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Chapter 11
IP Reassembly for Tunnels
Overview
This chapter describes IP packet reassembly for tunneled protocols on E Series
routers; it contains the following sections:
Overview on page 279
Platform Considerations on page 280
Configuring IP Reassembly on page 281
Monitoring IP Reassembly on page 282
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 on page 280, 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.
279
Overview
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