3

Routing protocol packets are IP packets and therfore they could have different DSCP values to have different priorities (which is implemented by QoS policy) during congestion. How about non-IP packets? For example, how to make STP BPDUs have more priority rather than some other packets? which field in the BPDU could "show" the level of priority? what do standards say about this problem?

5

There's an excellent post by a Cisco employee on Cisco support forums covering this:

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.

Source:

https://community.cisco.com/t5/switching/how-bpdu-packet-being-marked/td-p/1611229

  • I don't know 6500 platforms and its RBUS anyway.... but what about other platforms? How do they handle BPDU frames? In this way, each vendor could have its own way to handle BPDU frames, ya? No standard available for this problem? – A.A May 29 at 12:51
  • 1
    There are no standards describing this scenario, at least in the Cisco world. – user56700 May 29 at 12:53

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