1.4.3 MTU and Fragments
When IP packets are forwarded, larger packets are divided and sent according to the maximum transmission unit (MTU:Maximum Transfer Unit). This is called fragmentation. Packets that do not exceed the MTU are processed by the hardware. A packet that has been fragmented, however, is forwarded by the software in fragments, adversely affecting forwarding performance.
- <Structure of this section>
(1) Determining MTU
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Determining the MTU for VLAN interfaces
Of the MTUs for the Ethernet interfaces belonging to a VLAN, the system MTU information, and the IP MTU information, the smallest MTU is used as the MTU for the VLAN interface.
In IPv4/IPv6 communication, the Switch uses the MTU for VLAN interfaces.
The following table describes how the MTU is determined for a VLAN interface.
Table 1-7 VLAN Interfaces MTU Determination Matrix Settings
1
2
3
4
5
6
7
8
System MTU information
Set
Set
Set
Set
Omitted
Omitted
Omitted
Omitted
IP MTU information
Set
Set
Omitted
Omitted
Set
Set
Omitted
Omitted
Port MTU information
Set
Omitted
Set
Omitted
Set
Omitted
Set
Omitted
MTU value
A2
A1
A4
A1
A2
A3
A4
A5
- Legend
-
A1: The setting in the system MTU information and the setting in the IP MTU information are compared, and the smaller value is selected.
A2: The setting in the IP MTU information and the smallest value of the MTU values set for ports in the port MTU information are compared, and the smaller value is selected.
A3: The setting in the IP MTU information and 1500 are compared, and the smaller value is selected.
A4: Smallest value of the MTU values set for ports in the port MTU information
A5: 1500
NOTE When the line type is 10BASE-T, MTU is 1500 regardless of the setting.
Figure 1-10: Sample Configuration for VLAN Interfaces ![[Figure Data]](./GRAPHICS/ZU302120.GIF)
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When the IP MTU information does not exist
- Determination of MTUs
-
MTU value for VLAN 100: 1600
MTU number of VLAN 200: 1900
-
When the IP MTU information exists
When ip mtu 1000 is set for VLAN 100 and ip mtu 3000 is set for VLAN 200
- Determination of MTUs
-
MTU number of VLAN 100: 1000
MTU number of VLAN 200: 1900
(2) MTU and Fragments
A network can contain subnetworks with different MTUs. When a large IP packet passes through a network with a smaller MTU, the IP packet is fragmented before being forwarded.
The following figure illustrates fragmentation. In the figure, when a packet sent from network A is forwarded to network B, the packet is split into fragments because the MTU for network A is 1500 and the MTU for network B is 630.
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![[Figure Data]](./GRAPHICS/ZU302130.GIF)
(3) Generation of fragments
The data portion of an IP packet (the packet without the IP header) exceeding the MTU is divided into fragments that are multiples of 8.
Because the MTU of network B is 630, the size of a packet must be 610, excluding the IP header. Because the largest multiple of 8 that is less than 610 is 608, a packet is divided into 608-byte fragments, and each fragment is given an IP header. The following figure shows the fragmentation of a packet.
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![[Figure Data]](./GRAPHICS/ZU302140.GIF)
When an IP packet is split into fragments to fit the MTU, the fragmentation is described by the Fragment Offset field and the More Fragments bit in the IP header. The switch sets the same Identification field in all the IP headers and recalculates the checksum. The offset is calculated by dividing the data length from the beginning by 8.
(4) Reassembly of fragments
The fragments of an IP packet are reassembled at the end point based on the Identification and Fragment Offset fields and the More Fragments bit in the IP headers. Intermediate routers do not reassemble fragments. This is because routers are intended to route the individual fragments to the end point. If an intermediate router were to store fragments for reassembly into an IP packet, the stored fragments would be discarded if any fragments did not pass through the router.