MTU Considerations

An enterprise requires the service provider network to be able to carry IP packets of up to 1500 bytes presented to the service provider's PE router by the enterprise CE router. Network performance should not degrade for large numbers of 1500-byte packets.

It is desirable that the service provider network be able to support larger maximum transmission units (MTUs) than 1500 bytes presented by the CE to support encryption of IP traffic. IP GRE and IPsec tunnel mode encapsulations to support encryption add an additional 76 bytes to packets traveling through the network. Service providers' ability to handle 1580- to 1600-byte packets is desirable and should be investigated.

It is noted that the use of MVPN prepends an IP/GRE header to the multicast packet sent across the infrastructure, which may cause a different maximum IP MTU that could be used for multicast traffic. This extra header must be taken into consideration.

Here are some questions to ask the provider on this issue:

• Unicast MTU 1500 Can you carry 1500-byte packets presented by the CE router across your network?

• Unicast maximum MTU What is the maximum IP MTU of a packet presented by the CE router that can be carried across your network?

• Multicast/MVPN MTU What is the maximum IP MTU of a multicast packet presented by the CE router that can be carried across your network?