Service Provider Routing and Switching, Specialist (JNCIS-SP)

The show system switchover command is only available on the backup Routing Engine. This is because the kernel-replication process daemon, which supports this command, runs only on the backup Routing Engine and not on the primary Routing Engine.

The three well-known mandatory BGP attributes are origin, AS path, and next hop. These attributes are recognized by all BGP peers, are passed to all peers, and are present in all BGP Update messages. Origin indicates the origin of the route (IGP, EGP, or incomplete), AS path lists the ASes the route has traversed, and next hop specifies the next-hop IP address that should be used to reach the destination.

The correct order of BGP attributes for active route selection is: next hop, local preference, AS path, origin, and then MED. This sequence is commonly used when determining the best path in BGP routing.

Learning and aging are the two bridging concepts used to maintain an Ethernet switching table. Learning allows a switch to record the source MAC address and the port it arrived on when a frame is received, helping to update and build the switching table. Aging ensures that the table remains current by removing entries that have not been refreshed within a certain period, thus preventing the table from becoming cluttered with outdated data and freeing up space for new entries.

The AS path attribute in BGP is used to detect routing loops. It contains the sequence of Autonomous System (AS) numbers that the route has traversed. A BGP router examines the AS path for its own AS number; if found, it will reject the route to prevent loops. This mechanism ensures that a BGP router does not receive a route that it had previously advertised.