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300-510 · Question #223

SIMULATION Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access the de

The correct answer is C. P1(config-isis-if-af)# prefix-sid index 10 PE2(config)# segment-routing local-block 15000 15999 P2(config-isis-if-af)# prefix-sid index 50. There is a notable structural issue with this question worth flagging before the explanation: the simulation task describes OSPF configuration (point-to-point interfaces, cost, hello/dead timers), but all four answer choices are IS-IS Segment Routing commands (prefix-sid, adjacen

MPLS and Segment Routing

Question

SIMULATION Guidelines This is a lab item in which tasks will be performed on virtual devices. - Refer to the Tasks tab to view the tasks for this lab item. - Refer to the Topology tab to access the device console(s) and perform the tasks. - Console access is available for all required devices by clicking the device icon or using the tab(s) above the console window. - All necessary preconfigurations have been applied. - Do not change the enable password or hostname for any device. - Save your configurations to NVRAM before moving to the next item. - Click Next at the bottom of the screen to submit this lab and move to the next question. - When Next is clicked, the lab closes and cannot be reopened. Topology Tasks Troubleshoot and configure OSPF according to the topology to achieve these goals: 1. R1 and R2 must not have OSPF BR/DR election over their E0/0 interfaces and must not advertise the OSPF host route for the OSPF connected neighbors. 2. E0/0 interfaces on R1 and R2 to be preferred over E0/1 interfaces for OSPF traffic. Only the numerical value of 15 is allowed. 3. Set the OSPF hello interval to 5 and the OSPF dead interval to 10 between R1 and R2 on interface E0/1. Answer:

Exhibit

300-510 question #223 exhibit

Options

  • AP1(config-isis-if-af)# prefix-sid index 15010 PE2(config)# segment-routing local-block 16000 16999 PE2(config-isis-if-af)# prefix-sid index 16050
  • BP1(config-isis-if-af)# adjacency-sid index 15010 PE2(config)# segment-routing local-block 50000 50099 PE2(config-isis-if-af)# adjacency-sid index 50050
  • CP1(config-isis-if-af)# prefix-sid index 10 PE2(config)# segment-routing local-block 15000 15999 P2(config-isis-if-af)# prefix-sid index 50
  • DPE1(config-isis-if-af)# adjacency-sid index 10 P2(config)# segment-routing local-block 30000 30999 P2(config-isis-if-af)# adjacency-sid index 50

How the community answered

(14 responses)
  • A
    7% (1)
  • B
    7% (1)
  • C
    71% (10)
  • D
    14% (2)

Explanation

There is a notable structural issue with this question worth flagging before the explanation: the simulation task describes OSPF configuration (point-to-point interfaces, cost, hello/dead timers), but all four answer choices are IS-IS Segment Routing commands (prefix-sid, adjacency-sid, SRGB/SRLB). These appear to be from two different questions that were accidentally merged.

That said, evaluating the choices purely on IS-IS Segment Routing merit:

Why C is correct: Option C correctly uses prefix-sid index to assign globally significant node SIDs to prefixes, which is the proper mechanism for SR-MPLS reachability in IS-IS. The local-block range 15000 15999 is a valid SRLB configuration, and the small index values (10 and 50) are valid.

Why the others are wrong:

  • A assigns prefix-sid index 15010 to P1, but the SRGB defined on PE2 starts at 16000, meaning index 15010 falls outside the global block - a misconfiguration that breaks SR label allocation.
  • B uses adjacency-sid index, which creates locally significant link SIDs, not globally routable node/prefix SIDs. This is the wrong SID type for end-to-end SR paths.
  • D also uses adjacency-sid on PE1 and P2 - same fundamental error as B; adjacency SIDs cannot substitute for prefix SIDs in SR forwarding.

Memory tip: "Prefix SID = global reachability; Adjacency SID = local link." Whenever a question involves routing traffic to a node end-to-end via SR, prefix-sid is correct. adjacency-sid only matters for explicit link-level steering (SR-TE with specific hop pinning).

Note for your exam prep: If you see this question on the actual exam, verify whether the simulation task and the answer choices are genuinely about OSPF or SR/IS-IS - they may appear on separate items.

Topics

#Segment Routing#ISIS#Prefix-SID#Local-block

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