Exam 350-401 topic 1 question 563 discussion

Here is excerpt from 31 days before your CCNP exam - Day 30, page 29 CISCO Nonstop Forwarding: "Instead of waiting on any configured Layer 3 routing protocols to converge and rebuild the FIB, a router can use NSF to get assistance from other NSF-aware neighbors. The neighbors then can provide routing information to the standby supervisor, allowing the routing tables to be assembled quickly. In a nutshell, the Cisco NSF functions must be built into the routing protocols on both the router that will need assistance and the router that will provide assistance.

D is correct

From kevin wallace's course: SSO = maintains config and state information between RPs (such as routing protocol neighbourships). However, when the active RP fails, the routing table is erased NSF = CEF Data plane tables (FIB and Adjacency table) are maintained after switchover so the backup RP can use the routing information to forward packets until the Control Plane tables such as RIB is rebuilt. Therefore: A: wrong. The Standby RP forwards process before (using NSF CEF info) and after RIB is rebuilt. B: wrong: although stanby RP is update immediately, the RIB has to be rebuilt ("churning the network") C: wrong: same reason as B D: correct. "Along known routes" because RIB will be rebuild and new routes might me learned.

Perhaps it the word temporary in answer A that is the issue.

To me, both A and D can be correct. So, it is immediate, but there is convergence going on too. Cisco NSF with SSO allows for the forwarding of data packets to continue along known routes while the routing protocol information is being restored following a switchover. NSF works with SSO to minimize the amount of time that a Layer 3 network is unavailable following an active device election by continuing to forward IP packets. Reconvergence of Layer 3 routing protocols (BGP, OSPFv2, and EIGRP) is transparent to the user and happens automatically in the background. Routing protocols recover routing information from neighbor devices and rebuild the Cisco Express Forwarding table.

D Cisco Nonstop Forwarding does not maintain a continuously active control plane during switchover. Instead, the forwarding plane uses known routes while the routing protocol information is being restored after switchover https://www.cisco.com/en/US/technologies/tk869/tk769/technologies_white_paper0900aecd801dc5e2.html

answer is correct reference: https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_01100110.pdf