When R2 & R5 are route reflectors (RRs), routes from R4 & R8 are advertised to R5 and R5 advertises to R2. But R2 would drop them as R2 is also a RR. Therefore some routes are missing on R1 to advertise to eBGP peers.
Good reference:
https://www.ciscolive.com/c/dam/r/ciscolive/emea/docs/2015/pdf/TECRST-2310.pdf Route reflectors (RR) must be fully iBGP meshed so we cannot configure RR on both R1 and R5.
We should choose routers at the center of the topology RRs, in this case R4 & R5.

The -imp-null (implicit null) tag instructs the upstream router to pop the tag entry off the tag stack before forwarding the packet.
Note: pop means -remove the top MPLS label

The DHCPv6 Guard feature blocks reply and advertisement messages that come from unauthorized DHCP servers and relay agents.
Packets are classified into one of the three DHCP type messages. All client messages are always switched regardless of device role. DHCP server messages are only processed further if the device role is set to server. Further processing of server messages includes DHCP server advertisements (for source validation and server preference) and DHCP server replies (for permitted prefixes).
If the device is configured as a DHCP server, all the messages need to be switched, regardless of the device role configuration.