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NE40E V800R010C10SPC500 Feature Description - IP Services 01

This is NE40E V800R010C10SPC500 Feature Description - IP Services
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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
IP MTU Fragmentation

IP MTU Fragmentation

IP MTU Fragmentation Implementation

MTU Fragmentation Related Processes Process Location Notes
Original IPv4 packet sending Control plane

Original IPv4 packets indicate the protocol packets sent from control plane of local device. The source IP address of these IPv4 packets are local device.

Border Gateway Protocol (BGP) packets, Internet Control Message Protocol (ICMP) messages, and bidirectional forwarding detection (BFD) control packets, belong to protocol packets.

When the ping command is run on a device, the device sends ICMP request messages.

Original IPv6 packet sending Control plane

Original IPv6 packets indicate the protocol packets sent from control plane of local device. The source IPv6 address of these IPv6 packets are local device.

When the ping ipv6 command is run on a device, the device sends ICMPv6 request messages.

IPv4 packet forwarding Forwarding plane

IP fragmentation occurs when the device sends packet, but not when the device receives packet.

For NE40E device, the MTU configured in the interface (also called interface MTU) is IP MTU. IP MTU is a layer 3 definition. Therefor, interface MTU only takes effect on Layer 3 traffic, but not on Layer 2 traffic. Even when IP header + IP payload of L2 frame exceeds interface MTU, the L2 frame will not be fragmented.

NOTE:

Generally, only the source and destination nodes need to analyze the IPv6 extension headers. So fragmentation only occurs on the source node, which is different from IPv4.

Force-fragment

By default, when the IPv4 packet's length is larger than the interface MTU,

  • If DF=0, the packet is fragmented.
  • If DF=1, the packet is not permit to fragmented, and the device drops the packet and return a Packet-too-big message.

NE40E supports force-fragment function. If force-fragment is enabled, the board ignoring DF-bit all IPv4 large packets (size> MTU) will be cut into packets and be forwarded with DF=0.

To enable force-fragment function, run the ipv4 force-fragment enable command.

Force-fragment function is enabled only for IPv4 packets, not for other type packets.

By default, the force-fragment function is not enabled.

Fragmentation in Control Plane

As shown in Figure 7-3, control plane fragments IP packet and then encapsulate them with tunnel header (such as MPLS and L2TP) if needed before sending the packet to forwarding plane. Control plane fragmentation is implemented by software. Therefore, the fragmentation rules are the same in different board types.

Figure 7-3 Fragmentation in Control Plane

If the size (including the IP header and payload) of non-MPLS packets sent from control plane, is greater than the MTU value configured on an outbound interface:
  • If the DF field is set to 0 in a packet, the packet is fragmented. The size of each fragment is less than or equal to the interface MTU.
  • If the DF field is set to 1 in a packet, the packet is discarded.
  • If the DF field is set to 1 in a packet and the out interface is enabled with forcible-fragmentation, the packet is fragmented. Each fragment is forwarded with DF=0. (By default, forcible-fragmentation is not enabled for control plane. To enable forcible-fragmentation for control plane, run the clear ip df command in the out interface.

For the information about the fragmentation for MPLS packet, see the chapter MPLS MTU Fragmentation.

Protocol packets are usually allowed to be fragmented (DF=1), that is, the protocol packets are usually not be discarded in the original device even when they exceed the MTU. the protocol packets are not allowed to be fragment (DF=1) only when:
  • the device is implementing PMTU discover, such as IPv6 PMTU discover, or LDP/RSVP-TE PMTU negotiation.
  • the ping -f command is running on the local device.

Fragmentation in Forwarding Plane

Fragmentation in forwarding plane takes effort only on forwarding traffic. Forwarding traffic indicates the traffic passes through the local device, without being sent to the control plane. Forwarding traffic does not include the traffic sent from control plane.

The NE40E device provides rich board types. Different board types may have different MTU fragmentation rules, as shown in Figure 7-4.

Figure 7-4 Fragmentation on motherboards

Fragmentation on motherboards or integrated boards:

  • The fragmentation works only towards the IPv4 traffic to be forwarded (including the IPv4 traffic to be forwarded after decapsulation).

    For example, MPLS L3VPN traffic is MPLS-encapsulated before being forwarded from a network-to-network interface (NNI) to a user-to-network interface (UNI) on a provider edge (PE) node. PE fragments the traffic after removing MPLS labels.

    For another example, in L3VPN and high-speed Internet (HSI) scenarios, if a customer premises equipment (CPE) uses a Dot1q or QinQ termination sub-interface to access a PE, the packet sent from CPE to PE is VLAN tagged. In these scenarios, the packet is also fragmented on the PE after the VLAN tags are removed.

  • Fragmentation does not occur during L2 or MPLS forwarding behavior.
  • Fragmentation does not occur during IPv6 forwarding.
  • Force-fragment ( ipv4 force-fragment enable command ): By default, when the IPv4 packet's length is larger than the interface MTU, if DF=1, the packet is not permit to fragmented, and the device drops the packet and return a Packet-too-big message. If force-fragment is enabled, the board ignoring DF-bit all IPv4 large packets (size> MTU) will be cut into packets and be forwarded with DF=0. Force-fragment function is enabled only for IPv4 packets, not for other type packets.
Figure 7-5 Fragmentation on integrated boards

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Updated: 2019-01-03

Document ID: EDOC1100055041

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