taglinefilesource code
PPP_IPX53drivers/isdn/isdn_ppp.c#define PPP_IPX 0x002b 
PPP_IPX112drivers/net/ppp.c#define PPP_IPX 0x2b  /* IPX protocol over PPP */
PPP_IPX69include/linux/ppp_defs.h#define PPP_IPX    0x2b  /* IPX protocol */