In Figure: IP interworking VLL (Ipipe), PE 2 is manually configured with both CE 1 and CE 2 IP addresses. These are host addresses and are entered in /32 format. PE 2 maintains an ARP cache context for each IP interworking VLL. PE 2 responds to ARP request messages received on the Ethernet SAP. PE 2 responds with the Ethernet SAP configured MAC address as a proxy for any ARP request for CE 1 IP address. PE 2 silently discards any ARP request message received on the Ethernet SAP for an address other than that of CE 1. Likewise, PE 2 silently discards any ARP request message with the source IP address other than that of CE 2. In all cases, PE 2 keeps track of the association of IP to MAC addresses for ARP requests it receives over the Ethernet SAP.
To forward unicast frames destined for CE 2, PE 2 needs to know the CE 2 MAC address. When the Ipipe SAP is first configured and administratively enabled, PE2 sends an ARP request message for CE 2 MAC address over the Ethernet SAP. Until an ARP reply is received from CE2, providing the CE2 MAC address, unicast IP packets destined for CE2 are discarded at PE2. IP broadcast and IP multicast packets are sent on the Ethernet SAP using the broadcast or direct-mapped multicast MAC address.
To forward unicast frames destined for CE 1, PE 2 validates the MAC destination address of the received Ethernet frame. The MAC address should match that of the Ethernet SAP. PE 2 then removes the Ethernet header and encapsulates the IP packet directly into a pseudowire without a control word. PE 1 removes the pseudowire encapsulation and forwards the IP packet over the Frame Relay SAP using RFC 2427, Multiprotocol Interconnect over Frame Relay, routed PDU encapsulation.
To forward unicast packets destined for CE1, PE2 validates the MAC destination address of the received Ethernet frame. If the IP packet is unicast, the MAC destination must match that of the Ethernet SAP. If the IP packet is multicast or broadcast, the MAC destination address must be an appropriate multicast or broadcast MAC address.
The other procedures are similar to the case of communication between CE 1 and CE 2, except that the ATM SAP and the Ethernet SAP are cross-connected locally and IP packets do not get sent over an SDP.
A PE does not flush the ARP cache unless the SAP goes administratively or operationally down. The PE with the Ethernet SAP sends unsolicited ARP requests to refresh the ARP cache every âTâ seconds. ARP requests are staggered at an increasing rate if no reply is received to the first unsolicited ARP request. The value of T is configurable by the user through the mac-refresh command.