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How BPDU packet being marked

I have been searching for docs about how BPDU packet being marked in the Cisco network , could not find any.

Someone  says the BPDU packet has cos value of 6/7 by default in Cisco  switches,  however I could not find any Cisco document ever mentione  this default value.

Also,  only 802.1Q packet carry cos value. If there is no trunking at all in  the network. Where this QoS value being carried in the BPDU packet ?....  ( by going through the BPDU packet formate, I also do not see any field  could carry QoS marking value. ). In such network environment, how can  we make sure this BPDU packet being transimitted with high priority ?

This is something I really would like to understand more.

Any help will be very appreciated.

Angela

1 REPLY 1
Zizhen Gao
Cisco Employee

The QoS value is not part of BPDU frame, but an internal value assigned on the RBUS (assuming 6500 platform) for BPDU frames. BPDUs are marked with BPDU bit in the code and treated differently -- it always gets to the high priority queue (ingress & egress) on the switch, no matter what the qos setting is. If you are using a SUPERVISOR that supports ELAM (embedded logic analyzer module), you could capture a BPDU and see its DBUS/RBUS setting, an example is as below:

Router#show platform capture elam data
DBUS data:

STATUS_BPDU                      [1] = 1

DATA [592]
0000:  01 80 C2 00 00 00 00 24 14 0E E5 79 00 26 42 42   ".......$...y.&BB"
0010:  03 00 00 00 00 00 80 0A 00 23 04 0E 08 00 00 00   ".........#......"
0020:  00 00 80 0A 00 23 04 0E 08 00 80 02 00 00 14 00   ".....#.........."
0030:  02 00 0F 00 00 00 00 00 00 00 00 00 46 CD AD 5E   "............F..^"
0040:  00 00 00 00 00 00 00 00 00 00                     ".........."

RBUS data:

QOS                              [3] = 7

I cannot find a documentation explaining this either. But this implementation is dated back in 1999 and it has been this way since.

thanks

zz