Information Technology Reference
In-Depth Information
For the 2900XL/3500XL series, use these commands:
•
service timestamps debug uptime
—To show the time with the debug messages
•
service timestamps log uptime
—To show the time with the logging messages
•
debug spantree events
—So that we can see when the port moves through the spanning tree stages
•
show clock
—To see the current time (for the timing tests)
•
show spanning-tree interface FastEthernet 0/1
—To see whether PortFast is enabled on this port
(No mention of PortFast means that it is not enabled.)
•
shut
—To turn off a port from software
•
no shut
—To turn on a port from software
For the 1900/2800 series, use this command:
•
show running-config
—To see the current settings (Some commands are invisible when they
represent the default settings of the switch.)
Configuring and Troubleshooting IP Multilayer Switching
This document outlines basic troubleshooting of Multilayer Switching (MLS) for IP. This feature has
become a highly desired method of accelerating routing performance through the use of dedicated
application-specific integrated circuits (ASICs). Traditional routing is done through a central CPU and
software; MLS offloads a significant portion of routing (packet rewrite) to hardware and thus has also
been termed
switching
.
MLS
and
Layer 3 switching
are equivalent terms. The NetFlow feature of IOS is
distinct and is not covered in this document. MLS also includes support for IPX (IPX MLS) and
multicasting (MMLS), but this document will exclusively concentrate on basic MLS IP troubleshooting.
Introduction
As greater demands are placed on networks, the need for greater performance increases. More PCs are
being connected to LANs, WANs, and the Internet, and their users require fast access to databases,
files/web pages, networked applications, other PCs, and streaming video. To keep connections quick and
reliable, networks must be capable of rapidly adjusting to changes and failures and finding the best path,
all while remaining as invisible as possible to end users. End users who experience rapid information
flow between their PC and server with minimal network slowness are happy ones. Determining the best
path is the primary function of routing protocols, and this can be a CPU-intensive process; thus, a
significant performance increase is gained by offloading a portion of this function to switching hardware.
This is the point of the MLS feature.
There are three major components of MLS: Two of them are the MLS-RP and the MLS-SE. The MLS-RP
is the MLS-enabled router, performing the traditional function of routing between subnets/VLANs. The
MLS-SE is an MLS-enabled switch, which normally requires a router to route between subnets/VLANs
but, with special hardware and software, can handle rewriting of the packet. When a packet transverses
a routed interface, nondata portions of the packet are changed (rewritten) as it is carried to its destination,
hop by hop.
Confusion can arise here because it seems that a Layer 2 device is taking on a Layer 3 task; actually, the
switch is only rewriting Layer 3 information and is “switching” between subnets/VLANs—the router is
still responsible for standards-based route calculations and best-path determination. Much of this
confusion can be avoided by mentally keeping the routing and switching functions separate, especially
when, as is commonly the case, they are contained within the same chassis (as with an internal MLS-RP).
