Tunneling protocols provide a method of forwarding packets of a particular protocol through a network of a different protocol type. For example, L2TP can transport a protocol such as PPP through a routed IP network. This capability requires a pair of devices that define the endpoints of the tunnel. Packets entering the tunnel are processed and encapsulated at the ingress endpoint, and packets exiting the tunnel are processed and de–encapsulated at the egress endpoint.

When packets are tunneled through an IP network, simple IP forwarding is performed. The IP forwarding process might fragment packets in the tunnel. Tunnel processing requires each packet to exit the tunnel in the same form in which it entered. Fragmented packets that are not reassembled before the tunnel egress processing are dropped.

For example, in Figure 21, traffic is tunneled through an IP network that has four hops. Because the MTU of the link between routers B and C is smaller than that of previous hops, some packets are fragmented. Router D must reassemble the packets before tunnel egress processing and de-encapsulation are performed.

For more information about configuring tunnel-service interfaces, see Managing Tunnel Service and IPsec Service Interfaces in JunosE Physical Layer Configuration Guide.

Figure 21: Tunneling Through an IP Network That Fragments Packets

Tunneling Through an IP Network That
Fragments Packets