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Field-Tested Solutions to Cisco Router Problems
Cisco
Cookbook
Kevin Dooley & Ian J. Brown
Cisco Cookbook
Cisco Cookbook
Ian Brown and Kevin Dooley
Beijing
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Farnham
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CHAPTER 17
Simple Network Management
Protocol
Chapter 17
17.0 Introduction
Since its introduction in 1988, the Simple Network Management Protocol (SNMP)
has become the most popular network management protocol for TCP/IP based net-
works. The IETF created SNMP to allow remote management of IP based devices
using a standardized set of operations. It is now widely supported by servers, print-
ers, hubs, switches, modems, UPS systems, and (of course) Cisco routers.
The SNMP set of standards define much more than a communication protocol used
for management traffic. The standards also define how management data should be
accessed and stored, as well as the entire distributed framework of SNMP agents and
servers. The IETF has officially recognized SNMP as a fully standard part of the IP
protocol suite. The original SNMP definition is documented in RFC 1157.
In 1993, SNMP Version 2 (SNMPv2) was created to address a number of functional
deficiencies that were apparent in the original protocol. The added and improved
features included better error handling, larger data counters (64-bit), improved effi-
ciency (get-bulk transfers), confirmed event notifications (informs), and most nota-
bly, security enhancements. Unfortunately, SNMPv2 did not become widely
accepted because the IETF was unable to come to a consensus on the SNMP secu-
rity features.
So, a revised edition of SNMPv2 was released in 1996, which included all of the pro-
posed enhancements except for the security facility. It is discussed in RFCs 1905,
1906, and 1907. The IETF refers to this new version as SNMPv2c and it uses the
same insecure security model as SNMPv1. This model relies on passwords called
community strings
that are sent over the network as clear-text. SNMPv2c never
enjoyed widespread success throughout the IP community. Consequently, most
organizations continue to use SNMPv1 except when they need to access the occa-
sional large counter variable. The IETF recently announced that SNMPv3 would be
the new standard, with SNMPv1, SNMPv2, and SNMPv2c being considered purely
historical.
607
This is the Title of the Book, eMatter Edition
Copyright © 2003 O’Reilly & Associates, Inc. All rights reserved.
Cisco’s IOS supported SNMPv2 until Version 11.2(6)F, when Cisco began support-
ing SNMPv2c. Cisco continues to support SNMPv2c in every IOS version beginning
with 11.2(6)F. In addition, every version of IOS has supported SNMPv1 since the
earliest releases.
The compromise that became SNMPv2c left the management protocol without satis-
factory security features. So, in 1998, the IETF began working on SNMPv3, which is
defined in RFCs 2571–2575. Essentially, SNMPv3 is a set of security enhancements
to be used in conjunction with SNMPv2c. This means that SNMPv3 is not a stand-
alone management protocol and does not replace SNMPv2c or SNMPv1.
SNMPv3 provides a secure method for accessing devices using authentication, mes-
sage integrity, and encryption of SNMP packets throughout the network. We have
included a recipe describing how to use the SNMPv3 security enhancements (see
Recipe 17.21). Table 17-1 lists the three supported versions of SNMP and highlights
their security capabilities.
Table 17-1. SNMP versions supported by Cisco
Version
Authentication
Encryption
Description
v1
Community strings
None
Trivial authentication. Packets sent in clear-text.
v2c
Community strings
None
Trivial authentication. Packets sent in clear-text.
v3(noAuthNoPriv)
Username
None
Trivial authentication. Packets sent in clear-text.
v3(authNoPriv)
SHA or MD5 encrypted
pass phrase
None
Strong authentication. Packets sent in clear-text.
v3(authPriv)
SHA or MD5 encrypted
pass phrase
DES
Strong authentication. Packets are encrypted.
SNMP Management Model
SNMP defines two main types of entities,
managers
and
agents
. A manager is a server
that runs network management software that is responsible for a particular network.
These servers are commonly referred to as Network Management Stations (NMS).
There are several excellent commercial NMS platforms on the market. Throughout
this book we will refer to the freely distributed NET-SNMP system as a reference
NMS.
An agent is an embedded piece of software that resides on a remote device that you
wish to manage. In fact, almost every IP-capable device provides some sort of built-in
SNMP agent. The agent has two main functions. First, the agent must listen for
incoming SNMP requests from the NMS and respond appropriately. And second, the
agent must monitor internal events and create SNMP traps to alert the NMS that
something has happened. This chapter will focus mainly on how to configure the
router’s agent.
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608
Chapter 17:
Simple Network Management Protocol
This is the Title of the Book, eMatter Edition
Copyright © 2003 O’Reilly & Associates, Inc. All rights reserved.
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