If the IP packet was already at the maximum size possible for a certain data link (full MTU), it becomes too big to be sent on that data link because of the added labels. When a packet becomes labeled, the size increases slightly. London(config)# interface FastEthernet2/6
London# show mpls interfaces fastEthernet 2/6 detailĮnter configuration commands, one per line. Look at Example 3-10 to see how you can change the MPLS MTU on an interface in Cisco IOS. The default MPLS MTU value of a link equals the MTU value. Thus, all labeled packets of size 1508 bytes (labels included) can be sent on the link without fragmenting them. If, for example, you know that all packets that are sent on the link have a maximum of two labels and the MTU is 1500 bytes, you can set the MPLS MTU to 1508 (1500 + 2 * 4). This would lead to the need to fragment the packet.Ĭisco IOS has the mpls mtu command that lets you specify how big a labeled packet can be on a data link. However, when n labels are added, n * 4 bytes are added to an already maximum sized IP packet of 1500 bytes. For the Ethernet encapsulation, for example, MTU is by default set to 1500. The interface MTU command in Cisco IOS specifies how big a Layer 3 packet can be without having to fragment it when sending it on a data link. This parameter is used in the LFIB to keep track of how big labeled packets can be and still be forwarded without needing to fragment them. Finally, a new parameter is introduced: MPLS Maximum Receive Unit.
Furthermore, it explains what giant and baby giant frames are and how to ensure that Ethernet switches can handle them. This section explains that an MPLS MTU parameter pertains to labeled packets. So, if n is the number of labels, n * 4 bytes are added to the size of the packet when the packet is labeled. This does imply that the labeled packets are slightly bigger than the IP packets, because for every label, four bytes are added to the packet. All IPv4 packets have one or more labels. Take the case of an IPv4 network implementing MPLS. Data links in MPLS networks also have a specific MTU, but for labeled packets. It indicates the maximum size of the IP packet that can still be sent on a data link, without fragmenting the packet. Restart your PC.Maximum transmission unit (MTU) is a well-known parameter in the IP world. Add the following new DWORD values to the "0" subkey you created: "ProtocolType=800" (hexadecimal) "PPPProtocolType=21" (hexadecimal) "TunnelMTU=nnn" (decimal, where nnn is the desired MTU value) 4.
Add a "Protocols" subkey, add a "0" (zero) subkey to the Protocols subkey as well.
Navigate to Start > Run, type: regedit, find the following key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NdisWan\Parameters 2. To change the MTU setting for VPN connections: 1. Add the following new DWORD values to the "0" subkey you created: "ProtocolType=800" (hexadecimal) "PPPProtocolType=21" (hexadecimal) "ProtocolMTU=nnn" (decimal, where nnn is the desired MTU value) 4. To change the MTU setting for PPP connections: 1.
The following applies to Windows XP Pro, 2000 server, and 2003 server.