Two address-oriented table entries are used when routing into a VPLS service. On the routing side, an ARP entry is used to determine the destination MAC address used by an IP next-hop. In the case where the destination IP address in the routed packet is a host on the local subnet represented by the VPLS instance, the destination IP address is used as the next-hop IP address in the ARP cache lookup. If the destination IP address is in a remote subnet that is reached by another router attached to the VPLS service, the routing lookup returns the local IP address on the VPLS service of the remote router is returned. If the next-hop is not currently in the ARP cache, the system generates an ARP request to determine the destination MAC address associated with the next-hop IP address. IP routing to all destination hosts associated with the next-hop IP address stops until the ARP cache is populated with an entry for the next-hop. The dynamically populated ARP entries age out according to the ARP aging timer.
The second address table entry that affects VPLS routed packets is the MAC destination lookup in the VPLS service context. The MAC associated with the ARP table entry for the IP next-hop may or may not currently be populated in the VPLS Layer 2 FIB table. While the destination MAC is unknown (not populated in the VPLS FIB), the system is flooded with all packets destined to that MAC (routed or bridged) to all SAPs within the VPLS service context. When the MAC is known (populated in the VPLS FIB), all packets destined to the MAC (routed or bridged) is targeted to the specific SAP where the MAC has been learned. As with ARP entries, static MAC entries may be created in the VPLS FIB. Dynamically learned MAC addresses are allowed to age out or be flushed from the VPLS FIB while static MAC entries always remain associated with a specific virtual port. Dynamic MACs may also be relearned on another VPLS SAP than the current SAP in the FIB. In this case, the system automatically moves the MAC FIB entry to the new VPLS SAP.
In 7210 SAS, whenever a MAC entry is removed from the VPLS FIB (either explicitly by the user or due to MAC aging or mac-move), ARP entries which match this MAC address is removed from the ARP cache. Though the VPLS FIB entries are not removed; an ARP entry ages out and is removed from the ARP cache.
If the VPLS FIB limit is reached and the node is no longer able to learn new MAC address, ARP will also not be learned.