Module 14: Routing Concepts

Routing

When a router receives an IP packet on one interface, it determines which interface to use to forward the packet to the destination. This is known as _________.

Router

uses its IP routing table to determine which path (route) to use to forward a packet. R1 and R2 will use their respective IP routing tables to first determine the best path, and then forward the packet.

Longest Match

The best path in the routing table is also known as the _________________.

Routing Table

contains route entries consisting of a prefix (network address) and prefix length.

greatest number of far-left

The longest match is the route in the routing table that has the ____________ matching bits with the destination IP address of the packet. The longest match is always the preferred route.

Directly Connected Networks

Added to the routing table when a local interface is configured with an IP address and subnet mask (prefix length) and is active (up and up).

Remote Networks

Networks that are not directly connected to the router.

Static Routes

Added to the routing table when a route is manually configured.

Dynamic Routing Protocols

Added to the routing table when routing protocols dynamically learn about the remote network.

Default Route

Specifies a next-hop router to use when the routing table does not contain a specific route that matches the destination IP address. It can be entered manually as a static route, or learned automatically from a dynamic routing protocol.

ingress interface

The data link frame with an encapsulated IP packet arrives on the ______________.

directly connected network

If the route entry indicates that the egress interface is a _____________, the packet can be forwarded directly to the destination device.

/0 prefix length.

A default route has a __ prefix length. This means that no bits need to match the destination IP address for this route entry to be used.

destination MAC address

To encapsulate the packet in the Ethernet frame, the router needs to determine the ____________ associated with the destination IP address of the packet.

Remote Network

If the route entry indicates that the destination IP address is on a _____________, meaning a device on network that is not directly connected.

next-hop address

The__________ is indicated in the route entry.

ARP and ICMPv6 Neighbor Discovery

If the forwarding router and the next-hop router are on an Ethernet network, a similar process _____________ will occur for determining the destination MAC address of the packet as described previously.

the packet will be dropped.

•If there is no match between the destination IP address and a prefix in the routing table, and if there is no default route, __________________.

packet forwarding

The primary responsibility of the ___________ function is to encapsulate packets in the appropriate data link frame type for the outgoing interface. For example, the data link frame format for a serial link could be Point-to-Point (PPP) protocol.

Process Switching

An older packet forwarding mechanism still available for Cisco routers. 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. It is important to understand that the router does this for every packet, even if the destination is the same for a stream of packets.

Fast Switching

An older packet forwarding mechanism which was the successor to process switching. It 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. The flow information for the packet is then stored in the fast-switching cache. If another packet going to the same destination arrives on an interface, the next-hop information in the cache is re-used without CPU intervention.

Cisco Express Forwarding(CEF)

The most recent and default Cisco IOS packet-forwarding mechanism. It builds a Forwarding Information Base (FIB), and an adjacency table. The table entries are not packet-triggered like fast switching but change-triggered, such as when something changes in the network topology. When a network has converged, the FIB and adjacency tables contain all the information that a router would have to consider when forwarding a packet.

pipe (|) character

To enable the filtering command, enter a ___________ after the show command and then enter a filtering parameter and a filtering expression.

section

This displays the entire section that starts with the filtering expression.

include

This includes all output lines that match the filtering expression.

exclude

This excludes all output lines that match the filtering expression.

begin

This displays all the output lines from a certain point, starting with the line that matches the filtering expression.

L

Identifies the address assigned to a router interface.

C

Identifies a directly connected network.

S

Identifies a static route created to reach a specific network.

O

Identifies a dynamically learned network from another router using the OSPF routing protocol.

*

This route is a candidate for a default route.

Route source

This identifies how the route was learned.

Destination network (prefix and prefix length) 

This identifies the address of the remote network.

Administrative distance

This identifies the trustworthiness of the route source. Lower values indicate preferred route source.

Metric

This identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Next-hop

This identifies the IP address of the next router to which the packet would be forwarded.

Route timestamp

This identifies how much time has passed since the route was learned.

exit interface

This identifies the egress interface to use for outgoing packets to reach their final destination.

C

A directly connected network is denoted by a status code of ___ in the routing table. The route contains a network prefix and prefix length.

L

The routing table also contains a local route for each of its directly connected networks, indicated by the status code of _.

/32

For IPv4 local routes the prefix length is ____.

/128

IPv6 local routes the prefix length is ______.

Dynamic routing protocols

are used by routers to automatically share information about the reachability and status of remote networks.

O

The routing table entries use the status code of ___ to indicate the route was learned by the OSPF routing protocol.

0.0.0.0/0

A default route has an IPv4 route entry of _________ or an IPv6 route entry of ::/0.

child route

An indented entry is known as a ________ . A route entry is indented if it is the subnet of a classful address (class A, B or C network).

parent route

The classful network address of this subnet will be shown above the route entry, less indented, and without a source code. That route is known as a ________.

Administrative Distance

A route entry for a specific network address (prefix and prefix length) can only appear once in the routing table.

Directly Connected

Administrative Distance: 0

Static route

Administrative Distance: 1

EIGRP summary route

Administrative Distance: 5

External BGP

Administrative Distance: 20

Internal EIGRP

Administrative Distance: 90

OSPF

Administrative Distance: 110

IS-IS

Administrative Distance: 115

Routing Information Protocol(RIP)

Administrative Distance: 120

External EIGRP

Administrative Distance: 170

Internal BGP

Administrative Distance: 200

Static routes

are useful for smaller networks with only one path to an outside network. They also provide security in a larger network for certain types of traffic, or links to other networks that need more control.

Dynamic routing protocols

are implemented in any type of network consisting of more than just a few routers. Are scalable and automatically determine better routes if there is a change in the topology.

Interior Gateway Protocols (IGPs)

are routing protocols used to exchange routing information within a routing domain administered by a single organization.

BGP

is used to exchange routing information between different organizations, known as autonomous systems (AS).

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

a finite list of steps used to accomplish a task. Routing protocols use algorithms for facilitating routing information and for the best path determination.

Routing Information Protocol (RIP)

•The metric is “hop count”.

•Each router along a path adds a hop to the hop count.

•A maximum of 15 hops allowed.

Open Shortest Path First(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.

Enhanced Interior Gateway Routing Protocol (EIGRP)

•It calculates a metric based on the slowest bandwidth and delay values.

•It could also include load and reliability into the metric calculation.