C y D there is some documentation using bgp about Ce
and PE, CE doesn't need support NSR, then C describe peer PE (CE) does not need...
The BGP Support for Nonstop Routing (NSR) with Stateful Switchover (SSO) feature enables provider edge (PE) routers to maintain Border Gateway Protocol (BGP) state with customer edge (CE) routers and ensure continuous packet forwarding during a Route Processor (RP) switchover or during a planned In-Service Software Upgrade (ISSU) for a PE router. CE routers do not need to be Nonstop Forwarding (NSF)-capable or NSF-aware to benefit from BGP NSR capabilities on PE routers. Only PE routers need
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-sy/iro-15-sy-book/iro-nsr-ospf.pdf
B is wrong in my opinion. NSF does use a extension in OSPF, but not NSR.
B & C
Although OSPF Nonstop Routing (NSR) serves a similar function to OSPF Nonstop Forwarding (NSF), it
works differently. With NSF, OSPF on the newly active standby RP initially has no state information. OSPF
uses extensions to the OSPF protocol to recover its state from neighboring OSPF devices. For the recovery
to work, the neighbors must support the NSF protocol extensions and be willing to act as “helpers” to the
device that isrestarting.
With NSR, by contrast, the device that performs the switchover preserves its state internally, and in most cases the neighbors are unaware of the switchover. Because assistance is not needed from neighboring devices, NSR can be used in situations where NSF cannot be used; for example, in networks where not all neighbors
implement the NSF protocol extensions, or where network topology changes during the recovery making NSF unreliable, use NSR instead of NSF.
Question is about NSR, thus it must be C and D.
Seems to be B and C. https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/switches/lan/catalyst9600/software/release/17-3/configuration_guide/rtng/b_173_rtng_9600_cg/b_173_rtng_9600_cg_chapter_0101010.html.xml
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-sy/iro-15-sy-book/iro-nsr-ospf.pdf
• OSPF nonstop routing (NSR) can significantly increase the memory used by OSPF during certain phases of its operation. CPU usage also can be increased. You should be aware of router memory capacity and estimate the likely memory requirements of OSPF NSR. For more information see Configuring OSPF NSR.For routers where memory and CPU are constrained you might want to consider using OSPF NSF instead.
I think poorly written but the answer are correct C,D
https://www.cisco.com/c/dam/en/us/products/collateral/ios-nx-os-software/high-availability/product_data_sheet0900aecd8044e0ff.pdf
CE routers do not need to be NSF-capable or NSF-aware to benefit from BGP NSR capability on PE routers
C is for sure,, I am not sure for the second answer...
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