Managing Ingress Oversubscription at the PFE

Ingress oversubscription is a state where the transmission rate of the incoming packets is much higher than the rate that the Packet Forwarding Engine and router can handle, causing important packets to be dropped. If an oversubscribed link or service experiences an excess of traffic, it can result in traffic loss or delay that could potentially affect other services and links.

The Packet Forwarding Engine uses fixed rules to decide the priority of incoming packets. Based on these fixed rules, the Packet Forwarding Engine categorizes incoming packets into high-priority network control packets and low-priority best-effort packets. Packets with protocols such as routing protocols are classified as network control packets. Packets with protocols such as Telnet, FTP, and SSH are classified as best-effort packets.

The limitation of these fixed rules is that even if the trusted and non-network-control packets marked by a customer edge router are forwarded to the transit router, the transit router might drop these packets. This is because, according to the fixed rules, none of these packets are high-priority packets for the transit router.

To overcome this limitation, you can prioritize and classify the traffic entering a Packet Forwarding Engine by configuring a traffic class map based on CoS values and associating the values with a traffic class such as real-time, network control, or best-effort. You can associate the traffic class map with an interface on the transit router. During ingress oversubscription, the router interface uses this user-defined traffic class map to select the packet priority.