Exam 300-420 topic 1 question 28 discussion

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

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

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.

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

The answer is 100% D

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.

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.

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

I agree with C.

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

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/