450 5-5.3

Data Plane

The part of the network layer that physically moves packets from a router's input to its output. It's the "doing" part — actually forwarding the packet right now, at this router.

Control Plane

The part of the network layer that decides which route packets should take across the entire network. It's the "thinking" part — figuring out the best path before the packet even arrives.

Per-Router Control

The traditional approach where every router has its own routing brain. Each router runs its own routing algorithm and talks to nearby routers to figure out the best routes on its own.

SDN (Software-Defined Networking) Control

A newer approach where one powerful remote controller figures out all the routes for every router and tells each one exactly what to do. The routers just follow instructions instead of thinking for themselves.

Routing Protocol

A set of rules that routers follow to communicate with each other and figure out the best path to send packets from any source to any destination.

Link Cost

A number assigned to each connection between routers that represents how "expensive" it is to use that link. Cost can mean distance, speed, congestion level, or whatever the network operator decides.

Link-State Algorithm

A type of routing algorithm where every router gets a complete map of the entire network (all routers and all link costs), then uses that map to calculate the best paths.

Distance Vector Algorithm

A type of routing algorithm where each router only knows the costs to its immediate neighbors. Routers share their cost estimates with neighbors and slowly learn the best paths over many rounds.

Dijkstra's Algorithm

A step-by-step method used in link-state routing to find the cheapest path from one source router to every other router in the network.

D(v) in Dijkstra's

The current best estimate of the total cost to reach router v from the source. Gets updated every time a cheaper path is discovered.

p(v) in Dijkstra's

The predecessor — the router that comes just before v on the current best path from the source.

N' in Dijkstra's

The set of routers whose shortest path from the source has been fully confirmed and will not change.

Complexity of Dijkstra's Algorithm

O(n²) — meaning for n routers, it takes roughly n² comparisons to complete.

Oscillation Problem (Link-State)

When link costs depend on traffic levels, routers can keep flip-flopping their routes in response to each other.

Bellman-Ford Equation

Dx(y) = min over all neighbors v of { cost(x to v) + Dv(y) }.

Convergence (Distance Vector)

When all routers in the network have finished updating and their distance estimates are stable and correct.

Count-to-Infinity Problem

A bug in distance vector routing where bad news spreads extremely slowly.

Poisoned Reverse

A partial fix for count-to-infinity where a router lies to its neighbor about its costs.

Good News Travels Fast (DV)

When a link cost decreases, the improvement spreads quickly through the network.

Autonomous System (AS)

A group of routers all under the control of one organization.

Gateway Router

The border router of an AS that has connections to routers in other autonomous systems.

Intra-AS Routing

Routing that happens inside a single autonomous system.

Inter-AS Routing

Routing that happens between different autonomous systems.

OSPF (Open Shortest Path First)

The most common intra-AS routing protocol.

Link-State Advertisement (LSA)

A message a router sends out in OSPF announcing its own links and their costs.

Hierarchical OSPF

A scaled-down version of OSPF where the network is split into local areas.

Area Border Router (OSPF)

A router that summarizes routing information from its area to the backbone.

BGP (Border Gateway Protocol)

The protocol used for inter-AS routing.

eBGP (External BGP)

A BGP session between gateway routers in two different autonomous systems.

iBGP (Internal BGP)

A BGP session between routers inside the same autonomous system.

AS-PATH (BGP Attribute)

A BGP route attribute that lists every autonomous system the route advertisement has passed through.

NEXT-HOP (BGP Attribute)

A BGP route attribute that tells a router the IP address of the first router to send packets to.

BGP OPEN Message

The first BGP message sent to open a TCP connection.

BGP UPDATE Message

A BGP message that advertises a new available route or withdraws an old one.

BGP KEEPALIVE Message

A BGP message sent periodically to tell the neighbor "I'm still here."

BGP NOTIFICATION Message

A BGP message sent when an error occurs.

BGP Route Selection Order

The order routers use when picking among multiple paths.

Hot Potato Routing

A BGP strategy where a router forwards a packet to get it out quickly.

BGP Policy-Based Routing

The idea that ISPs use BGP rules to control which routes they advertise.

Why Intra-AS and Inter-AS Routing Are Different

Intra-AS focuses on performance while Inter-AS focuses on policy.

Forwarding Table

The lookup table inside a router that maps destination addresses to outgoing links.

Link-State vs Distance Vector — Key Difference

Link-State routers compute routes independently while Distance Vector routers rely on neighbors.