8.3. ISP Provider Edge (PE) Tunnel Routers

8.3. ISP Provider Edge (PE) Tunnel Routers

The use of ISP PE routers as tunnel endpoint routers is not the typical deployment scenario envisioned in this specification. This section attempts to capture some of the reasoning behind this preference for implementing LISP on CE routers.

The use of ISP PE routers as tunnel endpoint routers gives an ISP, rather than a site, control over the location of the egress tunnel endpoints. That is, the ISP can decide whether the tunnel endpoints are in the destination site (in either CE routers or last-hop routers within a site) or at other PE edges. The advantage of this case is that two tunnel headers can be avoided. By having the PE be the first router on the path to encapsulate, it can choose a TE path first, and the ETR can decapsulate and re-encapsulate for a tunnel to the destination end site.

An obvious disadvantage is that the end site has no control over where its packets flow or over the RLOCs used. Other disadvantages include difficulty in synchronizing path liveness updates between CE and PE routers.

As mentioned in earlier sections, a combination of these scenarios is possible at the expense of extra packet header overhead; if both site and provider want control, then Recursive or Re-encapsulating Tunnels are used.