[ovs-discuss] CDP (cisco discovery protocol) packets being dropped by OVS

[Ben Pfaff]

On Sun, Mar 29, 2015 at 01:44:58PM +0000, Field, Brian wrote:
> I’ve noticed the CDP frames are being dropped by OVS:
> 
> 
> # ovs-dpctl dump-flows
> 
> skb_priority(0),in_port(2),eth(src=52:54:00:d7:50:8e,dst=01:00:0c:cc:cc:cc),eth_type(0/0xffff), packets:0, bytes:0, used:never, actions:drop
> 
> 
> 
> In http://benpfaff.org/~blp/ovs-fields.pdf, it mentions “OpenFlow and Open vSwitch only support LLC headers with DSAP and SSAP 0xaa and control byte 0x03, which indicate that a SNAP header follows the LLC header. In turn, OpenFlow and Open vSwitch only support a SNAP header with organization 0x000000."
> 
> 
> CDP frames use LLC/SNAP encapsulation with an organization code of (0xc).  So it would appear this is why CDP frames are being dropped by OVS.

No.  I expect it's because forward-bpdu is disabled.  Please see
ovs-vswitchd.conf.db(5) for details.

> Is the limitation to only support LLC/SNAP with org values of 0x0 a limitation imposed on OVS because OpenFlow doesn’t support or… ?

It is true that OpenFlow does not support this, but the reason is
primarily that no one has found this important enough to implement.

For CDP, for example, you can alternatively match on the destination
Ethernet address, which is 01:00:0c:cc:cc:cc for CDP.