Mpls vpn Label Switching-label usage

Data Packets transmitted in the MPLS network can have multiple labels. Generally, there is only one label for data transmission in the MPLS network, and there are two labels in the mpls vpn. This is the label stack, they are called the top label and the bottom stack label respectively. The inner layer label should be the bottom stack label, and the outer layer label is the top label. In any case, when packets are transmitted in the MPLS network, A vro forwards data only by looking at one label, that is, only the top label is required for data packets to be forwarded.
Note that two items can be transmitted in the Network: data packets, user data, and route entries, such as OSPF and BGP Route entries; user data always has two labels in mpls vpn. One is used to guide how to transmit data from a PE to the peer PE in the MPLS network, this is the top (the outer tag you are talking about), and the bottom tag (the inner tag you are talking about) guides the PE to which CE the data is sent.

The two labels in the user data packet are the above content, there is another thing is the external attribute of the MP-BGP, this is not the label, the content of this attribute is to the route entry, and the label is irrelevant, it has nothing to do with user data, so the user data is tagged, the route entry is added with the BGP attribute, because with this attribute, it becomes the vpnv4 in the MPLS-VPN.

Additionally, LDP only tags IGP and does not tag BGP. Because BGP has the ability to tag, the BGP routing labels are customized by themselves, but the MP-BGP in mpls vpn does not need to label the route.