Routing
Build and update tables
Routing finds shortest or best paths from each source to destinations, then fills and updates routing tables.
Role
What Routing Protocols Are Responsible For
Routing protocols are network-layer software that implement routing algorithms and support forwarding decisions.
Forwarding
Use the table
Forwarding decides which output interface an incoming packet should use by referring to the routing/forwarding table.
Hierarchy
Why Internet routing is hierarchical
The Internet has too many routers and hosts for one flat table. Hierarchical routing divides the problem into manageable parts.
Mini summary
- Routing computes paths.
- Forwarding moves packets.
- Internet routing is hierarchical.
Graph
Modeling The Network As A Graph
Routing algorithms often treat the network as a graph so costs can be calculated systematically.
Nodes and edges
Graph representation
Routers are graph nodes. Physical links are edges. Each edge has a cost such as delay or bandwidth-based cost.
Cost
Shortest means least cost
The shortest path is the path with the least total cost. It does not always mean the fewest hops unless hop count is the metric.
Tree
Least-cost tree
A least-cost tree shows the lowest-cost paths from one router to destinations in the routing domain.
Mini summary
- Graphs model routers and links.
- Costs define best paths.
- Least-cost paths feed routing tables.
Tables
Forwarding Tables And Default Route
Destination-address forwarding applies masks to destination IP addresses and compares results with table entries.
Network-specific
Network route
A network-specific route lists a destination network address. It matches many hosts in that network.
Host-specific
Host route
A host-specific route lists one destination host address. It is more specific than a general network route.
Default
0.0.0.0/0
The default route is used when no more specific route is found. The route 0.0.0.0/0 can match any destination and acts as fallback.
Mini summary
- Network routes match groups.
- Host routes match one host.
- Default route is fallback.
Algorithms
Static, Distance Vector, And Link State
Routing algorithms differ in how routers learn and update route information.
Static
Precomputed or manually configured
Static routes change slowly and can be entered by an administrator or precomputed offline.
Distance vector
Neighbor distance exchange
Each node maintains a distance vector table containing distances to destinations. Routers share distance information with neighbors and update minimum costs.
Link state
Shared network map
Each router measures the cost to directly connected neighbors, creates link-state packets, floods them, and builds a link-state database. Routers then compute shortest paths.
| Concept | Meaning | Exam clue |
|---|---|---|
| Distance vector | Sends distance information to neighbors | Think table sharing |
| Link state | Floods link facts and builds LSDB | Think map building |
| Static | Configured and slow-changing | No automatic adaptation |
Mini summary
- Static is simple but manual.
- DV shares distances with neighbors.
- LS builds a shared database.
Internet
Autonomous Systems And Routing Protocols
The Internet is divided into autonomous systems to make global routing manageable.
AS
Autonomous system
An AS is a single network or group of networks under one administrator and assigned a globally unique AS number.
Interior
Inside one AS
Interior routing protocols operate inside one autonomous system. Examples listed in the slides include RIP, OSPF, and EIGRP.
Exterior
Between ASs
Exterior routing connects autonomous systems. BGP is the main protocol between ASs and border gateway routers.
Mini summary
- AS means one administrative domain.
- Interior protocols work inside AS.
- BGP works between ASs.
Revise
Final Cheat Sheet
Use this as the last-pass memory page before a quiz or exam.
- Routing Finds paths and fills tables.
- Forwarding Chooses output interface.
- Graph node Router.
- Graph edge Physical link.
- Default route 0.0.0.0/0.
- Network route Matches destination network.
- Host route Matches one host.
- DV Neighbor distance exchange.
- LS Flood link-state facts.
- AS Administrative routing domain.
- Interior RIP, OSPF, EIGRP.
- Exterior BGP.
Practice
Practice Questions With Answers
These questions target the facts, comparisons, and calculations that are easiest to test.
What does 0.0.0.0/0 represent?
Answer: The default route.
Explanation: A /0 mask can match any destination when no specific route matches.
What is the difference between distance vector and link state?
Answer: DV shares distance tables with neighbors; LS floods link facts and builds a shared database.
Explanation: They learn topology information differently.
Do hosts need full forwarding tables?
Answer: No, hosts normally send to a default router.
Explanation: Routers perform most forwarding decisions.
Which protocol is exterior routing?
Answer: BGP.
Explanation: BGP routes between autonomous systems.
What does shortest path mean?
Answer: Least total cost according to the metric.
Explanation: Cost may not equal hop count.