ECP-206 Exam Dumps : Ericsson Certified Associate - IP Networking

For IP destinations not found in the IS-IS Level 1 database, the Level 1 router must forward packets to the nearest Level 1-Level 2 router with the attach bit set. The attach bit is a flag in the IS-IS Level 1 LSP that indicates that the router is also a Level 2 router and can reach destinations outside the Level 1 area. The Level 1 router will install a default route pointing to the nearest Level 1-Level 2 router with the attach bit set. This way, the Level 1 router can forward packets to other areas without having to maintain a full Level 2 database. References: Ericsson IP Networking - Routing Protocols, Configure Attach Bit Set - Cisco

MTU mismatch would cause an IS-IS adjacency between two routers to fail. MTU stands for maximum transmission unit, which is the maximum size of a packet that can be transmitted on a link. IS-IS hellos are padded to the full MTU size, partly to allow a router to implicitly communicate its MTU to its neighbors. If two routers have different MTUs on their interfaces, they will not be able to exchange hellos properly, and their adjacency will not form or will be dropped34. To solve this problem, the MTUs on both interfaces must be configured to match.

SEL, context, and SysID are not fields that would cause an IS-IS adjacency failure. SEL stands for selector, which is a one-octet field that identifies the network layer service to which a packet should be sent. For IS-IS routers, the SEL must always be 005. Context is not a field in IS-IS packets, but rather a term used to describe the scope of routing information in different levels or areas6. SysID stands for system identifier, which is a six-octet field that uniquely identifies a router within an area. SysID does not have to match between two routers for them to form an adjacency7.

References: MTU Mismatch Problem in IS-IS - Cisco, IS-IS adjacency is not formed due to MTU mismatch - Cisco Community, IS-IS NSAP address - Cisco Community, Understanding IS-IS Routing Contexts - Juniper Networks, IS-IS - Nokia

In the exhibit, router R4 is performing a swap action. This means that router R4 is replacing the incoming label with a new label and forwarding the packet to the next hop along the LSP. In this case, router R4 receives a packet with label 20 from router R3 and swaps it with label 30 before sending it to router R5.

The exhibit shows an example of an MPLS network with four routers: R1, R2, R3, and R4. Router R1 is the ingress PE router and router R4 is the egress PE router. Router R2 and R3 are P routers. Router R1 assigns label 10 to the packet and sends it to router R2. Router R2 swaps label 10 with label 20 and sends it to router R3. Router R3 swaps label 20 with label 30 and sends it to router R4. Router R4 removes label 30 from the packet and forwards it based on its IP header or another label in the stack.

Therefore, the answer is D.

References: MPLS Label Switching | MPLS Operation | Push, Swap,Push ⋆ IPCisco, A Complete Guide to Multiprotocol Label Switching (MPLS) - G2, Multiprotocol Label Switching - Wikipedia