不是很同意楼上说法。
TS 29.303(v940)
5.1.1.3 Discovering a PGW or collocated PGW/GGSN for a 3GPP Access - S5/Gn intra-operator existing PDN
If the existing SGW hostname has "topoff" then the candidate list of PGW shall be used in the order given to try to contact a PGW, after moving any colocated SGW/PGW to the front of the candidate list while maintaining relative order within that set.