Chapter 6

IP routing Essentials

1) What is an AS?

An AS or Autonomous System (AS) is a connected group of IP networks managed by a single administrative entity, such as a university, government or a commercial organization.

2) What is an IGP?

An IGP or Interior Gateway Protocol is protocol that works best for the communication between IP networks within a single AS.

E.g: RIP, OSPF, EIFRP, ISIS

3) What is an EGP?

An EGP or Exterior Gateway Protocol is protocol that is suited for the communication between ASs.

4) What is the primary function of an IP routing protocol?

The primary function of an IP routing protocol is ensuring data packets can travel efficiently from source to destination.

#CategoriesDistance VectorLink-State
1Algorithm UsedUses the Bellman-Ford algorithm to calculate the best path based on distance (usually hop count) from a router to the destination.Uses Dijkstra’s algorithm to compute the shortest path based on a complete map of the network.
2Routing InformationRouters only share information with directly connected neighbors, and routers do not have a full view of the network. Instead, they rely on cumulative information passed through the network.Each router has a complete view of the network by building and maintaining a full topology map of all routers within the network.
3UpdatesRouters periodically send updates whether there is a change on the network or not. Each update always contains the entire routing table to their neighbors.Routers only send updates when there is a change in the network topology (link state updates), and they only send the changed information, not the entire routing table.
4Convergence TimeGenerally slower to converge due to periodic updates and reliance on incremental changes from neighbors.Faster convergence as routers have a complete view of the network and can independently calculate the shortest paths when a change occurs.
5ScalabilityLess scalable, especially in large networks, because it doesn’t have a full view of the topology and relies on periodic updates.More scalable in large networks due to faster convergence and efficient, event-driven updates.
6Loop PreventionRequires mechanisms like split horizon, route poisoning, and hold-down timers to prevent routing loops.Since routers have a full network map, routing loops are naturally prevented.
7Resource UsageLess CPU and memory intensive, as routers maintain only neighbor information and routing tables.More resource-intensive, requiring more memory and CPU to maintain and process the entire network topology.
8ExamplesRIP (Routing Information Protocol). EIGRP (Enhanced Interior Gateway Routing Protocol) – Cisco proprietary but adds advancedOSPF (Open Shortest Path First), IS-IS (Intermediate System to Intermediate System)
9Route CalculationRouters rely on neighbors to calculate routes. Each router trusts the information received from its neighbors without verifying the entire network topology.Each router calculates routes independently based on the complete network topology, resulting in a more accurate and reliable route selection.
10Network Type SuitabilitySuitable for smaller or less complex networks where simplicity and minimal resource usage are prioritized.Better suited for large, complex networks where fast convergence and scalability are required.
6) What algorithm is used by distance vector routing protocols to determine the best path?

The Bellman-Ford algorithm.

It uses Dijkstra’s algorithm to calculate the shortest path by constructing a full map of the network.

8) Define a path vector algorithm and provide an example of a protocol that uses it.

The path vector algorithm is used to determine the best route to a destination by tracking the entire path that routing information has traversed from source to destination. BGP uses a path vector algorithm called AS_PATH.

9) What is the significance of prefix length in routing?

It defines the size of the network portion of an IP address and helps in selecting the most specific route for packet forwarding.

10) How does a router use administrative distance (AD) to select the best route?

Administrative distance is a value that indicates the trustworthiness of a route source. In other words, when you have multiple protocol running on the same router and each learning a path towards a specific destination, the RIB (Router Iformation Base) table will only accept the route from the protocol with the lowest AD as it is deemed the most trustworthy.

Route OriginDefault Administrative Distance
Directly connected interface0
Static route1
EIGRP Summary route5
External BGP (eBGP) route20
EIGRP Internal route90
OSPF route110
IS-IS route115
RIPv2 route120
EIGRP (External) route170
Internal BGP (iBGP) route200
11) What factors contribute to a route’s metric, and why is it important in path selection?

Factors include bandwidth, delay, load, and hop count; the metric determines the “cost” of a route, helping the router choose the most efficient path.

12) Describe the concept of static routing and when it is typically used in a network.

Static routing involves manually configuring routes; it’s used in small, simple networks, for backup routes in larger networks or on device with low ressouces.

13) What is a floating static route, and how does it differ from a regular static route?

A floating static route is a static route for which the default AD of 1 has been changed to a value higher than that of the primary route to serve as a backup route.

14) How is a static route configured to direct traffic to a null interface?

This command which discards traffic, often for security or summarization purposes.

ip route <destination> <mask> null0
15) What are some advantages of using IPv6 static routes over IPv4 static routes?

IPv6 provides simplified routing with auto-configuration features, larger address space, and better hierarchical network design.

16) Explain the concept of policy-based routing (PBR) and provide a real-world example of its use.

By default, the router makes forwarding decisions based on the destination IP address. But with PBR, the router can make forwarding decisions based on other factors such as:

  • source IP address
  • protocol type (ICMP, TCP, UDP, etc…)
  • destination IP address
17) What role does Virtual Routing and Forwarding (VRF) play in modern networks?

VRF is a technology used in networking to create multiple, isolated instances of a routing table within the same router or Layer 3 device. Each VRF instance operates as a separate and independent logical router, allowing different networks or tenants to use the same IP address ranges without interfering with each other. VRF is commonly used in multi-tenant environments, service provider networks, and large enterprise networks.

18) In what scenarios would you implement static routes to null interfaces?

To prevent routing loops or to discard unwanted traffic, typically in route summarization or security implementations.

19) What steps are involved in configuring a basic static route?

Identify the destination network, subnet mask, and next-hop address or exit interface, then configure with the ip route command.

20) How does floating static routing provide redundancy?

It offers a backup route with a higher administrative distance that becomes active only when the primary route fails.

21) Why is the administrative distance of a route important in the context of static routing?

It determines the preference of routes when multiple routes to the same destination exist, ensuring the most reliable one is used.

22) What are some common issues that might arise when implementing policy-based routing, and how can they be resolved?

Misconfigurations leading to traffic being incorrectly routed; resolving requires careful review of access lists and route-maps.

23) How does IP routing interact with VRF to enable multiple virtual routing tables within a single router?

Each VRF maintains a separate routing table, allowing the router to handle different routing domains without overlap.