Management Interfaces
Believe it or not, one of the first things to think about when
configuring a new network is management, primarily because network management
typically is the last thing to be thought of when the network is implemented,
and seemingly one of the most tedious things to change or improve after the
network is operational. One item to consider is how to handle remote access to
the switch. Catalyst switches support both in-band and out-of-band management.
In-band management interfaces are connected to the switching fabric and
participate in all the functions of a switchport including spanning tree, Cisco
Discovery Protocol (CDP), and VLAN assignment. Out-of-band management interfaces
are not connected to the switching fabric and do not participate in any of these
functions.
Out-of-band management is achieved initially through the serial
console port on the Supervisor module. Each Catalyst switch ships with the
appropriate console cable and connectors to connect to a host such as a Windows
workstation or terminal server. Consult the Catalyst documentation at Cisco.com to determine
the kind of connectors and cables appropriate for each platform. After a
physical connection is made between the console port on a Catalyst switch and a
serial port on a workstation or terminal server, the administrator has full
access to the switch for configuration. At this point, the administrator can
assign an IP address to either an out-of-band management (sl0) interface via the
Serial Line Internet Protocol (SLIP), a predecessor to the Point-to-Point
Protocol (PPP), or assign an IP address to an in-band management interface (sc0
or sc1). Supervisors for the Catalyst 4500 series switches offer an additional
out-of-band management interface via a 10 Mbps or 10/100 Mbps Ethernet interface
(me1) depending on the Supervisor model.
The choice between out-of-band and in-band management is often
not an easy one because each has its pros and cons. An in-band management
connection is the easiest to configure and the most cost effective because
management traffic rides the same infrastructure as user data. Downsides to
in-band management include a potential for switches to be isolated and
unmanageable if connectivity to the site or individual device is lost, for
example in a spanning-tree loop or if fiber connections are cut accidentally. In
addition, if the management interface is assigned to a VLAN that has other ports
as members, any broadcast or multicast traffic on that VLAN is seen by the
management interface and must be processed by the supervisor.
As the speed of processors has improved with newer supervisors,
the risk of overwhelming a supervisor with broadcast/multicast traffic has
declined somewhat, but has not been eliminated completely. With these drawbacks
to in-band management, why doesn't everyone just use out-of band management? The
answer is simple: time and money. Out-of-band management requires a secondary
infrastructure to be built out around the devices such as terminal servers,
switches, and modems. The benefit of an out-of-band management solution is that
it offers a completely separate method of connecting to the devices for
management that does not rely upon a properly functioning data infrastructure to
work.
Many administrators find themselves implementing both in-band
and out-of-band management solutions depending on the reliability of the data
infrastructure between the networks that contain the management stations and the
devices being managed. For example, Catalyst switches in a typical headquarters
location are likely to be on reliable power grids, potentially with backup
power, and have redundant connections between devices. A Catalyst switch in a
remote office connected to headquarters via a router and a single nonredundant
Frame Relay connection might justify out-of-band management. The remote router
and switch could be connected to a terminal server and an analog dial-up
connection for configuration and remote management. In an ideal world,
networking devices would all be accessible via an out-of-band connection, if
possible. Sometimes it takes only a wake-up call at 3:00 a.m. or an unplanned
road trip to a remote location to compel an organization to install an
out-of-band management solution.
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