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CHAPTER 11
IP Reassembly for Tunnels
Overview
Copyright © 2010, Juniper Networks, Inc.
This chapter describes IP packet reassembly for tunneled protocols on E Series routers;
it contains the following sections:
Overview on page 269
Platform Considerations on page 270
Configuring IP Reassembly on page 271
Monitoring IP Reassembly on page 272
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 270, 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.
269
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