In deployment scenarios with multiple LNS nodes, a list of those LNS nodes can be presented to the LAC during the L2TP session instantiation process (either through CLI or RADIUS). An example of this would be a RADIUS Accept message with a list of tunnel peers:
tunnel.com Auth-Type := Local, Password == "tunnel1"
Tunnel-Type:1 += L2TP,
Tunnel-Medium-Type:1 += IP,
Tunnel-Client-Auth-Id:1 += lns_tun,
Tunnel-Assignment-Id:1 += 1,
Tunnel-Client-Endpoint:1 += 10.0.0.1,
Tunnel-Server-Endpoint:1 += 10.0.0.2,
Tunnel-Password:1 += TUNNELPASS,
Tunnel-Type:2 += L2TP,
Tunnel-Medium-Type:2 += IP,
Tunnel-Client-Auth-Id:2 += lns_tun,
Tunnel-Assignment-Id:2 += 2,
Tunnel-Client-Endpoint:2 += 10.0.0.1,
Tunnel-Server-Endpoint:2 += 10.0.0.3,
Tunnel-Password:2 += TUNNELPASS,
Tunnel-Type:3 += L2TP,
Tunnel-Medium-Type:3 += IP,
Tunnel-Client-Auth-Id:3 += lns_tun,
Tunnel-Assignment-Id:3 += 3,
Tunnel-Client-Endpoint:3 += 10.0.0.1,
Tunnel-Server-Endpoint:3 += 10.0.0.4,
Tunnel-Password:3 += TUNNELPASS,
Tunnel-Type:4 += L2TP,
Tunnel-Medium-Type:4 += IP,
Tunnel-Client-Auth-Id:4 += lns_tun,
Tunnel-Assignment-Id:4 += 4,
Tunnel-Client-Endpoint:4 += 10.0.0.1,
Tunnel-Server-Endpoint:4 += 10.0.0.5,
Tunnel-Password:4 += TUNNELPASS
If the tunnel or the session establishment attempt fails for any reason, a search for additional operational facilities (tunnels or peers) is made to complete the establishment of the tunnel or session that failed in the previous attempt. Sometimes it is required to go beyond this automatic search for the new facilities and place the tunnel or peer in question into a denylist. A tunnel timeout always forces the corresponding peer and the tunnel into the denylist. In addition, a tunnel can be forced into the denylist by specific explicit error codes (CDN, and Stop-CCN) during the tunnel or session initialization phase. A peer is never forced on a denylist because of explicit Result-Code sent by LNS.
Denylisted peers and tunnels are not eligible to serve new incoming L2TP session until they are removed from the denylist. The exception is when all tunnel specs evaluate into a denylisted item. Then, a denylist item (tunnel) is tried.