The ASA5510 and the ASA5510-K8 are the same hardware models. They both need to run the exact same OS for failover. Upgrade to 8.2(x) if possible.
FIREWALL# sh ver
Cisco Adaptive Security Appliance Software Version 8.2(1)
Compiled on Tue 05-May-09 22:45 by builders
System image file is "disk0:/asa821-k8.bin"
Config file at boot was "startup-config"
FIREWALL up 26 days 4 hours
failover cluster up 1 year 21 days
Hardware: ASA5510-K8, 256 MB RAM, CPU Pentium 4 Celeron 1600 MHz
Internal ATA Compact Flash, 256MB
Slot 1: ATA Compact Flash, 64MB
BIOS Flash AT49LW080 @ 0xffe00000, 1024KB
Encryption hardware device : Cisco ASA-55x0 on-board accelerator (revision 0x0)
Boot microcode : CN1000-MC-BOOT-2.00
SSL/IKE microcode: CNLite-MC-SSLm-PLUS-2.03
IPSec microcode : CNlite-MC-IPSECm-MAIN-2.04
0: Ext: Ethernet0/0 : address is 0019.e8d9.46f2, irq 9
1: Ext: Ethernet0/1 : address is 0019.e8d9.46f3, irq 9
2: Ext: Ethernet0/2 : address is 0019.e8d9.46f4, irq 9
3: Ext: Ethernet0/3 : address is 0019.e8d9.46f5, irq 9
4: Ext: Management0/0 : address is 0019.e8d9.46f1, irq 11
5: Int: Internal-Data0/0 : address is 0000.0001.0002, irq 11
6: Int: Internal-Control0/0 : address is 0000.0001.0001, irq 5
Licensed features for this platform:
Maximum Physical Interfaces : Unlimited
Maximum VLANs : 100
Inside Hosts : Unlimited
Failover : Active/Active
VPN-DES : Enabled
VPN-3DES-AES : Enabled
Security Contexts : 2
GTP/GPRS : Disabled
SSL VPN Peers : 2
Total VPN Peers : 250
Shared License : Disabled
AnyConnect for Mobile : Disabled
AnyConnect for Linksys phone : Disabled
AnyConnect Essentials : Disabled
Advanced Endpoint Assessment : Disabled
UC Phone Proxy Sessions : 2
Total UC Proxy Sessions : 2
Botnet Traffic Filter : Disabled
This platform has an ASA 5510 Security Plus license.
Serial Number: 99999999
Running Activation Key: XXXX XXXX XXXX XXXX XXXX
FW# sh fail state
State Last Failure Reason Date/Time
This host - Primary
Active None
Other host - Secondary
Standby Ready Comm Failure 06:10:17 CST May 17 2009
====Configuration State===
Sync Done
Sync Done - STANDBY
====Communication State===
Mac set
