A network consultant is troubleshooting IS-IS instances to identify why a routing domains is having communication problems between the two instances.
Which description of the possible cause of issues in the routing domain is true?
Click on the arrows to vote for the correct answer
A. B. C. D.A.
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-3s/irs-xe-3s-book/irs-instance-vrf.htmlIS-IS (Intermediate System to Intermediate System) is a protocol used for routing within a network. It is a link-state protocol that exchanges information about network topology with other routers to determine the best path to forward data packets. Each router running IS-IS belongs to an IS-IS instance, which is a logical representation of a routing domain.
In this scenario, the network consultant is troubleshooting IS-IS instances to identify why a routing domain is having communication problems between two instances. Let's evaluate each answer option to determine the possible cause of the communication problems:
A. The same interface cannot be advertised in two different IS-IS instances.
This statement is true. In IS-IS, a router interface can only belong to one IS-IS instance. If the same interface is advertised in two different instances, it can cause routing loops and connectivity issues between the instances. Therefore, this could be the possible cause of communication problems between the two IS-IS instances in the routing domain.
B. The IS-IS "ISP" and "ISP2" instances are unrelated and unable to intercommunicate.
This statement is not necessarily true. The names of the IS-IS instances are not relevant to their ability to intercommunicate. If the two instances have been configured to communicate with each other and the necessary routing information is being exchanged between them, they should be able to intercommunicate.
C. The configured IS-IS NSEL value is not allowing the routing systems to establish a neighborship.
IS-IS uses the Network Service Access Point (NSAP) to identify routers and establish neighborships between them. The IS-IS Network Service Access Point Identifier (NSAP-ID) is a hierarchical address used to identify a router's position within the network. The NSAP Selector (NSEL) is a field in the NSAP that indicates the level of routing protocol hierarchy. If the NSEL value is not configured correctly or is not compatible between the two instances, it could cause issues with establishing neighborships. Therefore, this could be a possible cause of communication problems between the two IS-IS instances.
D. The interface mode ip router is-is command was not included in the script.
This statement is not necessarily true. The interface mode ip router is-is command is used to enable IS-IS routing on a specific interface. If the command is not configured on an interface, the interface will not participate in IS-IS routing. However, this would not necessarily cause communication problems between two IS-IS instances in the routing domain.
In conclusion, based on the provided answer options, the possible cause of communication problems between the two IS-IS instances in the routing domain could be that the same interface is advertised in two different IS-IS instances or the configured IS-IS NSEL value is not allowing the routing systems to establish a neighborship.