R-VPLS Specific ARP Cache Behavior

In typical routing behavior, the system uses the IP route table to select the egress interface, and then at the egress forwarding engine, an ARP entry is used to forward the packet to the appropriate Ethernet MAC. With R-VPLS, the egress IP interface may be represented by a multiple egress forwarding engine (wherever the VPLS service virtual ports exist).

To optimize routing performance, the ingress forwarding engine processing has been augmented to perform an ingress ARP lookup to resolve which VPLS MAC address the IP frame must be routed toward. This MAC address may be currently known or unknown within the VPLS FDB. If the MAC is unknown, the packet is flooded by the ingress forwarding engine to all egress forwarding engines where the VPLS service exists. When the MAC is known on a virtual port, the ingress forwarding engine forwards the packet to the correct egress forwarding engine. Table 1 describes how the ARP cache and MAC FDB entry states interact at ingress and Table 2 describes the corresponding egress behavior.

Table 1. Ingress Routed to VPLS Next-Hop Behavior
Next-Hop ARP Cache Entry	Next-Hop MAC FDB Entry	Ingress Behavior
ARP Cache Miss (No Entry)	Known or Unknown	Flood to all egress forwarding engines associated with the VPLS or I-VPLS context.
	Unknown	Flood to all egress forwarding engines associated with the VPLS or I-VPLS context.
	Unknown	Flood to all egress forwarding engines associated with the VPLS for forwarding to all VPLS or I-VPLS virtual ports.

Table 2. Egress R-VPLS Next-Hop Behavior
Next-Hop ARP Cache Entry	Next-Hop MAC FDB Entry	Egress Behavior
ARP Cache Miss (No Entry)	Known	No ARP entry. The MAC address is unknown and the ARP request is flooded out of all virtual ports of the VPLS or I-VPLS instance.
ARP Cache Miss (No Entry)	Unknown	Request control engine processing the ARP request to transmit out of all virtual ports associated with the VPLS or I-VPLS service. Only the first egress forwarding engine ARP processing request triggers an egress ARP request.
ARP Cache Hit	Known	Forward out of specific egress VPLS or I-VPLS virtual ports where MAC has been learned.
ARP Cache Hit	Unknown	Flood to all egress VPLS or I-VPLS virtual ports on forwarding engine.