Fragmentation and Path Maximum Transmission Unit Discovery

As you learned in Chapter 6, "General Design Considerations," fragmentation can be dealt with by allow code 4 messages into your network. This allows a function called path MTU discovery (PMTUD) to functi segments. End-host-to-end-host MTU negotiation occurs at the beginning of a TCP communication with the TCP header. This doesn't take into account differences in the path MTU, though, as introduced by di This is where PMTUD is useful. It is helpful to first see a basic example of fragmentation without IPsec: I basic fragmentation as it occurs for UDP traffic and TCP traffic with the Don't Fragment (DF) bit not set;

the same example with the host using PMTUD and setting the DF bit for all communications.

Figure 10-18. IP Fragmentation

Figure 10-18. IP Fragmentation

Figure 10-19. Basic PMTUD

When using GRE, it is just like the preceding example except the MTU is set to 1476 when the actual tra 1500 (20 bytes for new IP header, 4 bytes for GRE encapsulation). GRE fragmentation occurs before en for the tunneled traffic (to catch lower MTU links along the path) is turned off by default on Cisco router with the following command on the interface used by the tunnel:

Router(config-if)# tunnel path-mtu-discovery

When using IPsec, fragmentation occurs after encryption and PMTUD is on by default. If a packet arrive gateway and needs fragmentation but the DF bit is set, the gateway drops the packet and sends a type just as in traditional WAN fragmentation.

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