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Frequently Asked Questions - Q—From where did RMONv2 derive?
- A—The IETF (Internet Engineering Task Force) developed RMONv2 to provide Network Managers the ability to view data rates on their network and the network layer. The built-in feature set is going to enable network managers to more accurately see the complete picture of the network. Many manufactures are currently pursuing this.
- Q—Will I have to change all IP addresses on my network if my company adopts IPv6?
- A—No. The new standard defines how Ipv4 and IPv6 are interoperable.
- Q—When should I migrate from IPv4 to IPv6?
- A—First of all, relax. You should not have to worry about this for a while yet. In the meantime, Cisco Systems, in conjunction with other network vendors, have already incorporate many of the new and exciting features of IPv6. Things such as classless interdomain routing (CIDR) and Network Address Translation (NAT) provide the means to resolve the current limitations of IP address assignments.
- Q—Where can I find more information about SNMPv2?
- A—Refer to the following sites for more information:
www.snmp.cs.utwente.nl/int www.simple-times.org/pub/simple-times/usec www.snmp.com www.aw.com/cp/stallings3.html - Q—Where can I find more information on SNMPv3?
- A—The best, and most up to date source can be found at:
http://www.snmp.com/v2status.html
To join the SNMPv3 mailing list, send mail to: snmpv3-request@tis.com with “subscribe” in the mail message.
The mailing list archive can be found at ftp://ftp.tis.com/pub/ietf/snmpv3 - Q—What do Cisco routers do for the following SNMP MIB variables: ifInNUcastPkts, ifInDiscards, ifInErrors, ifInUnknownProtos, ifOutOctets, ifOutUcastPkts, ifOutDiscards, ifOutErrors, and ifOutQLen?
- A—The following list provides details on these SNMP MIB variables.
- • ifInNUcastPkts. Counts of inbound broadcast and multicast packets
- • ifInDiscards. Counts as no buffers as reflected in show int
- • ifInErrors. Counts of all input errors as reflected in show int
- • ifInUnknownProtos. Counts as unclassified errors
- • ifOutOctets. Counts of the number of bytes output by the interface as shown in show int
- • ifOutUcastPkts. Counts of outbound broadcast and multicast packets
- • ifOutDiscards. Counts as output drops as shown in show int
- • ifOutErrors. Counts as output errors as shown in show int
- • ifOutQLen. The number of packets allowed to be on the output queue as shown in show int
The variables previously listed that do not say they appear in show int are not available anywhere other than SNMP. - Q—Explain the relationship between the show interface statements no buffers and input queue drops. Why do the inDiscards of SNMP give no buffers counts and not input queue drops, even though the outDiscards of SNMP do give output queue drops?
- A—The locIfInputQueueDrops/ifInDiscards work differently than locIfOutputQueueDrops/ifOutDiscards. The ifInDiscards counts the number of packets that were thrown away for lack of a system resource such as a buffer. This is generally a subset of the locIfInputQueueDrops. You will often see locIfInputQueueDrops = ifInDiscards. However, locIfInputQueueDrops also counts the number of packets dropped because of hitting the input queue limit. So more generally, you will see locIfInputQueueDrops > ifInDiscards.
To recap: locIfInputQueueDrops = Queue Limit Drops + No Buffer Drops ifInDiscards = No Buffer Drops (and is a subset of locIfInputQueueDrops)
The locIfOutputQueueDrops and ifOutDiscards are always equal when they count the same events. (These events are hitting the output queue limit, and do not have a hardware tx buffer when a packet is fast-switched from one interface to another.) - Q—Can I poll “no buffers” on a router?
- A—Yes. You can poll no buffers by polling for ifInDiscards, you can also use the MIB OLD-CISCO-MEMORY-MIB.my to get all of the buffer information on a router. You can use a short script to place these into a table that can be read through a browser or imported into a spreadsheet.
- Q—How do I poll “Queue Limit Drops” on a router?
- A—There is no way via SNMP or the show interface commands to break out the individual elements that go into the output drops. Here is some new information about what goes into the output drops counter:
Input drops == Queue limit drops + Throttling drops + SMT queue full drops + RSRB drops + no buffer drops
In addition, SNMP counters are never cleared, even if interfaces have been cleared. - Q—How can you identify what interface number to use in the MIB name?
- A—Each row of the interfaces table has an associated number, called an ifIndex. You use the ifIndex number to get a specific instance of an interface group object. For example, ifInNUcastPkts.1 would find the number of broadcast packets received on interface number one. You can then find the description of interface number one by looking at the object, which holds the interface description (from MIB-II) ifDescr.1.
- Q—Why would I need the snmp-server enable traps <trap-type> <trap- option> command?
- A—To enable all traps of a given type or types (that is, if the given trap type is generating noise in which you aren’t interested).
- Q—Why does the snmp-server enable traps <trap-type> <trap-option> command only have a subset of the traps?
- A—Most of the trap types in the snmp-server enable traps <trap-type> <trap-option> command have associated MIB notificationEnable objects that map to this CLI command. No effort has been made as yet to provide this global enable option to traps that do not have this equivalent MIB control.
- Q—Would the above commands ever be used together?
- A—It is up to you. For example, you could enable all Frame Relay traps, and not want trap receiver xxyy to see them, but only trap receiver bumble.
- Q—Where can I get a better definition of the trap types listed under the snmp-server host <host> <com-string> <trap-type> command? (For example, the documentation explains “config” as: Send configuration traps.) I would like to know what events send a trap.
- A—Each of the trap type labels is supposed to reflect the name of the MIB in which the associated traps are defined. For instance, config would be from the CISCO-CONFIG-MAN-MIB.my, envmon would be from the CISCO- ENVMON-MIB.my, etc.
- Q—Does Cisco implement 64-bit counters, especially for the IF-MIB? If not, when is 64-bit counter support expected?
- A—Cisco does not support 64 bit counters yet. This is being worked on now.
- Q—What version of SNMP is required for 64-bit counter support?
- A—SNMPv2C is required for Counter64.
- Q—Does the Cisco IOS support sub-interfaces in the ifTable?
- A—Generic support for sublayers in the ifTable has been present since Cisco IOS 11.1(1). For any given media type, it is up to the groups that support that media to determine the following factors:
- 1. If sublayers are appropriate (with direction from IETF).
- 2. How to support those sublayers.
- Q—What is the minimum Cisco IOS version I should be running to see sub-interfaces in SNMP tables?
- A—Cisco IOS 11.1.
- Q—Are the SNMP counters ifInOctects and ifOutOctets the same as the show interface in/out counters?
- A—Yes.
- Q—Do the ifInOctets and ifOutOctets counters include framing overhead (PPP, HDLC)?
- A—Yes.
- Q—On an ATM interface, do the counters include the cell header?
- A—ATM counters do not include ATM overhead (cell headers and AAL5 padding).
- Q—Some ifTable columns do not show up for certain interface types. Why? Is this a bug?
- A—This is not a bug. ifTable based on RFC 1573 is designed specifically so that some columns in a given row will not be instantiated based on ifType. Please read the RFC compliance statement for further clarification for which columns to expect for different media groups. An example of this would be ATM, which is a fixed length packet. As such, rows in the ifTable, etc. would be based on ifFixedLengthGroup.
- Q—In which release is SNMPv2C support available?
- A—Cisco IOS 11.3 will be the first shipping release with SNMPv2C support.
- Q—Is there a way to tell the router to load a specific configuration file via TFTP from a specific host?
- A—Yes. ftp://ftpeng.cisco.com/pub/mibs/app_notes/configset
- Q—Is there an SNMP MIB to grab arp table information? We need both the IP and MAC address in the same table.
- A—Yes. ipNetToMediaTable in MIB-II (RFC 1213)
- Q—I activated Silicon Switching and now MIB values for interface statistics are only updated every 10 seconds. SNMP GETs for MIB values show no change if polled more often (in this case, 8 seconds). Is this is a bug?
- A—This is expected (it’s not a bug). Part of the tradeoff for allowing the box to dedicate more resources to actually switching traffic is to poll less often for interface statistics. “Show interface” should present the same behavior.
- Q—Is there a tool for extracting MIBs from an RFC?
- A—Yes, try premosy < rfc > mib, where rfc is the text document from which mib is to be extracted.
- Q—I would like to capture SNMP traps on my workstation. What tool can I use for this?
- A—On SunOS machines, use /usr/local/bin/tcpdump.
On Solaris, use /sw/current/solaris2bin/traprcv.
On Windows, use: http://www.mg-soft.com. - Q—Is there a way to automatically route IP accounting information to a host’s SYSLOG file? What are some guidelines/performance issues related to setting the accounting threshold and transits?
- A—If you want the IP accounting info, you need to use SNMP to get the information. The router will not “send” the IP accounting info to a SYSLOG server. I don’t think there are any set guidelines for setting the thresholds and transits in IP accounting. Performance is based on how often you use SNMP to get the IP accounting info and how much info you have to get. Be sure to set your SNMP packet-size to 8192 when getting this info.
- Q—What is the best way to block SNMP from coming in on a serial port of a Cisco 2501 that is attached to the Internet? I have an access list for other restrictions already.
- A—If you already have an extended access list, you only need to block UDP ports 161 and 162.
- Q—How do you reload a Cisco 2511 using the SNMP protocol?
- A—Use the MIB variable tsMsgSend with an instance of 0 and a value of 2. Then issue the SET. You also need to add the command snmp-server system- shutdown in the router, as well as have a RW community string.
- Q—What’s the difference between SNMP and SNMP2 configuration in Cisco IOS? Which version of Cisco IOS supports SNMP2?
- A—These are two different SNMP versions. The SNMP packet format is different in each, but there is no difference in configuring them in Cisco IOS. SNMPv2 provides more flexibility and is actually an enhancement to SNMPv1. SNMPv2 is supported as of Cisco IOS 10.2. Cisco supports both. Some NMS stations do not support SNMPv2 yet, so the SNMPv2 MIBs cause an error on the management platform (SNM, HPOV, NetView) when compiled
- Q—Can SNMP give a non-privileged user access to a “privileged” command, such as clear counter?
- A—Yes, you can do a privileged command via SNMP sets. If you have an NMS, SNMP can be used to execute some privileged commands.
- Q—Can Cisco VLANs be configured via SNMP with Cisco IOS 11.0(3)?
- A—You are able to set up different bridge groups, but not via SNMP. You cannot configure a Cisco router at all via SNMP. There is nothing in the Cisco MIBs to enable you to configure via SNMP; you can only monitor. This might change in the future, however.
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