routes%20Based%20on%20Tags,-There%20is%20a
As you said answer is A this config shows that clearly:
router ospf 1
redistribute ospf 2 subnet tag 1 route-map filter_domain2
distance ospf 2 external 200
!
route-map filter_domain2 deny 10
match tag 2
route-map filter_domain2 permit 20
router ospf 2
redistribute ospf 1 subnet tag 2 route-map filter_domain1
distance ospf 1 external 200
!
route-map filter_domain1 deny 10
match tag 1
route-map filter_domain1 permit 20
This is a good example of I'm redistributing my ospf process into another and I put a tag on it, and in the route-map I'm gonna deny any kind of ospf redistribution from the other process that got this tag.
It's kinda same as redistribution between routing protocols, the teory is basically the same.
I feel this question is more of "how good is your english"
when we redistribute, it gets into that domain(IN) so we deny it on its way OUT of that current OSPF domain.
my understanding with my level of english
It is Either A or B. However, for the routing to work between the two domains, the tagged routes need to be advertised first.
So, for A, it says we are Denying the tag in to the new domain. While B says, we are Denying the Tag from the other domain coming back to the origin domain.
To me, this is language question and not technical.
I dont see where it states in the question that the routes are redistributed, I think it is just an adjacency made with the network command. therefore D seems right.
Most commends forgot to read the question properly it says "between multiple OSPF domains". Since redistribution is used to inject between different routing protocols we will need to configure a route-map that will match and deny the same incoming route from different OSPF domain.
Redistribute Path (See a similar figure 17-6 on Page 678)
EIGRP 100 R1 ---> R2 OSPF 1 (Redistribute into OSPF from EIGRP)
EIGRP 100 R3 <---- R4 OSPF 1 (The same redistribute packets will be introduce into EIGRP from OSPF)
R2
====
router ospf 1
redistribute eigrp 100 subnet route-map FROM-EIGRP-TO-OSPF
route-map FROM-EIGRP-TO-OSPF permit
ip address match prefix-list PREFIX-TAG
set tag 100
ip prefix-list PREFIX seq 10 permit 10.1.1.0/24
R4
===
router eigrp 100
redistribute ospf metric 1000000 100 255 1 1500 route-map FROM-OSPF-TO-EIGRP
route-map FROM-EIGRP-TO-OSPF deny
tag match 100
route-map FROM-EIGRP-TO-OSPF permit
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