Identifying A Layer 2 Path; Using Ip Traceroute; Understanding How Ip Traceroute Works - Cisco WS-C6506 Software Manual

Using IP Traceroute

Identifying a Layer 2 Path

To identify a Layer 2 path, perform one of these tasks in privileged mode:
Task
(Optional) Trace a Layer 2 path using MAC
addresses.
(Optional) Trace a Layer 2 path using
IP addresses or IP aliases.
This example shows the source and destination MAC addresses specified, with no VLAN specified, and
the detail option specified. For each Catalyst 5000 and 6500 series switch found in the path, the output
shows the device type, device name, device IP address, in port name, in port speed, in port duplex mode,
out port name, out port speed, and out port duplex mode.
Console> (enable) l2trace 00-01-22-33-44-55 10-22-33-44-55-66 detail
l2trace vlan number is 10.
00-01-22-33-44-55 found in C5500 named wiring-1 on port 4/1 10Mb half duplex
C5500:wiring-1:192.168.242.10:4/1 10Mb half duplex -> 5/2 100MB full duplex
C5000:backup-wiring-1:192.168.242.20:1/1 100Mb full duplex -> 3/1 100MB full duplex
C5000:backup-core-1:192.168.242.30:4/1 100 MB full duplex -> 1/1 100MB full duplex
C6000:core-1:192.168.242.40:1/1 100MB full duplex -> 2/1 10MB half duplex.
10-22-33-44-55-66 found in C6000 named core-1 on port 2/1 10MB half duplex.
Using IP Traceroute
The IP Traceroute utility allows you to identify the path that packets take through the network at Layer 3
on a hop-by-hop basis. The command output displays all network layer (Layer 3) devices, such as the
routers, that the traffic passes through on the way to the destination.
These sections describe how to use IP Traceroute:
•
•

Understanding How IP Traceroute Works

The traceroute command uses the Time To Live (TTL) field in the IP header to cause the routers and
the servers to generate specific return messages. Traceroute starts by sending a User Datagram Protocol
(UDP) datagram to the destination host with the TTL field set to 1. If a router finds a TTL value of 1 or
0, it drops the datagram and sends back an Internet Control Message Protocol (ICMP) time-exceeded
message to the sender. The traceroute facility determines the address of the first hop by examining the
source address field of the ICMP time-exceeded message.
To identify the next hop, traceroute sends a UDP packet with a TTL value of 2. The first router
decrements the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL
value of 1, discards the datagram, and returns the time-exceeded message to the source. This process
continues until the TTL is incremented to a value that is large enough for the datagram to reach the
destination host (or until the maximum TTL is reached).
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
20-18
Understanding How IP Traceroute Works, page 20-18
Executing IP Traceroute, page 20-19
Chapter 20
Command
l2trace {src-mac-addr} {dest-mac-addr} [vlan] [detail]
l2trace {src-ip-addr} {dest-ip-addr} [detail]
Checking Status and Connectivity
OL-8978-04