Missing MAC Address

[Peter Fiers]

Hi,

I've got four switches in a line. On the fourth, an AP is connected. A PC, associated with this AP, sends ethernet frames through the switches, but its MAC address doesn't show up in the MAC tables like this:

--- snip ---

Switch1#ping 10.149.251.33

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.149.251.33, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms
Switch1#
Switch1#sh ip arp | in 10.149.251.33
Internet  10.149.251.33          93   0016.6f75.861e  ARPA   Vlan251
Switch1#sh mac add add 0016.6f75.861e
No entries present.

--- snap ---

Only the switch which the access point is connected to knows the MAC address.

The port in the first switch leading to the host in question is configured like this:

--- begin config ---

interface GigabitEthernet1/45
switchport access vlan 7
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 7,251
switchport mode trunk
no snmp trap link-status
spanning-tree portfast
end

--- end config ---

Other MAC addresses ARE present on the port in VLAN 251

Any suggestions?

Thanks

[Peter Fiers]

Hi Peter,

I've got the trace now. In the attachment, you can see two packets: DHCP Req and DHCP Ack, captured on the ethernet interface of the notebook. You can learn the MAC address of this interface from the first packet: 00:0b:97:a2:3d:b1. Note that the interface uses the IP address of the wireless interface (10.149.251.33). Its own IP is 10.149.251.35. The entry 'client hw address' in the data field contains the MAC address of the wireless interface (00:16:6f:75:86:1e). The answer from the DHCP server is sent to this MAC address and broadcasted by the switches as unknown unicast, so it reaches the ethernet interface. Besides, a trace made on the wireless interface shows that no frame other than for 802.1X ever leaves the notebook on this interface and therefore the network never gets a chance to learn the MAC address of this interface.

Thanks,

Peter

[Peter Paluch]

Hi Peter,

Thank you for updating this thread! Interesting... you have seen the issue happening with DHCP packets. The MAC of the wireless interface was used in the hwaddr field of the DHCPREQ packet that was sent out the wired interface. Seems to be very consistent with the ARP issue I have described earlier. It could be interesting to see whether the ARPs behave in the same way but the DHCP does indeed.

Thank you again, Peter. This is really an issue worth having bookmarked somewhere.

Best regards,

Peter

[vinod.agrahari]

Great,

I am not getting any reason to not get mac address through the command(sh mac).

Very interesting discussion.

Please just apply authentication to connect with AP and probably then you could get mac address from sh mac command.

Regards

Vinod Agrahari