No relevant resource is found in the selected language.

This site uses cookies. By continuing to browse the site you are agreeing to our use of cookies. Read our privacy policy>Search

Reminder

To have a better experience, please upgrade your IE browser.

upgrade

Feature Description - LAN Access and MAN Access 01

NE05E and NE08E V300R003C10SPC500

This is NE05E and NE08E V300R003C10SPC500 Feature Description - LAN Access and MAN Access
Rate and give feedback:
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).
MPLS MTU Fragmentation

MPLS MTU Fragmentation

MPLS MTU Definition

The Multiprotocol Label Switching (MPLS) MTU defines the maximum number of bytes in a labeled packet that an MPLS device can forward without fragmenting the packet. In NE, the MPLS MTU is the maximum length of the sum of the following fields:

  • MPLS label stack
  • IP header
  • IP payload
Figure 7-6 MPLS MTU example

MPLS MTU Usage Scenarios

On NE, the MPLS MTU takes effect only on Layer 3 traffic traveling from IP networks to MPLS tunnels. The MPLS packets on which a specified MPLS MTU takes effect must contain labels next to the IP headers. MPLS MTU usage scenarios are as follows:

  • MPLS L3VPN scenario, the traffic forwarding from User-to-network interface (UNI) Network-to-network interface (NNI).
  • IP traffic on PEs or Ps is directed into LSPs using policy-based routing (PBR), the redirection function, static routes, Interior Gateway Protocol (IGP) shortcuts, and forwarding adjacency.
  • Packets originate from the control plane and are directed to LSPs. For example, run the ping -vpn-instance or ping lsp command is executed on the device, the device originates ICMP Request messages. These messages are IP packets and will be sent to MPLS tunnels.

MPLS MTU Value Selection

The basic MPLS MTU formula is:

MPLS MTU = IP MTU + Number of labels x 4 bytes

On NE, the following parameters may affect MPLS MTU selection:

  • configured interface MTU on the physical out interface

  • configured MPLS MTU on the physical out interface

  • PMTU negotiated by LDP signalling (for details about this parameter, see chapter Protocols MTU Negotiation)

  • PMTU negotiated by RSVP-TE signalling (for details about this parameter, see chapter Protocols MTU Negotiation)

  • configured interface MTU on the tunnel interface

For detailed rules of MPLS MTU Value Selection, see Table 7-1.

Table 7-1 MPLS MTU Value Selection Rules
Scenarios Parameters which may affect MPLS MTU value selection ("Y" indicates affect, "N" indicates no affect, the smallest value among the affecting parameters is selected as the MPLS MTU)
interface MTU on the physical out interface MPLS MTU on the physical out interface PMTU negotiated by LDP signalling PMTU negotiated by RSVP-TE signalling interface MTU on the tunnel interface
LDP LSP Y Y Y N N
MPLS-TE Y Y N Y N
LDP over TE Y Y Y N Y
NOTE:

In LDP over TE scenario, interface MTU on the tunnel interface affects MPLS MTU value selection, because the LDP LSP is over TE tunnel and the TE tunnel interface is an out interface of the LDP LSP.



According to the above rules, the selected MPLS MTU on NE is impossible larger than the physical interface MTU. Therefore, the size of the MPLS-labeled packets are less than or equal to the physical interface MTU and will not be discarded by the local device if DF=0.

Fragmentation Implementation for IP Packets That Enter MPLS Tunnels

For the MPLS packets sent from the control plane of the NE device, if the size of the IP datagram and label in MPLS packets is greater than the MPLS MTU value,
  • if DF=0, the packet is fragmented. Each fragment (including the IP header and label) is less than or equal to the MPLS MTU value.
  • if DF=1, the packet is discarded and an ICMP Datagram Too Big message is sent to the source end.

For the MPLS packets received from the physical interface of the NE device, different board types may have different MPLS MTU fragmentation modes, as shown in Table 7-2.

Table 7-2 Fragmentation implementation for IP packets entering MPLS tunnels (just for the packet received from the physical interface)
Fragmentation mode Board Type MPLS Fragmentation Rules on the Forwarding Plane
Figure 7-7 Mode A

-

MPLS fragmentation is implemented only on the ingress for IP packets entering an MPLS tunnel.

When the total length of the IP datagram and label of a packet exceeds a specified MPLS MTU, and DF is set to 0, the IP datagram is fragmented. Each fragment is attached one or more MPLS labels and then forwarded.

When the total length of the IP datagram and label of a packet exceeds a specified MPLS MTU, and DF is set to 1,

  • if forcible-fragmentation is disabled: the IP datagram is attached with one or more MPLS labels and then forwarded, without being fragmented.
  • if forcible-fragmentation is enabled: the IP datagram is fragmented, attached one or more MPLS labels, and then forwarded.
Translation
Download
Updated: 2019-01-14

Document ID: EDOC1100058932

Views: 6638

Downloads: 11

Average rating:
This Document Applies to these Products
Related Documents
Related Version
Share
Previous Next