DHCP Scope Exhausted — No IPs Available
Presenting Symptom
Clients in the branch office cannot obtain IP addresses via DHCP; they receive APIPA addresses (169.254.x.x) or no IP at all.
Network Context
A small branch office with 200+ users connects to the corporate network via a Cisco 4321 ISR router running IOS XE 16.9. The router acts as the DHCP server for the local VLAN 10 (192.168.10.0/24). The DHCP pool is configured with a /24 subnet, but due to a recent expansion, the number of clients has exceeded the available addresses. The router is the default gateway and also provides DHCP services.
Diagnostic Steps
Check DHCP pool utilization
show ip dhcp poolPool VLAN10 : Utilization mark (high/low) : 100 / 0 Subnet size (first/next) : 0 / 0 Total addresses : 254 Leased addresses : 254 Pending event : none Automatic bindings : 254 Declined addresses : 0 Expired addresses : 0 Malformed messages : 0
If 'Leased addresses' equals 'Total addresses', the pool is exhausted. This confirms the root cause: no free IP addresses available.
Check DHCP binding table for active leases
show ip dhcp bindingBindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
192.168.10.1 0100.1a2b.3c4d.5e Mar 01 2020 12:00 AM Automatic
192.168.10.2 0100.1a2b.3c4d.5f Mar 01 2020 12:00 AM Automatic
... (254 entries)A full list of 254 bindings confirms all addresses are leased. Look for any expired or declined addresses that might indicate problems, but here all are active.
Check DHCP server statistics for conflicts
show ip dhcp conflictIP address Detection method Detection time 192.168.10.50 Ping Mar 01 2020 10:00 AM 192.168.10.100 Gratuitous ARP Mar 01 2020 11:00 AM
Conflicts indicate IP address duplication. If conflicts exist, the DHCP server excludes those addresses, reducing available pool size. However, in this scenario, conflicts are not the primary issue; pool exhaustion is.
Check DHCP server configuration for pool size
show running-config | section dhcpip dhcp pool VLAN10 network 192.168.10.0 255.255.255.0 default-router 192.168.10.1 dns-server 8.8.8.8 lease 7
The pool is configured with a /24 subnet, providing only 254 usable addresses. If the number of clients exceeds 254, the pool will be exhausted. This confirms the design flaw: insufficient address space.
Root Cause
The DHCP pool is configured with a /24 subnet (192.168.10.0/24), providing only 254 usable IP addresses. The branch office has grown to over 254 clients, exhausting the pool. No additional scopes or superscopes are configured, and the lease time is long (7 days), preventing quick reuse of addresses.
Resolution
Verification
1. Verify the new pool size: show ip dhcp pool Expected: 'Total addresses: 510' (for /23) 2. Verify that clients can obtain IP addresses: show ip dhcp binding | count Expected: count less than total addresses. 3. On a client, run 'ipconfig /renew' (Windows) or 'dhclient' (Linux) and confirm it receives an IP in the new range.
Prevention
1. Use subnet sizing based on actual client count with growth margin (e.g., /23 for up to 500 clients). 2. Implement DHCP lease time management: shorter leases (e.g., 1 day) for high-density areas to allow quicker reuse. 3. Monitor DHCP pool utilization via SNMP or syslog alerts when utilization exceeds 80%.
CCNA Exam Relevance
On the CCNA 200-301 exam, DHCP troubleshooting scenarios often appear as multiple-choice or drag-and-drop questions. The exam tests the ability to identify DHCP pool exhaustion by interpreting 'show ip dhcp pool' output. Key fact: When 'Leased addresses' equals 'Total addresses', the pool is full. Candidates must know how to expand the subnet or reduce lease time.
Exam Tips
Memorize the output of 'show ip dhcp pool' — specifically the 'Leased addresses' and 'Total addresses' fields.
Remember that APIPA (169.254.x.x) indicates DHCP failure; the first step is to check the DHCP server's pool utilization.
On the exam, if a question shows a DHCP pool with all addresses leased, the solution is to increase the subnet size or reduce lease time.
Commands Used in This Scenario
show ip dhcp binding
Displays the current DHCP binding table, showing which IP addresses have been leased to clients, along with their MAC addresses, lease expiration, and type of binding.
show ip dhcp conflict
Displays IP address conflicts detected by the DHCP server, helping administrators identify and resolve duplicate IP assignments on the network.
show ip dhcp pool
Displays the configuration and utilization statistics of a DHCP pool, used to verify pool settings and address allocation status.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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