Ensuring Smooth Packet Delivery for Inter-Provider MPLS IP-VPN Service
Question
Two MPLS service providers (SP1 and SP2) are offering inter-provider RFC 2547/4364-based IP-VPN service to an enterprise customer.
IP traffic among some of the customer's sites has to traverse both of the service providers.
The service providers and the enterprise do not know what the minimum MTU is along the end- to-end path.
What could be done to guarantee that large packets are not dropped for MTU-related reasons?
Answers
Explanations
Click on the arrows to vote for the correct answer
A. B. C. D.B.
The correct answer is C. Configure the IP sender or senders to set the DF (Don't Fragment) flag in the IP header.
When IP packets are sent across a network, they can be fragmented if their size exceeds the MTU (Maximum Transmission Unit) of a particular link in the path. Fragmentation can cause problems, such as packet loss and poor performance, especially in situations where packets have to traverse multiple links with different MTUs, as in the case of the inter-provider RFC 2547/4364-based IP- VPN service.
To avoid these problems, the IP sender can set the DF flag in the IP header of packets. The DF flag instructs routers along the path not to fragment the packets, but instead to drop them and send an ICMP (Internet Control Message Protocol) message back to the sender indicating that the packet was too large and could not be forwarded.
By setting the DF flag, the sender can ensure that packets are not fragmented and are not dropped due to MTU-related reasons. However, this approach assumes that all devices along the path support ICMP messages and that the sender can receive them.
Enabling Path MTU Discovery (PMTUD) is another option, as suggested in answers A and B. PMTUD is a mechanism that allows devices along the path to dynamically discover the MTU of the next link and adjust the size of packets accordingly. PMTUD works by setting the DF flag and relying on ICMP messages to inform the sender of the MTU of the next link. However, PMTUD can be unreliable in some cases, such as when ICMP messages are blocked by firewalls or when there are misconfigured routers along the path.
In summary, the best option to guarantee that large packets are not dropped for MTU-related reasons in this scenario is to configure the IP sender or senders to set the DF flag in the IP header.
