Dynamic Route Flashcards
What is Dynamic routing protocols ?
• When a routing protocol is used, routers automatically advertise their best paths to known networks to each other.
• Routers use this information to determine their own best path to the known destinations.
• When the state of the network changes, such as a link going down or a new subnet being added, the routers update each other.
• Routers will automatically calculate a new best path and update the routing table if the network changes.
Summary routes in dynamic routing protocols
• Summary routes lead to less memory usage in routers as their routing tables contain less routes
• They also lead to less CPU sage as changes in the network only affect other routers in the same area
Dynamic Routing Protocols vs Static Routes
• Routing protocols are more scalable than administrator defined static routes.
• Using purely static routes is only feasible in very small environments.
• The routers automatically advertise available subnets to each other without the administrator having to manually enter every route on every router.
• If a subnet is added or removed the routers will automatically discover that and update their routing tables.
• If the best path to a subnet goes down routers automatically discover that and will calculate a new best path if one is available.
• Using a combination of a dynamic routing protocol and static routes is very common in real world environments.
• In this case the routing protocol will be used to carry the bulk of the network information.
• Static routes can also be used on an as needed basis. For example for backup purposes or for a static route to the Internet (which will typically be injected into the dynamic routing protocol and advertised to the rest of the routers.)
RIP protocol command
router rip
version 2
no auto-summary
network 10.0.0.0|
Routing protocols type
• Routing protocols can be split into two main types:
• Interior gateway protocols (IGPs)
• Exterior gateway protocols (EGPs)
• Interior gateway protocols are used for routing within an organisation
• Exterior gateway protocols are used for routing between organisations
over the Internet
• The only EGP in use today is BGP (Border Gateway Protocol)
Routing Protocols in Real Life
- Interior Gateway Protocols (IGPs) – Used Inside an Organization
• IGPs are used within a company, university, or government network to manage internal traffic.
• Example: A large company like Google has multiple offices and data centers worldwide. Inside each office, routers need to communicate efficiently. They use IGPs like:
• OSPF (Open Shortest Path First) – Used in many corporate networks for dynamic routing.
• EIGRP (Enhanced Interior Gateway Routing Protocol) – Used by Cisco-based networks.
• RIP (Routing Information Protocol) – Older but sometimes used in smaller networks. - Exterior Gateway Protocols (EGPs) – Used Between Organizations
• EGPs manage traffic between different organizations over the internet.
• The only EGP used today is BGP (Border Gateway Protocol).
• Example:
• Imagine a person in Germany visits a website hosted in the USA.
• The request must travel across different ISPs (Internet Service Providers) like Deutsche Telekom, AT&T, and Cloudflare.
• These ISPs use BGP to exchange routing information and find the best path for the data.
Why is This Important?
• IGPs keep internal networks running smoothly. Example: Your company’s internal emails and shared files don’t need to go through the public internet.
• BGP (EGP) keeps the internet connected. Without BGP, different networks wouldn’t know how to send data to each other.
- Interior Gateway Protocols (IGPs) – Inside an Organization
Example 1: A Company’s Office Network
Imagine you work at Siemens in Berlin. The office has:
• Hundreds of computers
• Wi-Fi routers
• Servers for emails, files, and printers
The company needs routing to send data efficiently between departments.
• They use OSPF (Open Shortest Path First) to find the best way between offices.
• If one router fails, OSPF quickly finds another route.
• This prevents delays when employees send emails or access shared documents.
👉 Without OSPF or EIGRP, internal communication would be slow and inefficient.
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Example 2: A University Campus
A large university like TU Berlin has:
• Multiple buildings
• Wi-Fi hotspots
• A data center for student records
They need routing inside the university to:
• Manage internet access
• Connect different faculties (IT, Engineering, Business)
They use IGPs like OSPF or RIP to ensure smooth internal connectivity.
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- Exterior Gateway Protocols (EGPs) – Between Organizations
Example 3: How Google and Facebook Communicate Over the Internet (BGP in Action)
When you open Facebook.com, your request travels across multiple ISPs.
What Happens?
1. You type facebook.com in Berlin.
2. Your internet provider (e.g., Deutsche Telekom) checks BGP routes to Facebook’s servers.
3. BGP finds the best path:
• It might go through Cloudflare (a global network provider).
• Then pass through AT&T (a U.S. provider).
• Finally reach Facebook’s data center in the U.S.
4. Your page loads in seconds because BGP chose the fastest route!
👉 Without BGP, there would be no global internet connectivity!
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Example 4: What Happens When BGP Fails?
In June 2021, Facebook, WhatsApp, and Instagram went down worldwide.
• The reason? Facebook accidentally deleted BGP routes that connected them to the internet.
• No ISP knew how to reach Facebook, so all services were offline for six hours.
👉 This proves how important BGP is for the internet.
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Summary
• IGP (like OSPF, EIGRP, RIP) = Used inside a company, university, or data center.
• EGP (like BGP) = Used between organizations, ISPs, and on the global internet.
• BGP connects the world—without it, major websites and businesses would be unreachable!
Interior Gateway Protocols
• Interior gateway protocols can be split into two main types:
• Distance Vector routing protocols
• Link State routing protocols
Distance Vector Routing Protocols
• In Distance Vector protocols, each router sends its directly connected neighbours a list of all its known networks along with its own distance to each of those networks
• Distance vector routing protocols do not advertise the entire network topology
• A router only knows its directly connected neighbours and the lists of networks those neighbours have advertised. It doesn’t have detailed topology information beyond its directly connected neighbours
• Distance Vector routing protocols are often called ‘Routing by rumour’
Link State Routing Protocols
• In Link State routing protocols, each router describes itself and its interfaces to its directly connected neighbours
• This information is passed unchanged from one router to another
• Every router learns the full picture of the network including every router, its interfaces and what they connect to
Routing protocols abbreviations
RIP
EIGRP
OSPF
IS-IS
BGP
RIP: Routing Information Protocol
EIGRP: Enhanced Interior Gateway Routing Protocol
OSPF: Open Shortest Path First
IS-IS: Intermediate System - Intermediate System
BGP: Border Gateway Protocol:
Dynamic Routing Protocols type
2- Exterior Gateway Protocols (EGPs)
#Path VectorRouting Protocol
>BGP
1- Interior Gateway Protocols (IGPs)
# Distance Vector Routing Protocol
>RIP
Advanced EIGRP
#Link State Routing Protocol
>OSPF
>IS-IS
Deference between Interior Gateway Protocols
• All of the IGPs do the same job, which is to advertise routes within an organisation and determine the best path or paths
• An organisation will typically pick one of the IGPs
• If an organisation has multiple IGPs in effect (for example because of a merger), information can be redistributed between them. This should generally be avoided if possible
- Distance Vector Protocols
• Examples: RIP (Routing Information Protocol), IGRP (Interior Gateway Routing Protocol)
• How they work: Routers share information with their directly connected neighbors and rely on hop count or a similar metric to determine the best route.
• Key characteristics:
• Periodic updates (broadcasting entire routing table)
• Slower convergence (risk of routing loops)
• Simpler configuration
• Less scalable for large networks - Link-State Protocols
• Examples: OSPF (Open Shortest Path First), IS-IS (Intermediate System to Intermediate System)
• How they work: Each router builds a complete map of the network topology and calculates the shortest path using Dijkstra’s algorithm.
• Key characteristics:
• Faster convergence
• Uses Link-State Advertisements (LSAs) to exchange information
• More scalable and efficient for large networks
• Requires more memory and processing powerDistance Vector (e.g., RIP) Link-State (e.g., OSPF) Convergence Speed. Slower. Faster Routing Updates Entire routing table. Only topology changes Scalability. Limited (small networks). High (large networks) Memory & CPU Usage. Lower. Higher Algorithm Use. Bellman-Ford. Dijkstras Algorithm
What is metrics in routing?
• A router may receive multiple possible paths to get to a destination network
• Only the best path will make it into the routing table and be used
• The different IGPs use different methods to calculate the best path to a destination network
• Each possible path will be assigned a ‘metric’ value by the routing protocol which indicates how preferred the path is
• The lowest metric value is preferred
• Distance Vector routers advertise to each other the networks they know about, and their metric to get to each of them
• Link State routers advertise all the links in their area of the network to each other
• Each router will take this information and then make an independent calculation of its own best path to get to each destination
• If the best path to a destination is lost (for example because a link went down) it will be removed from the routing table and replaced with the next best route
RIP Metric - Hop Count
• RIP uses Hop Count as the metric
• The maximum hop count by default is 15. Paths which are more than 15 hops away are marked as unreachable
• RIP is typically used only in small or test environments
OSPF Metric - Cost
• OSPF uses ‘Cost’ as the metric, which is automatically derived from interface bandwidth by default
• You can manually configure the cost of links if you want to manipulate the path
OSPF Metric (Cost) Explanation
In OSPF (Open Shortest Path First), the cost is the metric used to determine the best path to a destination. It is based on the bandwidth of the links rather than hop count (as in RIP). Lower cost values indicate better paths.
How is OSPF Cost Calculated?
The cost of an OSPF link is calculated using the formula:
Cost= Reference Bandwidth/ Interface Bandwidth
• Reference Bandwidth: Default is 100 Mbps in OSPF, but it can be changed.
• Interface Bandwidth: The actual bandwidth of the link in Mbps.
Default OSPF Costs for Common Link Speeds
Interface Bandwidth
OSPF Cost (Default)
10 Mbps
10
100 Mbps
1
1 Gbps
1
10 Gbps
1
How OSPF Uses Cost in Routing
• OSPF sums up the costs of all links in a path to determine the shortest path.
• The route with the lowest total cost is chosen as the best route.
• If multiple paths have the same cost, OSPF can use equal-cost multipath (ECMP) to load-balance traffic.
IS-IS Metric - Cost
• IS-IS also uses ‘Cost’ as the metric, but it is not automatically derived from interface bandwidth. All links have an equal cost by default
• You can manually configure the cost of links if you want to manipulate the path
• If you do not manually set the link costs then the path with the lowest hop count will be used
EIGRP Metric
• EIGRP uses the bandwidth and delay of links to calculate the metric
• (Load and reliability can also be considered but are ignored by default)
• A fixed delay value is used based on the interface bandwidth, the protocol does not dynamically measure current delay
• You can manually configure the delay on links if you want to manipulate the path
Choosing a Routing Protocol
• RIP uses hop count and has a default maximum metric of 15. It is not usually used in production networks because of its scalability limitations.
• EIGRP is very simple to maintain, calculates changes very quickly and its metric calculation will normally choose the best path by default. It is typically only supported on Cisco routers however.
• OSPF’s metric calculation will typically choose the best path by default. It is an open standard which is supported by all vendor’s routers and is the most commonly deployed IGP today. It is however more complicated to maintain than EIGRP.
• IS-IS links need to be manually configured or it will use hop count to determine the best path. It is typically only used in Service Provider networks or large organisations with their own MPLS network who choose it because of its scalability.
