IP Services MODULE 07
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IP Services

The support infrastructure every network relies on — addressing, time, redundancy, and discovery.

DHCP — Dynamic Host Configuration Protocol
RFC 2131 · UDP 67 (server) / UDP 68 (client)

DHCP automatically assigns IP addresses, subnet masks, default gateways, and DNS servers to hosts. The four-step DORA process is essential exam knowledge.

D
DISCOVER
Client broadcasts to 255.255.255.255 UDP 67
"Is there a DHCP server?"
O
OFFER
Server unicasts an IP address offer with lease time and options
R
REQUEST
Client broadcasts acceptance of the offered IP (may notify other servers)
A
ACK
Server confirms the lease. Client applies the IP configuration.
DHCP Relay Agent (ip helper-address): When the DHCP server is on a different subnet, configure ip helper-address <server-ip> on the router interface facing the clients. The router unicasts the broadcast to the server.
DHCP Server Config
ip dhcp excluded-address 192.168.1.1 192.168.1.10 ip dhcp pool LAN_POOL network 192.168.1.0 255.255.255.0 default-router 192.168.1.1 dns-server 8.8.8.8 lease 7 ! 7 days
DHCP Relay (on router)
interface GigabitEthernet0/0 ip address 192.168.1.1 255.255.255.0 ip helper-address 10.0.0.100 ! 10.0.0.100 = DHCP server IP
DNS — Domain Name System
UDP/TCP 53 · Hierarchical distributed database

DNS translates human-readable names (cisco.com) into IP addresses. The hierarchy: Root → TLD (.com, .org) → Authoritative Name Servers → Resolving Resolvers → Client.

Key Record Types
AIPv4 address
AAAAIPv6 address
CNAMEAlias (canonical name)
MXMail server
PTRReverse lookup (IP→name)
NSName server for zone
Cisco DNS Client Config
ip domain-name corp.example.com ip name-server 8.8.8.8 8.8.4.4 ip domain-lookup ! enabled by default ! Disable lookup (stops DNS on typos) no ip domain-lookup
NTP — Network Time Protocol
UDP 123 · Stratum-based time hierarchy

Accurate time is critical for logging, certificates, and AAA. NTP uses a stratum hierarchy — lower stratum = closer to the reference clock = more accurate.

0
Stratum 0 — Reference Clock
GPS, atomic clocks. Never on the network directly.
1
Stratum 1 — Primary NTP
Syncs directly to Stratum 0. Public NTP servers (time.google.com).
2
Stratum 2 — Secondary NTP
Enterprise NTP servers. Syncs to Stratum 1.
3–15
Stratum 3–15
Further downstream. Most end devices are Stratum 3–5.
NTP Client Config
ntp server 216.239.35.0 ntp server 216.239.35.4 ! show ntp status ! show ntp associations
NTP Master (internal stratum)
ntp master 3 ! Makes this router a Stratum 3 ! source for other devices
FHRP — First Hop Redundancy Protocols
Provides a virtual gateway IP shared between multiple routers

When the default gateway fails, traffic stops. FHRPs create a virtual IP and virtual MAC address shared between two or more routers — clients only see the virtual IP, so failover is transparent.

HSRP
Cisco Proprietary
Protocol: UDP 1985 (Hello)
Roles: Active / Standby
Virtual MAC: 0000.0c07.acXX
Version: v1 (IPv4) / v2
Election: Priority (default 100)
Only Active router forwards traffic. Standby is idle but ready.
VRRP
IEEE Standard
Protocol: Multicast 224.0.0.18
Roles: Master / Backup
Virtual MAC: 0000.5e00.01XX
RFC: RFC 5798
Election: Priority (default 100)
Open standard equivalent of HSRP. Works across vendor equipment.
GLBP
Cisco Proprietary
Protocol: UDP 3222 (Hello)
Roles: AVG + up to 4 AVFs
Load Balancing: Yes (multiple MACs)
Election: AVG by priority
AVG: Active Virtual Gateway
Unique: actively load-balances traffic across multiple routers simultaneously.
HSRP Configuration Example
! On the primary router (higher priority = becomes Active) interface GigabitEthernet0/1 ip address 192.168.1.2 255.255.255.0 standby 1 ip 192.168.1.1 ! Virtual IP standby 1 priority 110 ! Default=100; higher=Active standby 1 preempt ! Take back Active if priority recovers standby 1 track GigabitEthernet0/0 20 ! Decrement by 20 if G0/0 fails
CDP & LLDP — Neighbor Discovery
Layer 2 discovery protocols — they do NOT use IP
CDP (Cisco Discovery Protocol) is Cisco proprietary. It runs at Layer 2 and discovers directly connected Cisco devices: hostname, IOS version, IP address, platform, and interface.
CDP Commands
show cdp neighbors ! Summary show cdp neighbors detail ! Full info + IP show cdp interface ! Per-interface status no cdp run ! Disable globally no cdp enable ! Disable on interface
LLDP (Link Layer Discovery Protocol, IEEE 802.1AB) is the vendor-neutral open standard. Disabled by default on most Cisco devices — you must enable it manually.
LLDP Commands
lldp run ! Enable globally show lldp neighbors show lldp neighbors detail ! interface-level control: lldp transmit ! send LLDPDUs lldp receive ! receive LLDPDUs
Syslog — System Logging
UDP 514 · 8 severity levels (0–7)

Syslog sends device log messages to a centralized server. The severity level controls which messages are sent. Memorise: "Every Awesome Cisco Engineer Will Need Daily Inspection"

LevelNameMnemonicMeaning
0EmergencyEverySystem unusable
1AlertAwesomeImmediate action required
2CriticalCiscoCritical conditions
3ErrorEngineerError conditions
4WarningWillWarning conditions
5NoticeNeedNormal but significant
6InformationalDailyInformational messages
7DebuggingInspectionDebug-level messages (verbose!)
Syslog Config
logging host 10.0.0.200 ! Send to syslog server logging trap informational ! Send severity 0–6 logging buffered 4096 debugging ! Buffer locally (size, level) service timestamps log datetime msec ! Add timestamps show logging ! View buffered logs

IP Services Drills

Spaced-repetition MCQ — wrong answers come back more often.

30s
QUESTION 1 / 18
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IP Services Flashcards

Click the card to flip. Use arrows to navigate. Great for quick reviews before bed!

CONCEPT
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IP Services Labs

Hands-on Packet Tracer configurations. Type every command — muscle memory matters.

Lab A — DHCP Server on a Router
Topology: R1 (DHCP server) → SW1 → PC1, PC2
1

Exclude reserved addresses

Keep the first 10 IPs free for static assignments (router, servers, printers).

ip dhcp excluded-address 192.168.10.1 192.168.10.10
2

Create the DHCP pool

Define the network, gateway, DNS, and lease time.

ip dhcp pool LAN network 192.168.10.0 255.255.255.0 default-router 192.168.10.1 dns-server 8.8.8.8 lease 7
3

Verify DHCP bindings

Check which IPs have been leased and to which MAC addresses.

show ip dhcp binding show ip dhcp pool show ip dhcp conflict
Lab B — Configure HSRP Active/Standby
Topology: R1 (Active) and R2 (Standby) sharing virtual IP 192.168.1.1
1

Configure R1 (Active — higher priority)

interface GigabitEthernet0/1 ip address 192.168.1.2 255.255.255.0 standby 1 ip 192.168.1.1 standby 1 priority 110 standby 1 preempt
2

Configure R2 (Standby — default priority)

interface GigabitEthernet0/1 ip address 192.168.1.3 255.255.255.0 standby 1 ip 192.168.1.1 standby 1 priority 100 ! No preempt on standby router
3

Verify HSRP state

show standby show standby brief ! R1 should show: Active local 192.168.1.1 ! R2 should show: Standby local 192.168.1.2
Lab C — NTP and Syslog
1

Configure NTP client

ntp server 216.239.35.0 prefer ntp server 216.239.35.4 show ntp status show ntp associations
2

Configure Syslog

logging host 10.0.0.200 logging trap informational service timestamps log datetime msec localtime show logging

FHRP Topology

HSRP ACTIVE/STANDBY TOPOLOGY Internet 203.0.113.0/30 R1 192.168.1.2/24 HSRP: Active ✓ R2 192.168.1.3/24 HSRP: Standby Virtual IP: 192.168.1.1 Clients use this as default gateway SW1 Layer 2 Switch PC1 PC2 PC3 HSRP Hello

IP Services Cheatsheet

Key Ports & Protocols
DHCP ServerUDP 67
DHCP ClientUDP 68
DNSUDP/TCP 53
NTPUDP 123
SyslogUDP 514
SNMPUDP 161/162
HSRP HelloUDP 1985
GLBP HelloUDP 3222
FHRP Comparison
HSRPCisco / Active-Standby
VRRPIEEE / Master-Backup
GLBPCisco / Load-Balances
Default Priority100 (all three)
HSRP vMAC0000.0c07.acXX
VRRP vMAC0000.5e00.01XX
Syslog Levels (Memorise!)
0 EmergencyEvery
1 AlertAwesome
2 CriticalCisco
3 ErrorEngineer
4 WarningWill
5 NoticeNeed
6 InformationalDaily
7 DebugInspection
Key Commands
show ip dhcp binding show standby brief show ntp status show ntp associations show cdp neighbors detail show lldp neighbors detail show logging debug ip dhcp server events

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