Improve fabric.p4 with BNG support

Change-Id: I08f8991d8cf432785e0c409354b6301c8983bfb4
diff --git a/pipelines/fabric/src/main/resources/include/define.p4 b/pipelines/fabric/src/main/resources/include/define.p4
index 8f70fb4..8058416 100644
--- a/pipelines/fabric/src/main/resources/include/define.p4
+++ b/pipelines/fabric/src/main/resources/include/define.p4
@@ -115,6 +115,7 @@
 
 const bit<16> PPPOE_PROTOCOL_IP4 = 0x0021;
 const bit<16> PPPOE_PROTOCOL_IP6 = 0x0057;
+const bit<16> PPPOE_PROTOCOL_MPLS = 0x0281;
 
 const bit<8> PROTO_ICMP = 1;
 const bit<8> PROTO_TCP = 6;