For a better understanding here are the picture of my iBGP Mesh:

DN42 Basic Network

Every Peering has its own Bird Routing Table and i use pipes to leak from the Peering Tables to master. Following are the details of my iBGP Peerings/Templates and Filters. The lines indicates WireGuard VPN Links (purple) and iBGP Peerings (green dotted).

Used Bird Templates for iBGP Peerings and Pipes

# Template for iBGP Peerings
template bgp iBGP_Peer {
  local as MYAS;
  # Table should be set within each Peer to T_PEERNAME
  igp table T_OSPF;
  path metric on;                               # Enable comparison of path lengths when deciding which BGP route is the best one
  import keep filtered;                         # Keep Filtered Routes for this peering.
  import where iBGP_import_peer_policy();       # iBGP Import Peer Policy
  export where iBGP_export_peer_policy();       # iBGP Export Peer Policy
  source address MYIPv4;                        # iBGP Peering with MYIPv4 to add backup pathes
  next hop self;                                # Set Next Hop Self for clean routing
};

# Template Pipe for iBGP Peerings
template pipe iBGP_Pipe {
  # Table had to be set in each Peer Definition to T_IBGP_PEERNAME
  peer table master;
  
  import where iBGP_routing_policy();
  export where iBGP_routing_policy();
};

With these Templates, a iBGP Peering consists of one file in peers4/6 directory, for example Peering Definition on dn42-uk01 for dn42-de02 aka dn42-gw:

table T_IBGP_DE02;

protocol bgp B_IBGP_DE02 from iBGP_Peer {
  table T_IBGP_DE02; 
  neighbor 172.20.175.211 as 4242423905;
};

protocol pipe P_IBGP_DE02 from iBGP_Pipe {
  table T_IBGP_DE02; 
};

BGP Communities for AS4242423905

I added the following communities to routes i learned:

BGP Community	Desription
MYPeerID	PeerID of System who learns the route( PeerID == iBGP Loopback IP)
1000 + MYPeerID	Peering Originates from an Peer on System with PeerID
7000 + MYPeerID	Route was learned from a "Customer/Peering" on System with PeerID
8000 + MYPeerID	Route was learned from an IXP on System with PeerID
9000 + MYPeerID	Route was learned from an "Upstream" on System with PeerID

Based on this communities i update local_pref on iBGP egress per System. Below you find my current Filter/Routing Polica on iBGP Links.

iBGP Routing Policy and import/export filter

#
##
## iBGP Policies
##
#

# iBGP Peer Policies
function iBGP_import_peer_policy() {

  if bgp_path.len > 64 then return false;       # Reject too long BGP Paths
  if local_nets() then return false;            # Reject local used networks (IXP, Peerings) from iBGP Peers
  if ( bgp_local_pref > 1000 ) then {           # Reset local pref on iBGP Links, to do AS Path Metric only
	bgp_local_pref = 100;
  }

  return true;                                  # Allow all on iBGP Links
};

function iBGP_export_peer_policy() {

  if proto = "dn42_static" then return true;    # Allow Static announced routes, should be renamed to S_STATIC

  if source != RTS_BGP then return false;
  if bgp_path.len > 64 then return false;

  if ( bgp_ext_community ~ [ (rt, MYAS, MYPeerID + 9000 ) ] ) then {
    bgp_local_pref = 100;
  }

  if ( bgp_ext_community ~ [ (rt, MYAS, MYPeerID + 8000 ) ] ) then {
    bgp_local_pref = 110;
  }

  if ( bgp_ext_community ~ [ (rt, MYAS, MYPeerID + 6000 ) ] ) then {
    bgp_local_pref = 115;
  }

  if ( bgp_ext_community ~ [ (rt, MYAS, MYPeerID + 7000 ) ] ) then {
    bgp_local_pref = 145;
  }

  return true;                                  # Allow all on iBGP Links
};

# iBGP Routing Policies
function iBGP_routing_policy() {
  
  if source = RTS_OSPF then return false;       # For iBGP i don't want OSPF in my Routing Policy
  if source = RTS_DEVICE then return false;     # For iBGP i don't want DEVICE ROUTES in my Routing Policy

  krt_prefsrc = MYIPv4;

  return true;                                  # Allow all on iBGP Links
};