+## FIB Table Lookup for Inner IPv4/IPv6 packet
+
+SRv6 Mobile functions of `t.m.gtp4.dt*` and `end.m.gtp6.dt*` support decapsulating outer IP/UDP/GTP-U headers and forwarding inner IP packet based on specific fib table.
+
+In case of the both outer and inner IP address families are IPv4, `t.m.gtp4.dt4` function supports GTP-U decapsulation and fib lookup for inner IPv4 with an associated steering policy and the following parameters:
+
+- SID: A SRv6 SID to represents the function
+- FIB: fib-table number for inner IPv4 packet lookup and forwarding
+
+The following command instantiates a new T.M.GTP4.DT4 function.
+
+```
+sr policy add bsid SID behavior t.m.gtp4.dt4 fib-table FIB
+```
+
+For example, the below commands configure D5:: as the SID instantiates `t.m.gtp4.dt4` function. A steering policy for packets destine to 172.20.0.1 binds to the SID.
+
+```
+sr steer l3 172.20.0.1/32 via bsid D5::
+sr policy add bsid D5:: behavior t.m.gtp4.dt4 fib-table 0
+```
+
+In addition, inner IPv6, or mix of IPv4 and IPv6 inner packet cases require the function to be configured with local-fib table.
+
+- LOCAL-FIB: fib-table number for lookup and forward GTP-U packet based on outer IP destination address
+
+This is inner IPv6 case specific. The reason is that GTP-U encapsulates link local IPv6 packet for NDP (Neighber Discovery Protocol). Outer GTP-U header should be kept until the packets reach to the node responsible for NDP handling. It is typically UPF(User Plane Function) node.
+
+The following command instantiate a new T.M.GTP4.DT6 function.
+
+```
+sr policy add bsid D5:: behavior t.m.gtp4.dt6 fib-table 0 local-fib-table LOCAL-FIB
+```
+
+Following example configures fib 0 for inner packet and fib 1 for outer GTP-U packet forwarding:
+
+```
+sr policy add bsid D5:: behavior t.m.gtp4.dt6 fib-table 0 local-fib-table 1
+```
+
+If you need to suport both IPv4 and IPv6 inner packet lookup with just one SID, you can configure `t.m.gtp4.dt46` function:
+
+```
+sr policy add bsid D5:: behavior t.m.gtp4.dt46 fib-table 0 local-fib-table 1
+```
+
+In case of GTP-U over IPv6 case, `end.m.gtp6.dt4`, `end.m.gtp6.dt6` and `end.m.gtp6.dt46` functions support inner IPv4, IPv6 and IPv4/IPv6 lookup and forwarding respectively. Specifiyng fib table for inner IP packet forwarding is required as same as GTP-U over IPv4 case, and local-fib table for inner IPv6 and IPv4/IPv6 cases as well.
+
+```
+sr localsid prefix D::/64 behavior end.m.gtp6.dt46 fib-table 0 local-fib-table 0
+```
+