In a traditional MMP deployment, the stack group tunnel servers use L2TP or L2F tunnels to deliver MLP links to the bundle owner. This architecture does not easily scale beyond a few routers per tunnel server stack, and inherently adds hops and latency variations between links in a bundle.
Enabling L2TP redirect allows a stack group member to send a redirect message to the NAS if it receives a link that is owned by another stack group member. L2TP redirect increases the scalability of MMP deployments, load balances sessions across the stack group tunnel servers, and smooths traffic as all links in a multilink bundle experience the same delay and latency.
The figure below shows a network scenario using L2TP redirect for an MMP deployment.
Figure 2. L2TP Redirect Scenario
When tunnel server 1 answers the initial call, SGBP bidding is performed by all stack group members to determine which device owns the call. If the call is owned by a different tunnel server, such as tunnel server 2, the call must be passed from tunnel server 1 to the owner.
In a traditional multihop SGBP deployment, tunnel server 1 would establish an L2F or L2TP tunnel to to tunnel server 2 and forward the call over that tunnel.
With L2TP redirect enabled, instead of establishing a new tunnel to tunnel server 2, tunnel server 1 sends a redirect message to the NAS informing it that tunnel server 2 actually owns the call. The NAS then tears down the initial connection to tunnel server 1 and establishes a new L2TP tunnel directly to tunnel server 2.
How L2TP Redirect Works
In a traditional SGBP multihop VPDN deployment, if a stack group member receives a call that belongs to a different stack group member, it forwards the call to the bundle owner over an L2TP or L2F tunnel. When L2TP redirect is configured, instead of forwarding the call to the bundle owner the stack group member will send a redirect message to the NAS. The redirect message includes the IP address or redirect identifier of the bundle owner. The NAS will terminate the initial connection, and initiate a new connection directly to the bundle owner.
For L2TP redirect to function, it must be enabled on both the NAS and the stack group tunnel servers. If the NAS is not configured for L2TP redirect, the tunnel server will forward the call to the bundle owner using traditional multihop technology. This maintains interoperability with non-Cisco devices and Cisco devices running older versions of Cisco IOS software.
In order to redirect the call, the NAS must perform redirect authorization for the bundle owner. If a redirect identifier has been configured on the bundle owner, the NAS uses that identifier to get redirect authorization information. Otherwise, the IP address of the bundle owner must be configured on the NAS.