bit indexed explicit replication is a 256 bit header after the mpls labels stack.
(anyway it have other encapsulations, not just mpls)
as name implies, each bit describes the correspondig pe router a given packet is
targeted to or not. additionally it carries a protocol indicator, source bier id, and some
other various fields too. you configure the bier id to every router's loopback in your core,
and this info is flooded within isis or ospf
with the label base of the bier of the given router. from that point, you can address
any combination of pe routers by setting the corresponding bits of them in the bier header
and finally sending out the resulting big bier packet to the next hop p with it's advertised
bier label. the core will replicate the packet to all the pe routers
whose bit was set, so it could be considered as an one to many tunneling technique.
this is achieved by every p router examines the bier header and replicate the packet
if needed to multiple interfaces, keeping only those bits on a given interface whose pe
routers are reachable through that interface.
it could be used with vpn services or to tunnel pim. with layer3 mvpn
to provide multicast services or with layer2 vpn to eliminate the burden of ingress replicating
packets that need to be flooeded within the layer2 domain.
if you have a lot of pe routers in the core, it can support bigger bit strings,
or the sending pe router can send out more copies of the given packet to various
indexes in the bier label block to address higher numbered targets.
the main advantage of using bier is that it have no state at the p routers
and gives you ecmp, frr and some other advantages.