Firewalls can participate in SNMP by maintaining several MIBs. The MIB values are constantly updated with the current values that are in use. For example, one MIB parameter records the average firewall CPU load over a 5-second period. This is based on the CPU usage measurements that can also be shown from the firewall CLI.
SNMP MIBs represent data as a hierarchical tree structure; each MIB variable is referenced by its object identifier (OID). OIDs are formed by concatenating the name or number of a tree branch as the tree is followed from the root to the object's location in dotted notation.
Figure 4-12 shows the top layers of the standard MIB tree, along with the lower layers that apply to firewalls. The root layer is unnamed. All MIB variables that are useful for network management are located under the internet subtree. Following the tree structure downward, internet is referenced as OID iso.org.dod.internet or 188.8.131.52.
Figure 4-12. SNMP MIB Structure
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Your SNMP management station needs to have several firewall-specific MIBs compiled into its database. Make sure you find these MIBS: IF-MIB, RFC1213-MIB, CISCO-MEMORY-POOL-MIB, CISCO-PROCESS-MIB, ENTITY-MIB, CISCO-SMI, and CISCO-FIREWALL-MIB.
ASA also adds CISCO-IPSEC-FLOW-MONITOR-MIB, CISCO-FIPS-STAT-MIB, and ALTIGA-SSL-STATS-MIB.
These can all be obtained for free from Cisco.com at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
A firewall uses the mgmt subtree (iso.org.dod.internet.mgmt or 184.108.40.206.2) to contain several useful objects, all organized under the mib-2 subtree (220.127.116.11.2.1). These objects are defined in the RFC1213-MIB file (18.104.22.168.2.1.11). They fall into these categories:
• system— Descriptions of the firewall, uptime, and network services
• interfaces— Parameters and counters for each interface
• ip— IP addresses, subnet masks, and broadcast addresses assigned to each interface
Many of the values maintained in the mib-2 subtree can also be seen with the show snmp-server (PIX 6.x), show running-config snmp-server (ASA), show version, and show interface EXEC commands.
The EntityMIB subtree (22.214.171.124.2.1.47) is also included. It is defined by the ENTITY-MIB file, which is based on RFC 1212. This was added to ASA 7.0(1) to support the firewall chassis and field-replaceable units (FRU) available on the ASA platforms.
The private (126.96.36.199.4) subtree contains one subtree, enterprise (188.8.131.52.4.1), where all network vendor-specific objects are located. The Cisco private MIB structure is contained in the cisco subtree (184.108.40.206.4.1.9). The set of specific MIBs that are included under the cisco MIB tree varies according to the hardware platform (router, switch, firewall, and so on).
A firewall maintains several subtrees under iso.org.dod.internet.private.enterprise.cisco.mgmt, as follows:
• The ciscoMemoryPool subtree (220.127.116.11.18.104.22.168.48) has objects that are defined in the CISCO-MEMORY-POOL-MIB file. These describe the current status of firewall memory. It can also be seen with the show blocks EXEC command.
• The ciscoProcess subtree (22.214.171.124.126.96.36.199.109) is defined by the CISCO-PROCESS-MIB file. These values describe the firewall's CPU usage over 5-second, 1-minute, and 5-minute periods. The same values can be seen with the show cpu usage EXEC command.
• The ciscoFirewall subtree (188.8.131.52.184.108.40.206.147) is defined by the CISCO-FIREWALL-MIB file. A number of values are maintained that describe the current memory buffer usage (cfwBufferStat) and the connection usage (cfwConnectionStat) in the firewall. These correspond to the output of the show memory and show conn count EXEC commands, respectively.
• The ciscoIpSecFlowMonitorMIB subtree (220.127.116.11.18.104.22.168.171) is defined by the CISCO-IPSEC-FLOW-MONITOR-MIB file. This was added to ASA and FWSM platforms to support IPSec VPN functionality to report on tunnel statistics.
• The ciscoRemoteAccessMonitorMIB subtree (22.214.171.124.126.96.36.199.392) is defined by the CISCO-REMOTE-ACCESS-MONITOR-MIB file. This was added to ASA 7.0(1) to support VPN client session statistics, but removed in ASA 8.0.
• The ciscoFipsStatsMIB subtree (188.8.131.52.184.108.40.206.999999) is defined by the CISCO-FIPS-STAT-MIB file. This was added to ASA 7.0(1) to support reporting on IPSec cryptographic engine operations, but removed in ASA 8.0.
• The altigaSSLstats subtree (220.127.116.11.4.1.3076.2.1.2.26) is defined by the ALTIGA-SSL-STATS-MIB file. This was added to ASA 7.0(1) to support reporting on SSL VPN session statistics.
A firewall can send notification or trap messages to SNMP management stations when certain events occur. This allows the management station to receive alerts in real time and relay them to the appropriate networking personnel.
Generic traps are sent when firewall links (interfaces) go up or down, when the firewall is reloaded (a "warm start") or booted up (a "cold start" after power is applied) for some reason, and when an SNMP poll has been received with an incorrect community string. Syslog messages can also be sent as SNMP traps if the firewall is configured to do so.
When SNMP traps are sent, the firewall's OID is included. This allows the SNMP management station to determine what type of device has sent the trap. Cisco firewall models use the unique OIDs shown in Table 4-9. Notice that the OIDs use most of the same tree hierarchy as SNMP MIBs. For example, 18.104.22.168.4.1.9. would lead to the private.enterprise.cisco. subtree. This is followed by .1., which points to the Cisco products subtree, which is followed by a number that uniquely identifies the firewall model.
Table 4-9. Firewall OID Values Used in SNMP Traps
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