Refer to the exhibit. A network administrator implemented MPLS routing between routers R1, R2, R3, and R4. AToM is configured between R1 and R4 to allow
Layer 2 traffic from hosts on RA1 and RA2. A targeted MPLS session is established between R1 and R4. Which additional action must the administrator take on all routers so that LDP synchronization occurs between connected LDP sessions?
To ensure LDP synchronization between connected LDP sessions, configure OSPF or IS-IS as the routing protocol. LDP-IGP synchronization is designed to work specifically with OSPF and IS-IS, ensuring that LDP and the IGP are synchronized before any routing updates are processed. This helps to prevent routing loops and blackholing of traffic.
Restrictions for MPLS LDP IGP Synchronization • This feature does not support targeted Label Distribution Protocol (LDP) sessions. Therefore, Any Transport over MPLS (AToM) sessions are not supported. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_ldp/configuration/xe-3s/mp-ldp-xe-3s-book/mp-ldp-igp-synch.pdf
Thanks chst! Another bizarre question not covered in the Cisco's official SP training material
what is the Answer for this question A or B ?
what is the Answer for this question A or B ?
If we configure OSPF or IS-IS (Answer B) on all routers, how does LDP IGP synchronization work when there is the AToM and a targeted MPLS session between R1 and R4? Cisco say "This feature does not support targeted Label Distribution Protocol (LDP) sessions. Therefore, Any Transport over MPLS (AToM) sessions are not supported."? Can somebody explain this?
I don't think it matters because the question is asking for LDP sync "between connected LDP sessions", not targeted sessions. So I don't think the restriction applies in this scenario.
The answer is B because LDP IGP Sync is only supported on OSPF and IS-IS enabled interfaces. With EIGRP not supported, answer C is incorrect. LDP-IGP sync works like this: When a new LDP neighbor is discovered on an interface, LDP checks if the remote LDP neighbor loopback IP-address is reachable via any other interface. - If yes, LDP is first established via that other interface before IGP is allowed to setup an adjacency on this interface. - If no, the IGP adjacency is established on the interface to learn the route for the LDP neighbor loopback address, but the link is advertised with max-metric to minimize the risk of blackholing user traffic. Once LDP has established, the IGP readvertises the link with its normal metric, allowing traffic to flow over the link. Disabling the holddown only limits the length of time the IGP will wait before establishing a session on the interface, so answer A is incorrect. Sync delay is used to delay the "synchronized" message from LDP to the IGP. This is not necessary for LDP-IGP sync to function, so answer D is incorrect. https://www.cisco.com/c/en/us/td/docs/routers/ios/config/17-x/mpls/b-mpls/m_mp-ldp-igp-synch.html
I think OSPF or IS-IS must already be configured for targeted session to be established R1 - R4?
i think is D When LDP is fully established and synchronized, LDP checks the delay timer: If you configured a delay time, LDP starts the timer. When the timer expires, LDP checks that synchronization is still valid and notifies the Open Shortest Path First (OSPF) process. If you did not configure a delay time, if synchronization is disabled or down, or if an interface was removed from an IGP process, LDP stops the timer and immediately notifies the OSPF process. https://www.cisco.com/content/en/us/td/docs/ios-xml/ios/mp_ldp/configuration/15-s/mp-ldp-15-s-book/mp-ldp-igp-synch.html
I think the keyword used in the quesiton here is "connected LDP sessions" thus indicating the directly connected neighborships... therefore B would be the answer. I don't think its asking about targeted session. I think that is there to throw us off... Also none of the other answers really make sense imo.
If the LDP peer is reachable, the IGP waits indefinitely (by default) for synchronization to be achieved. To limit the length of time the IGP session must wait, enter the no mpls ldp igp sync holddown command. If the LDP peer is not reachable, the IGP establishes the adjacency to enable the LDP session to be established. A is correct
When the router has at least one IGP adjacency, the LDP peer will be reachable, the LDP session will be established, so the IGP session will go up eventually (no need to disable holddown). If the router is booting, it has no IGP session up, and the LDP peer is not reachable, the IGP session will go up in order to get the reachability to the LDP peer. If the router is booting and there is a static route that makes the router believe the the LDP peer is reachable, but it is actually not, then the IGP sessions will never go up. This is the case named "race condition" when you need to disable the holddown.
I asked the question to AI, and here is the answer: Microsoft Copilot answered: B. Configure OSPF or IS-IS as the routing protocol. Google Gemini answered: D. Enable MPLS LDP sync delay timers. ChatGpt answered: A. Disable the MPLS LDP IGP sync holddown. Stupid Cisco question!!
D: at least this "Enable MPLS LDP sync dely timers" ensure that it is enabling the LDP sync. B is not correct: Since there is already AToM, it presume that IGP is already there. Even, if we add iGP, still it doesn't say whether it is enabling the LDP sync. C is not correct: EIGRP and stub area have nothing to do with LDP sync A: it is also very close answer, since sync holddown is not enabled by default, why to bother
A is not mandatory to be configured for LDP IGP sync C is excluded D Is not mandatory to be configured for LDP IGP sync. Configuring the timer does not mean that it will enable the sync on the interface, because it is already enabled by default on the interface and disabled by default on the ISIS/OSPF.
This is the best answer as we need to activate the LDP Sync. B cannot be the correct answer, because based on the question, we are supposed to have the IGP implemented in the network.