Two address-oriented table entries are used when routing into a VPLS service. An ARP entry is used on the routing side 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. 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 ARP cache may be populated with a static ARP entry for the next-hop IP address. While dynamically populated ARP entries age out according to the ARP aging timer, static ARP entries never age out.
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 FDB table. While the destination MAC is unknown (not populated in the VPLS FDB), the system floods all packets destined for that MAC (routed or bridged) to all virtual ports within the VPLS service context. When the MAC is known (populated in the VPLS FDB), all packets destined for the MAC (routed or bridged) are targeted to the specific virtual port where the MAC has been learned.
As with ARP entries, static MAC entries may be created in the VPLS FDB. Dynamically learned MAC addresses are allowed to age out or be flushed from the VPLS FDB, while static MAC entries always remain associated with a specific virtual port. Dynamic MACs may also be relearned on another VPLS virtual port than the current virtual port in the FDB. In this case, the system automatically moves the MAC FDB entry to the new VPLS virtual port.
The MAC address associated with the R-VPLS IP interface is protected within its VPLS service such that frames received with this MAC address as the source address are discarded. VRRP MAC addresses are not protected in this way.