Q1

Which statement is correct?

1. In a datagram network, routers keep per-flow state, while virtual-circuit routers are stateless.
2. In a datagram network, routers forward by destination address lookup; in a virtual-circuit network, they forward by swapping a label per hop.
3. Virtual-circuit and datagram approaches cannot be combined in one network.
4. In a datagram network, the source must choose the entire path using source routing.

B - Briefly: datagram routers do a destination-IP lookup each hop; VC routers switch on a short per-hop label/VCI set up during connection establishment.

Q2

In a pure link-state routing protocol, after a single link cost change, what must happen before all routers can compute new shortest paths?

1. Only the routers adjacent to the change update their forwarding tables; no flooding is needed.
2. The router(s) detecting the change flood an updated LSP; once everyone has a consistent LS database, each runs Dijkstra.
3. A route-poisoning message propagates hop-by-hop and routers update distances iteratively.
4. Each router queries neighbors for their forwarding tables (path-vector style).

B - In link-state, the change is turned into an updated LSP and flooded; once everyone’s LSDB matches, each router reruns Dijkstra.

Q3

In distance-vector routing, which technique specifically guarantees that a simple two-node loop after a link failure is avoided?

1. Split horizon
2. Split horizon with poisoned reverse
3. Triggered updates
4. Holddown timers