When the number of L2TP sessions reaches the configured maximum value, the LNS sends an out-of-resource Result Code (4 or 5) in a CDN (Call-Disconnect-Notify) message to the LAC. This would trigger the LAC to fail over to another LNS that has the resources available. Similarly, when the tunnel is not usable because of the invalid destination CDN error, the Result-Code 6 is sent from the LNS.
Certain third-party LAC implementations trigger tunnel failover only when they receive Result Code 2 in CDN messages (and not 4,5 or 6). To support those scenarios, the LNS in the 7750 SR can overwrite result Codes 4, 5 and 6 with result Code 2 just before they are sent to the LAC. Result Codes can be overwritten only during the L2TP session initialization phase. These codes have the following meanings and are described in RFC 2661, 4.4.2:
2
Call disconnected for the reason indicated in error code
4
Call failed because of the lack of appropriate facilities being available (temporary condition)
5
Call failed because of the lack of appropriate facilities being available (permanent condition)
6
Invalid Destination
This functionality is enabled on LNS with the following CLI hierarchy:
configure router l2tp
configure service vprn <id> l2tp
replace-result-code {cdn-tmp-no-facilities | cdn-prem-no-facilities | cdn-inv-
dest}
no replace-result-code