Cisco IOS — Router Configuration

Overview

A Cisco router connects networks by forwarding IP packets between interfaces. Each interface belongs to a different subnet, and the router maintains a routing table that maps destination prefixes to outgoing interfaces or next-hop IP addresses. Static routes manually define these mappings; dynamic routing protocols build the table automatically.

This article covers router interface configuration, serial and loopback interfaces, the routing table, static route syntax, floating static routes, the default route, Administrative Distance, and how to verify routing with ping and traceroute.

Router Interface Configuration

Ethernet Interfaces

Router Ethernet interfaces are administratively shut down by default. You must assign an IP address and enable the interface.

interface GigabitEthernet0/0/0
 ip address 192.168.1.1 255.255.255.0
 description LAN-Facing-Interface
 no shutdown

The IP address becomes the default gateway for hosts on that subnet. no shutdown removes the administrative shutdown. Without it, the interface stays in administratively down / down state regardless of cabling.

Checking Interface Status

show ip interface brief

The most important verification command. Displays every interface with its assigned IP, line status (Layer 1), and protocol status (Layer 2).

Detailed Interface Statistics

show interfaces GigabitEthernet0/0/0

Displays: bandwidth, encapsulation, duplex, speed, MTU, input/output packet counts, error counters (input errors, CRC, frame errors, output drops). High input errors combined with CRC errors typically indicate a duplex mismatch or a bad cable.

Serial Interfaces

Serial interfaces are used for legacy WAN connections (T1/E1, Frame Relay). They are less common in modern networks but still appear in CCNA lab equipment.

DCE vs DTE

A serial link has one DCE (Data Communications Equipment) end and one DTE (Data Terminal Equipment) end. The DCE side provides the clocking signal. In labs, the back-to-back serial cable includes a DCE connector on one end — that router must provide the clock rate.

interface Serial0/0/0
 ip address 10.0.0.1 255.255.255.252
 clock rate 128000
 bandwidth 128
 no shutdown

clock rate is only needed on the DCE side. bandwidth does not change the physical speed — it tells routing protocols the logical bandwidth of the link for metric calculations (OSPF cost, EIGRP composite metric).

The /30 subnet (255.255.255.252) is standard for point-to-point WAN links — only 2 usable host addresses needed.

Loopback Interfaces

A loopback interface is a logical, software-only interface. It has no physical connector and is always in up/up state as long as it is not administratively shut down. Loopbacks are used for:

• Stable OSPF/EIGRP router IDs (a loopback IP never goes down)
• Management access that is always reachable as long as the router is up
• Testing and simulation in labs

interface Loopback0
 ip address 1.1.1.1 255.255.255.255

The /32 mask is typical for loopbacks — there is only one host (the router itself), so no subnet range is needed.

The Routing Table

The routing table lists every prefix the router knows how to reach, how it learned about it, and how to forward packets toward it.

show ip route

Route Source Codes

Connected (C) and Local (L) routes are created automatically when an interface comes up/up. Every configured interface generates one C route (the subnet) and one L route (/32 host route for the router’s own IP).

Reading a Route Entry

O    10.2.0.0/24 [110/2] via 192.168.1.2, 00:05:14, GigabitEthernet0/0/0

• O — learned by OSPF
• 10.2.0.0/24 — destination prefix
• [110/2] — [Administrative Distance / Metric]
• via 192.168.1.2 — next-hop IP
• 00:05:14 — age of route
• GigabitEthernet0/0/0 — outgoing interface

Filtering the Routing Table

show ip route connected
show ip route static
show ip route ospf
show ip route 10.2.0.0
show ip route 10.2.0.5          # Shows which route would be used for this destination

Static Routes

A static route manually tells the router how to reach a specific network.

Syntax

ip route <network> <mask> {<next-hop-ip> | <outgoing-interface>} [distance]

Examples

Route via next-hop IP (recommended on Ethernet)

ip route 10.2.0.0 255.255.255.0 192.168.1.2

Route via outgoing interface (acceptable on serial point-to-point)

ip route 10.2.0.0 255.255.255.0 Serial0/0/0

On Ethernet, using only an outgoing interface creates a “proxy ARP” dependency — the router ARP-requests for every destination. Use next-hop IP on Ethernet.

Fully-specified static route (both next-hop and interface — preferred)

ip route 10.2.0.0 255.255.255.0 GigabitEthernet0/0/1 192.168.1.2

Specifying both eliminates the recursive lookup (router must resolve next-hop to an interface anyway) and avoids proxy ARP issues. This is the most explicit and recommended form.

Static Host Route (/32)

Routes to a single specific host:

ip route 10.1.1.100 255.255.255.255 192.168.1.2

Default Route

The default route matches any destination not covered by a more specific route. It is the “gateway of last resort.”

ip route 0.0.0.0 0.0.0.0 192.168.1.1

This appears in the routing table as S* 0.0.0.0/0 (the asterisk means “candidate default”). The router also prints Gateway of last resort is 192.168.1.1 to network 0.0.0.0.

Administrative Distance

Administrative Distance (AD) is a rating of trustworthiness assigned to each route source. When two routing sources have routes for the same prefix/mask, the lower AD wins — that source’s route is installed in the routing table.

AD 255 means the route is never installed in the routing table. AD 0 (connected) means the interface is directly attached — cannot be better than that.

Metrics are only compared within the same routing protocol. You cannot compare an OSPF cost of 10 against an EIGRP metric of 100,000 — they measure different things. AD decides which protocol “wins” first; the metric within that protocol selects the best path.

Floating Static Routes

A floating static route has a higher AD than the primary dynamic route. It sits in the configuration but does not appear in the routing table while the dynamic route exists. If the dynamic route disappears (link failure, neighbour lost), the floating static takes over.

ip route 10.2.0.0 255.255.0.0 192.168.2.1 130

With AD 130, this static route is less preferred than OSPF (AD 110). As long as OSPF has a route to 10.2.0.0/16, the static is invisible. When OSPF loses the route, the static with AD 130 becomes the best (only) route for that prefix and is installed.

Floating statics are commonly used to provide a dial-backup or cellular link that activates automatically when the primary WAN path fails.

Routing Logic and Longest Prefix Match

When a packet arrives, the router does two things:

1. Finds all routes whose prefix matches the destination
2. Among matching routes, uses the most specific match (longest prefix length)

Example: destination is 172.16.10.4. Routing table contains:

• S 0.0.0.0/0 via 192.168.1.1 (default)
• O 172.16.0.0/16 via 10.1.1.1 (OSPF)
• S 172.16.10.0/24 via 10.1.2.1 (static)

All three match. The static /24 is most specific (longest prefix), so the router uses 10.1.2.1 as the next hop.

This means you can have a general OSPF route for a whole supernet and override specific subnets within it with more specific static routes — longest prefix match handles the selection automatically.

Ping and Traceroute

Standard Ping

ping 10.1.2.1

Sends 5 ICMP Echo Request packets, each 100 bytes, from the closest interface to the destination. !!!!! means all 5 replies received. U means unreachable. . means timeout.

Extended Ping

Extended ping lets you control every aspect of the test — including the source address:

R1# ping
Protocol [ip]:
Target IP address: 10.1.2.1
Repeat count [5]:
Datagram size [100]:
Extended commands [n]: y
Source address or interface: 10.1.1.1

Specifying a source address tests the full round-trip including the remote host’s ability to route back to that specific source. This catches asymmetric routing problems that standard ping misses.

Or in one line on IOS XE:

ping 10.1.2.1 source GigabitEthernet0/0/0
ping 10.1.2.1 source 10.1.1.1

Traceroute

traceroute 10.1.2.1

Uses incrementing TTL values (1, 2, 3…) to map each hop. Each router that receives a packet with TTL=0 returns an ICMP Time Exceeded message, revealing its IP address. Cisco IOS traceroute uses UDP probes (unlike Windows tracert which uses ICMP).

Output shows three round-trip times for each hop. * means no response (host is not responding to ICMP TTL expired or is rate-limiting).

Quick Reference