Module #3 Routing Decisions

Vocabulary flashcards covering key routing-decision terms: processes, protocols, route types, metrics, and IOS commands.

Router

A Layer 3 device that examines destination IP addresses, selects best paths, and forwards packets between networks.

Layer 2 Switch

A device that forwards frames based on destination MAC addresses; operates at the data-link layer and does not make routing decisions.

Encapsulation

The process of placing a Layer 3 packet inside a new Layer 2 frame before forwarding it out an interface.

De-encapsulation

The removal of Layer 2 headers and trailers to expose the Layer 3 packet for processing.

Routing Table

A data file in RAM that stores paths to directly connected and remote networks, including next-hop information.

Static Route

A manually configured path to a destination network; identified with code “S” in the routing table.

Dynamic Routing Protocol

Software (e.g., RIP, OSPF, EIGRP) that automatically learns and advertises routes, updating the routing table without manual input.

Address Resolution Protocol (ARP)

Protocol used by routers and hosts on Ethernet networks to map an IPv4 address to its corresponding MAC address.

Next-Hop

The IP address of the router interface to which a packet is forwarded on its way to the destination network.

Administrative Distance (AD)

A value that rates the trustworthiness of a route source; lower ADs are preferred when multiple routes exist.

Metric

A numerical value a routing protocol uses to choose the best path; lower metrics indicate better routes.

Best Path

The optimal route selected by a router based on the lowest metric and/or administrative distance.

Equal-Cost Load Balancing

Technique where a router distributes traffic across multiple paths that have identical metrics to the same destination.

Unequal-Cost Load Balancing

EIGRP feature that allows traffic distribution across paths with different metrics according to a configured ratio.

Directly Connected Route

Network automatically entered in the routing table when an interface is up/up with an assigned IP; marked with code “C.”

Local Route

An entry for the router’s own interface address; marked with code “L” in the routing table.

Remote Route

Path to a network not directly attached; learned through static configuration or a dynamic routing protocol.

Gateway of Last Resort

The default route a router uses when no specific path to the destination exists (0.0.0.0/0).

Default Static Route

A static route with network 0.0.0.0/0 (or ::/0 for IPv6) that directs unknown traffic to a specified next hop or interface.

ip route Command

Cisco IOS syntax used to configure an IPv4 static route: ip route network mask {next-hop-ip | exit-intf}.

ipv6 route Command

IOS syntax to configure a static IPv6 route: ipv6 route prefix/prefix-length {next-hop-ipv6 | exit-intf}.

show ip route

IOS command that displays the IPv4 routing table, route sources, metrics, and outgoing interfaces.

Code "S"

Routing-table prefix indicating the route was learned via a static configuration.

Code "D"

Routing-table prefix for routes learned through EIGRP (Enhanced Interior Gateway Routing Protocol).

Code "O"

Routing-table prefix for routes learned via OSPF (Open Shortest Path First).

Routing Information Protocol (RIP)

Distance-vector protocol that uses hop count as its sole metric.

Open Shortest Path First (OSPF)

Link-state protocol that selects paths based on cost, calculated from cumulative interface bandwidth.

Enhanced Interior Gateway Routing Protocol (EIGRP)

Cisco proprietary protocol that calculates metrics using bandwidth, delay, load, and reliability.

Interior Gateway Routing Protocols

Protocols like RIP, OSPF, EIGRP used for routing within a single autonomous system.

Convergence

The state in which all routers have consistent, up-to-date routing tables after exchanging information.

Exit Interface

Router interface through which a packet is forwarded toward its next hop or final destination.

Serial Interface

Point-to-point link type without MAC addresses; routers use data-link address equivalents instead of Ethernet MACs.

MAC Address

Layer 2 hardware address used on Ethernet networks; changes at every hop during packet forwarding.

ARP Cache

Table stored in RAM that holds recently resolved IP-to-MAC address mappings to avoid repeat ARP requests.

Hop Count

Number of routers a packet traverses to reach a destination; used as RIP’s metric.

Cost (OSPF)

OSPF metric based on cumulative bandwidth from source to destination; lower cost equals better path.

Load (EIGRP)

Dynamic component of EIGRP’s metric reflecting traffic utilization on an interface.

Delay (EIGRP)

Time (in tens of microseconds) for a packet to traverse an interface; part of EIGRP’s composite metric.

show ipv6 route

IOS command used to view the IPv6 routing table, including static and dynamic IPv6 routes.

Up/Up Interface

Interface state indicating protocol and line status are both active, allowing its network to appear in the routing table.