CCNAv2 - Module 14, Routing Concepts

What is the purpose of a dynamic routing protocol?

A dynamic routing protocol is used to perform network discovery and maintain the routing table. It also chooses the best path, and chooses a new best path when there is a topology change.

What is a stub network?

A network that has only one entry and exit point

Explain how CEF works

Cisco Express Forwarding is a layer 3 routing technology used to enhance the performance of layer 3 routing by reducing the amount of time it takes to packet match. CEF uses 2 data structures: FIB The Forwarding Information Base is somewhat of a cached mirror of the routing table, that only stores the next hop for an address. Adjacency table The adjacency table maintains layer 2/switching informaton for FIB entries. This eliminates the need to conduct an ARP request for each table lookup. The adjacency table is pre-populateed by the ARP table.

Explain the difference between an IGP and EGP

Interior Gateway Protocol (IGP) is a Routing Protocol which is used to find network path information within an Autonomous System. Exterior Gateway Protocol (EGP) is a Routing Protocol which is used to find network path information between different Autonomous Systems.

What are the 3 primary advtanges of static routing?

It provides ease of routing table maintenance in smaller networks that are not expected to grow significantly. It uses a single default route to represent a path to any network that does not have a more specific match with another route in the routing table. It routes to and from stub networks.

Explain the difference between the control plane and the data plane

Control Plane Refers to functions/processes that determine what path should be used to send a packet. This includes building the routing table and how packets should be forwarded Data Plane Refers to functions/processes that actually forward a packet out an interface based off of control plane logic. Dependent on control plane Refer here: https://www.geeksforgeeks.org/difference-between-control-plane-and-data-plane/

Explain the 3 main components of a routing protocol: Data structures, routing protocol messages, and algorithms.

Data structures - Routing protocols typically use tables or databases for their operations. This information is kept in RAM. Routing protocol messages - Routing protocols use various types of messages to discover neighboring routers, exchange routing information, and other tasks to learn and maintain accurate information about the network. Algorithm - An algorithm is a finite list of steps used to accomplish a task. Routing protocols use algorithms for facilitating routing information and for the best path determination.

What are the 3 route types? (i.e. directly connected, etc.)

Directly Connected Networks Networks that are configured on the interfaces of the router Remote Networks Networks that are not directly connected to the router Configured statically or through a dynamic routing protocol Default Route Next-hop when there are no matches Default for IPv4 is 0.0.0.0/0, IPv6 is ::/0 If there are no routes with a longer match than 0 bits, then the default route is used to forward the packet

What are local routes in the routing table? What are their purpose?

Local routes simply refer to the interface address of a router on a given network. The purpose of a local route is it allows the router to efficiently determine when it receives a packet for the interface instead of being forwarded. Prefix of /32 and /128 for ipv4 and ipv6

Explain the concept of longest match

Longest match is the method routers use to determine which route entry to use when forwarding a packet. Longest match works by analyzing the destination IP address of a packet in binary form. A router compares this binary digit value to route entries. The route entry that matches the highest number of binary digits, starting from the left, will be the matching route. Note, longest match only compares binary digits that are masked by the prefix. For example Destination IP: 2001:db8:acad:8::1 - 2001:db8:acad:8::/48 - 2001:db8:acad:8::/64 - 2001:db8:acad:8::/90 The matching route entry would be the middle one, since this route entry has the highest number of matching leftmost bits that are masked by the prefix

Explain the difference between process switching, fast switching, and CEF

Process Switching When a packet arrives on an interface, it is forwarded to the control plane where the CPU matches the destination address with an entry in its routing table, and then determines the exit interface and forwards the packet Inefficient since this is done for every single packet Fast Switching uses a fast-switching cache to store next-hop information. when a packet arrives on an interface, it is forwarded to the control plane where the CPU searches for a match in the fast-switching cache. If it is not there, it is process-switched and forwarded to the exit interface CEF (Cisco Express Forwarding) CEF builds a Forwarding Information Base (FIB), and an adjacency table table entries are not packet-triggered like fast switching but change-triggered, such as when something changes in the network topology Refer to the figure Common Analogical Comparison Process switching solves a problem by doing math long hand, even if it is the identical problem that was just solved. Fast switching solves a problem by doing math long hand one time and remembering the answer for subsequent identical problems. CEF solves every possible problem ahead of time in a spreadsheet.

What is a metric? What is the metric based on for RIP, OSPF, and EIGRP?

Quantitative value used to determine the distance to a network RIP: The metric is "hop count". Each router along a path adds a hop to the hop count. OSPF: The metric is "cost" which is the based on the cumulative bandwidth from source to destination. Faster links are assigned lower costs compared to slower (higher cost) links. EIGRP: It calculates a metric based on the slowest bandwidth and delay values. It could also include load and reliability into the metric calculation.

Explain the structure of an IP routing table entry For example R 172.16.4.0/28 [120/2] via 209.165.200.226, 00:00:12, Serial0/0/0

Route source: Identifies how the route was learned. Destination network: Identifies the address of the remote network. Administrative distance: Identifies the trustworthiness of the route source. Metric: Identifies the value assigned to reach the remote network. Lower values indicate preferred routes. Next hop: Identifies the IPv4 address of the next router to forward the packet to. Route timestamp: Identifies from when the route was last heard. Outgoing interface: Identifies the exit interface to use to forward a packet toward the final destination.

What is distance vector, link-state, and path vector?

These simply refer to routing algorithms using by different protocols to determine a best path route.

Why are IPv4 route entries indented?

This has to do with classful IP addressing which was developed in the 1980s. Parent routes (non-indented lines) are simply classful network addresses that are one of the A, B, or C classes Child routes (indented lines) are subnets of one of the classful addresses.

What is administrative distance?

When an identical route is learned from 2 routing sources (static and dynamic, or 2 dynamic protocols), the administrative distance tells the router which learned route to use. A lower administrative distance means a higher precedence

What is load balancing? What routing protocol supports unequal cost load balancing?

When there are 2 equal cost paths to a destination network, packets are forwarded across both paths equally Only EIGRP supports unequal cost load balancing Static routes also use equal cost loading balancing