LDP communities provide separation between groups of FECs at the LDP session level. LDP sessions are assigned a community value and any FECs received or advertised over them are implicitly associated with that community.
The community value only has local significance to a node. The user must therefore ensure that communities are assigned consistently to sessions across the network.
SR OS supports multiple targeted LDP sessions over a specified network IP interface between LDP peer systems, each with its own local LSR ID. This makes it especially suitable for building multiple LDP overlay topologies over a common IP infrastructure, each with their own community.
LDP FEC resolution per specified community is supported in combination with stitching to SR or BGP tunnels as follows.
Although a FEC is only advertised within a specific LDP community, FEC can resolve to SR or BGP tunnels if those are the only available tunnels.
If LDP has received a label from an LDP peer with an assigned community, that FEC is assigned the community of that session.
If no LDP peer has advertised the label, LDP leaves the FEC with no community.
The FEC may be resolvable over an SR or BGP tunnel, but the community it is assigned at the stitching node depends on whether LDP has also advertised that FEC to that node, and the community assigned to the LDP session over which the FEC was advertised.