Mpls Mtu
The maximum transmission unit (MTU) of a link determines whether a labeled packet is too big to be transmitted across a link. Note that a link has a physical MTU value and an IP MTU value. Take the example of an Ethernet link on a Cisco IOS router. The MTU value is 1500 bytes. That is the Layer 3 MTU value of the packet. Therefore, an IP packet up to 1500 bytes can be transmitted across the Ethernet link without having to be fragmented. Typically, when MPLS is used, the IP packet of 1500 bytes receives one or two labels. It might be more, such as when CsC is deployed, but two labels are the maximum for this example. Therefore, 2 labels are added to the IP packet, or 2 times 4 bytes, which makes 8 bytes. The maximum size of the labeled packet is 1508 bytes. However, the MTU of the Ethernet link was 1500 bytes, meaning that IP packets of 1493 to 1500 bytes now have to be fragmented because they are labeled. Because sending maximum-sized packets is common (for example, for FTP), this might lead to fragmenting a huge part of the traffic and problems down the road.
To prevent fragmentation in the first place, you can do three things: lower the MTU for the IP packets to 1492 bytes, use Path MTU discovery, or create a new MTU, specifically for MPLS. The first solution works only if the MTU is lowered consistently throughout the network. Also, if you do not lower the MTU values of the end stations throughout the network, the traffic that enters the MPLS cloud might still be 1500 bytes, leading to fragmentation on the edge LSRs. Lowering the MTU value on the end stations is usually not practical because of their abundance. Also, the end stations usually belong to a different company than the one owning the MPLS network, as with the example of an MPLS VPN network, in which you usually have a service provider and customers.
The second solution, using Path MTU discovery, involves the end stations sending packets and lowering the maximum size of the packet when necessary. However, this is a viable solution only if all the end stations use Path MTU discovery. Even if the end stations do use Path MTU discovery, the ICMP messages needed to make it work might be dropped by access lists or firewalls, which occurs more and more these days. Fortunately, most end stations now support Path MTU discovery and have it enabled by default. The remark regarding the end stations belonging to a different company from the one owning the MPLS network also applies here.
Cisco IOS has the third solution as a new MTU value was introduced for the labeled packets: MPLS MTU. This MTU is the size of the Layer 3 packet, including the labels stack. Therefore, in our example, for an IP packet that is 1500 bytes and has two labels in the label stack, you need to set the MPLS MTU to 1508 on the Ethernet interface. This solution has one problem. The Ethernet specification allows for a Layer 3 payload of only 1500 bytes. With the addition of two labels, the total size of the packet is 1508 bytes. Cisco IOS now allows a slightly bigger packet
than the 1500 bytes maximum. These frames that are slightly bigger than the maximum allowed size are called baby giant frames. The advantage of setting the MPLS MTU is that you need to configure it only in the MPLS network.
NOTE MPLS MTU is a configurable value in Cisco IOS. It is the maximum allowed size for a packet after labeling. The Internet Engineering Task Force (IETF) specified a value called maximum initially labeled IP datagram size (MILIDS). However, this is the maximum allowed size of the packet before labeling.
MPLS MTU is an interface configuration command. If you do not configure it, the default MPLS MTU value is the same as the MTU value of the interface.
Continue reading here: Debug MPLS Packets
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