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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter27 Troubleshooting
Using the show forward Command
This is an example of the output from the show forward privileged EXEC command for Fast Ethernet
port 8, where VLAN ID, source and destination MAC addresses, and source and destination IP addresses
were provided.
Switch# show forward fa0/8 vlan 8 0000.1111.2222 0022.3355.9800 ip 8.8.8.10 4.4.4.33 255
signature:00000007, comparison ind:10, control info:2000941A control map:00000000
vlan:8, vlanid entry:000C0012 00000000 00000000 04620000
vlan:8, vlanid entry:000C0012 00000000 00000000 04620000
lookup key bk adata rawoff secoff sec
qos 940808080A04040421 800000000000FF0000 0 00000000 006304 004064 4
acl 940808080A04040421 800000000000FF0000 1 00000082 045408 002016 1
learn 187008000011112222 801008002233559800 0 80010003 002176 002176 0
forw 187008000011112222 801008002233559800 1 40020000 043328 010560 5
bridgeDestMap: 00000000 00000000 0000FFFF FFFFFFC7
vlanMask: 00000000 00000000 0000FFFF FFFFFE7F
portMask: 00000000 00000000 00000000 00000080
sourceMask: 00000000 00000000 00000000 00000000
globalMap: 00000000 00000000 00000000 00000000
globalMask: 00000000 00000000 0002FFFF EFFFFC03
forwMap: 00000000 00000000 00000000 00000100
frame notifies:
src u_dat vlan fl q-map
2 00 8 00 00000000 00000000 00000000 00000100
Egress q 8
signature:00000007, comparison ind:10, control info:2000941A control map:00000000
vlan:8, vlanid entry:000C0012 00000000 00000000 04620000
FastEthernet0/9 vlan 8, dst 0022.3355.9800 src 0000.1111.2222, cos 0x0, dscp 0x0
Much of this information is useful mainly for Technical Support personnel, who have access to detaile d
information about the switch application-specific integrated circ uits (ASICs). However, you can look at
the Egress q section to get information about the output interface. There is an egress section for each
separate destination port. The important information is in the line containing the name of the output
interface, output VLAN ID, and rewritten destination MAC address for the frame. The example shows
that the output interface is Fast Ethernet port 9 and the output VLAN is VLAN 8 and provides the
rewritten source and destination MAC address for the frame.
If the output interface is a trunk port that needs to transmit multiple copies of the frame on different
VLANs (for example, for IP multicast frames), several lines might contain the same output interface
name, but different output VLANs. If output security access control lists (ACLs) are present, it is
possible that one or more of these Egress q sections will not contain a line listing an output port. This
happens when the output ACL denies the packet.
When the CPU is one of the destinations for a packet, a Cpu q section is displayed, followed by a queue
name. This name should correspond to one of the queue nam es in the output from the show controllers
cpu-interface privileged EXEC command, where statistics are displayed for the number of packets
received at each queue.
This is an example of the Cpu q section display:
Cpu q:100 - routing queue