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Exam 300-420 topic 1 question 28 discussion

Actual exam question from Cisco's 300-420

Question #: 28
Topic #: 1

An engineer is creating a design to enable IPv6 to run on an existing IPv4 IS-IS network. The IPv4 and IPv6 topologies will match exactly, and the engineer plans to use the same IS-IS router levels for each protocol per interface. Which IS-IS design is required?

A. multi topology without enabling transition feature
B. multi topology with transition feature enabled
C. single topology without enabling transition feature
D. single topology with transition feature enabled

Show Suggested Answer

Suggested Answer: C 🗳️

by Gabi512 at Dec. 9, 2021, 3:20 p.m.

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Gabi512

Highly Voted 3 years ago

Selected Answer: C

I would say C as both IPV4 and IPV6 are sharing the exact same topology. No need of multipology nor transition mode: https://www.ws.afnog.org/afnog2011/are/ipv6-presentations/4-isis-for-ipv6.pdf

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neiker45

Most Recent 12 months ago

Selected Answer: C

https://datatracker.ietf.org/meeting/105/agenda/v6ops-drafts.pdf Single Topology: 4.1 Third Paragraph Transition: 4.2.1 First Paragraph

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LSLS55

1 year, 1 month ago

Selected Answer: B

According to OCG, page 120: "IS-IS support IPv6 as a separate protocol(...)" and page 121 "IS-IS provides support for IPv4 and IPv6 as separate topologies" -> this means it needs two separate topologies for different address families = MULTITOPOLOGY.

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draxon

1 year, 4 months ago

Selected Answer: B

https://rayka-co.com/lesson/isis-ipv6-multi-topology/

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Clauster

1 year, 5 months ago

Selected Answer: D

The answer is 100% D

upvoted 1 times

Clauster

1 year, 5 months ago

Sorry folks the answer is indeed C

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SpicyMochi

1 year, 8 months ago

Selected Answer: C

C. single topology without enabling transition feature In a single topology design, both IPv4 and IPv6 addresses can be carried within the same IS-IS domain without creating separate topologies. IS-IS, being a Layer 2 protocol, can natively support both IPv4 and IPv6. By utilizing a single topology design without any transition feature, the engineer can enable IPv6 on the existing IPv4 IS-IS network with the same router levels for each protocol per interface, while still maintaining a single, integrated topology.

upvoted 2 times

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andrewChan

2 years, 2 months ago

Selected Answer: D

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-3s/irs-xe-3s-book/ip6-route-isis-xe.html#GUID-04990BCF-8228-459E-AFAA-9FAF1E2136E9 single topology with no adjacency-check, so D is correct For single-topology IS-IS IPv6, routers must be configured to run the same set of address families. IS-IS performs consistency checks on hello packets and will reject hello packets that do not have the same set of configured address families. For example, a router running IS-IS for both IPv4 and IPv6 will not form an adjacency with a router running IS-IS for IPv4 or IPv6 only. In order to allow adjacency to be formed in mismatched address-families network, the adjacency-check command in IPv6 address family configuration mode must be disabled. Enter the no adjacency-check command only when you are running IPv4 IS-IS on all your routers and you want to add IPv6 IS-IS to your network but you need to maintain all your adjacencies during the transition.

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iLikeHamburgers

2 years, 3 months ago

The key thing here is that it states there is a currently deployed IPV4 network. Once you configure IPv6 on an interface that is currently running IPv4, you break the requirement that states "all interfaces on which IS-IS is configured must support the identical set of network address families." There is also a requirement that states "all routers in the IS-IS area (for Level 1 routing) or domain (for Level 2 routing) must support the identical set of network layer address families." I don't have the ability to lab this at the moment, however I would be curious to see if you had a network running single topology IPv4 only, and then started configuring IPv6 on the interfaces, would it break anything. I would question whether C is correct or not. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-3s/irs-xe-3s-book/ip6-route-mult-isis-xe.html#GUID-EA369F57-430A-4F30-B467-4529140CB0B6

upvoted 2 times

andrewChan

2 years, 2 months ago

yes it would break adjanency when address familty is different during transition, according to cisco doc: For single-topology IS-IS IPv6, routers must be configured to run the same set of address families. IS-IS performs consistency checks on hello packets and will reject hello packets that do not have the same set of configured address families. For example, a router running IS-IS for both IPv4 and IPv6 will not form an adjacency with a router running IS-IS for IPv4 or IPv6 only. In order to allow adjacency to be formed in mismatched address-families network, the adjacency-check command in IPv6 address family configuration mode must be disabled. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-3s/irs-xe-3s-book/ip6-route-isis-xe.html#GUID-04990BCF-8228-459E-AFAA-9FAF1E2136E9

upvoted 1 times

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python_tamer

2 years, 6 months ago

Selected Answer: C

I agree with C.

upvoted 1 times

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oaban

2 years, 7 months ago

Selected Answer: C

I thinks C. Topologies match exact, no need multitopology.

upvoted 2 times

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certstudent2016

2 years, 8 months ago

Selected Answer: B

https://www.ws.afnog.org/afnog2011/are/ipv6-presentations/4-isis-for-ipv6.pdf toward the ends its given answer - B is correct https://datatracker.ietf.org/doc/rfc5120/

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XalaGyan

1 year, 11 months ago

Again i agree with you certstudent2016. At some point in the configuration we have to introduce IPv6 to an already running and converged IPv4 IS-IS single topology. Single topology mandates the use of a single address family. ==> this brings adjacencies down Solution is to go MULTITOPOLOGY as MT dont bother about Address Families. Then you have to have transition feature enabled due to the EXACT SAME TOPOLOGY part of the question. later when you have multitopology and ipv4 and ipv6 then you disable transition feature.

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XalaGyan

1 year, 11 months ago

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-mt/irs-15-mt-book/ipv__routing__is-is_multitopology_support_for_ipv_.pdf Transition from Single-Topology to Multitopology Support for IPv6 All routers in the area or domain must use the same type of IPv6 support, either single-topology or multitopology. A router operating in multitopology mode will not recognize the ability of the single-topology mode router to support IPv6 traffic, which will lead to holes in the IPv6 topology. To transition from single-topology support to the more flexible multitopology support, a multitopology transition mode is provided.

upvoted 1 times

XalaGyan

1 year, 11 months ago

The multitopology transition mode allows a network operating in single-topology IS-IS IPv6 support mode to continue to work while upgrading routers to include multitopology IS-IS IPv6 support. While in transition mode, both types of TLVs (single-topology and multitopology) are sent in LSPs for all configured IPv6 addresses, but the router continues to operate in single-topology mode (that is, the topological restrictions of the single-topology mode are still in effect). After all routers in the area or domain have been upgraded to support multitopology IPv6 and are operating in transition mode, transition mode can be removed from the configuration. Once all routersin the area or domain are operating in multitopology IPv6 mode, the topological restrictions of single-topology mode are no longer in effect.

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