Courseiva
RedundancyCCNA 200-301

GLBP Not Load Balancing as Expected

Presenting Symptom

Clients in VLAN 10 are not load balancing across two GLBP routers; all traffic goes to the active AVG router.

Network Context

A small branch office with two Cisco routers (R1 and R2) running GLBP for VLAN 10. Both routers are configured as GLBP members with the same virtual IP (10.10.10.1). The network uses IOS 15.x. Clients are connected via a Layer 2 switch. The expected behavior is load sharing, but only R1 is forwarding traffic.

Diagnostic Steps

1

Check GLBP status on both routers

show glbp
Interface GigabitEthernet0/0, Group 10, state Active
  Virtual IP address is 10.10.10.1
  Hello time 3 sec, hold time 10 sec
  Next hello sent in 0.256 secs
  Redirect time 600 sec, forwarder time-out 14400 sec
  Preemption disabled
  Active is 10.10.10.2, priority 100 (expires in 9.856 sec)
  Standby is 10.10.10.3, priority 100
  There is 1 forwarder (1 active)
  Forwarder 1
    State is Active
    1 state change, last state change 00:05:12
    MAC address is 0007.b400.0101
    Owner is 10.10.10.2
    Redirection enabled, preemption enabled
    Active is 10.10.10.2, weighting 100 (expires in 9.856 sec)
  Forwarder 2
    State is Listen
    MAC address is 0007.b400.0102
    Owner is 10.10.10.3
    Redirection enabled, preemption enabled
    Active is 10.10.10.3, weighting 100 (expires in 9.856 sec)

Check if both routers are in the GLBP group and their states. Look for the Active Virtual Gateway (AVG) and Active Virtual Forwarders (AVFs). If only one forwarder is active, load balancing is not occurring.

2

Verify GLBP load balancing method

show glbp 10 load-balancing
Group 10 load-balancing method: host-dependent

The load-balancing method determines how traffic is distributed. 'host-dependent' may cause uneven distribution if client MACs are not diverse. The default is 'round-robin'. If set to 'host-dependent', check if it's intended.

3

Check GLBP forwarder weights and thresholds

show glbp 10 forwarders
Forwarder 1
  State is Active
  MAC address is 0007.b400.0101
  Owner is 10.10.10.2
  Weighting 100 (configured 100)
  Active is 10.10.10.2, weighting 100
Forwarder 2
  State is Listen
  MAC address is 0007.b400.0102
  Owner is 10.10.10.3
  Weighting 100 (configured 100)
  Active is 10.10.10.3, weighting 100

Verify that both forwarders have equal weighting and that the second forwarder is in Listen state (not Active). If one forwarder is missing or has lower weight, it may not be used.

4

Check GLBP interface configuration on both routers

show running-config interface GigabitEthernet0/0
interface GigabitEthernet0/0
 ip address 10.10.10.2 255.255.255.0
 glbp 10 ip 10.10.10.1
 glbp 10 priority 100
 glbp 10 preempt
 glbp 10 load-balancing round-robin

Compare configurations on both routers. Ensure both have the same GLBP group number, virtual IP, and load-balancing method. Check if 'preempt' is enabled (should be for load balancing). Look for missing or mismatched commands.

Root Cause

The GLBP load-balancing method is set to 'host-dependent' on both routers, which assigns a specific forwarder based on the client MAC address. Since all clients in VLAN 10 have similar MAC prefixes (same vendor), they are all mapped to the same forwarder (R1), causing no load balancing.

Resolution

Change the load-balancing method to 'round-robin' on both routers: On R1: interface GigabitEthernet0/0 glbp 10 load-balancing round-robin On R2: interface GigabitEthernet0/0 glbp 10 load-balancing round-robin This ensures that each new client ARP request is answered by a different router in a round-robin fashion.

Verification

Run 'show glbp 10 load-balancing' on either router. Expected output: 'Group 10 load-balancing method: round-robin'. Then generate traffic from multiple clients and run 'show glbp 10 forwarders' to see both forwarders active with traffic counters increasing.

Prevention

1. Use the default 'round-robin' load-balancing method unless host-dependent is specifically required. 2. Ensure GLBP preempt is enabled so that a router with higher priority can take over. 3. Monitor GLBP forwarder states regularly to detect uneven distribution early.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why GLBP is not load balancing. The exam tests understanding of GLBP load-balancing methods (round-robin, host-dependent, weighted) and the 'show glbp' command output. A candidate must know that 'host-dependent' can cause uneven distribution if client MACs are similar.

Exam Tips

1.

Memorize the three GLBP load-balancing methods: round-robin, host-dependent, weighted.

2.

Remember that 'show glbp' displays the AVG and AVF states; if only one forwarder is active, check the load-balancing method.

3.

In the exam, if a question shows GLBP not load balancing, look for the load-balancing method in the configuration or output.

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