Different encoding of PW label between LDP-VPLS and BGP-VPLS

[ShahriarBasiri]

Dear Community,

While using Wireshark for checking VPLS control packets, I noticed that the format of encoding PW label in LDP-VPLS is different than BGP-VPLS.

In LDP-VPLS, as shown below, a label mapping message is transferred between PEs targeted session. There is a generic label in this control message which is 4 bytes but 20 lowest bits are actually used for label encoding.

 

But in the case of BGP-VPLS, as shown below, 3 bytes are considered for representing PW label block base in BGP update message, and ironically, 3 experimental bits and 1 bottom of stack bit are also included in this message (while they are not included in LDP-VPLS case). It is also interesting that Wireshark understand this behavior and it discards 4 lowest bits when calculating label base.

Interestingly, the format of encoding PW label is not stated in RFC 4761 and RFC 4762 and I know at least one implementation that is different than Cisco which considers the whole 3 bytes as label base in BGP-VPLS case.

Best Regards.

[ShahriarBasiri]

Dear @MHM Cisco World  ,

Thanks. Do you know if there was also a behavior change for VPLS label encoding?

It seems that there was a similar problem with EVPN ESI encoding but there is still two differences between VPLS label encoding and EVPN ESI encoding: 1- EVPN ESI is encoded in extended community but VPLS label is encoded in NLRI and in this "behavior change" it is stated that: "Cisco is changing the way MPLS label being encoded/decoded from Extended Community" 2- In the case of EVPN ESI, RFC 8317 explicitly states that ESI should be encoded in high-order 20 bits but I think there isn't such a rule for VPLS label in any RFC.

Regards.