Maximum transmission unit (MTU) is a well-known parameter in the IP world. It indicates the maximum size of the IP packet that can still be sent on a data link, without fragmenting the packet. Data links in MPLS networks also have a specific MTU, but for labeled packets. Take the case of an IPv4 network implementing MPLS. All IPv4 packets have one or more labels. 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. 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 section explains that an MPLS MTU parameter pertains to labeled packets. Furthermore, it explains what giant and baby giant frames are and how to ensure that Ethernet switches can handle them. Finally, a new parameter is introduced: MPLS Maximum Receive Unit. 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.
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. For the Ethernet encapsulation, for example, MTU is by default set to 1500. However, when n labels are added, n * 4 bytes are added to an already maximum sized IP packet of 1500 bytes. This would lead to the need to fragment the packet.
Cisco IOS has the mpls mtu command that lets you specify how big a labeled packet can be on a data link. 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). Thus, all labeled packets of size 1508 bytes (labels included) can be sent on the link without fragmenting them. The default MPLS MTU value of a link equals the MTU value. Look at Example 3-10 to see how you can change the MPLS MTU on an interface in Cisco IOS.
london#show mpls interfaces fastEthernet 2/6 detail Interface FastEthernet2/6: IP labeling enabled LSP Tunnel labeling not enabled BGP labeling not enabled MPLS not operational MTU = 1500 london#configure terminal Enter configuration commands, one per line. End with CNTL/Z. london(config)#interface FastEthernet2/6 london(config-if)#mpls mtu 1508 london(config-if)#^Z london# london#show mpls interfaces fastEthernet 2/6 detail Interface FastEthernet2/6: IP labeling enabled LSP Tunnel labeling not enabled BGP labeling not enabled MPLS not operational MTU = 1508
When a packet becomes labeled, the size increases slightly. 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. Therefore, the frame at Layer 2 becomes a giant frame. Because the frame is only slightly bigger than the maximum allowed, it is called a baby giant frame.
Take the example of Ethernet: The payload can be a maximum of 1500 bytes. However, if the packet is a maximum sized packet and labels are added, the packet becomes slightly too big to be sent on the Ethernet link. It is possible to close one eye and allow frames that are bigger (perhaps by just a few bytes) to be sent on the Ethernet link, even though it is not the correct thing according to the Ethernet specifications, which say that such frames should be dropped. This is, of course, possible only if the Ethernet hardware in the router and all switches in the Ethernet network support receiving and sending baby giant frames.
On Ethernet data links on LSRs, you can set the MPLS MTU to 1508 bytes to allow IP packets with a size of 1500 bytes with two labels to be received and forwarded. If, however, the hardware of the router does not support this, or if an Ethernet switch exists in between, dropping baby giant frames, you can lower the MPLS MTU parameter on the LSRs. When you set the MPLS MTU to 1500, all the IP packets with a size of 1492 bytes are still forwarded, because the size of the labeled packet then becomes 1500 (1492 plus 8) bytes at Layer 3. However, all IP packets sized between 1493 through 1500 bytes (or more) are fragmented. Because of the performance impact of fragmentation, you should use methods to avoid it, such as path MTU discovery.
NOTE
In some Cisco IOS releases, you cannot configure the MPLS MTU to be bigger than the interface MTU.
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