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Configuration Guide - VPN

CloudEngine 12800 and 12800E V200R005C10

This document describes the configurations of VPN, including GRE, BGP/MPLS IP VPN, BGP/MPLS IPv6 VPN, VLL, PWE3, and VPLS.
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MAC Address Management

MAC Address Management

Background

A characteristic of the Ethernet is that an interface sends unicast packets with unknown destination MAC addresses, broadcast packets, and multicast packets to all other interfaces on the Ethernet. As an Ethernet-based technology, VPLS emulates an Ethernet bridge for user networks. To forward packets on a VPLS network, PEs must establish MAC address tables and forward packets based on MAC addresses or MAC addresses and VLAN tags.

MAC Address Learning and Flooding

MAC address learning

PEs create MAC address tables based on dynamic MAC address learning and associates destination MAC addresses with PWs.

Table 6-4 describes MAC address learning modes.

Table 6-4 MAC address learning modes

MAC Address Learning Mode

Description

Characteristic

Qualified

A PE learns the MAC addresses and VLAN tags of received Ethernet frames. In this mode, each user VLAN is an independent broadcast domain and has independent MAC address space.

The broadcast domain is confined to each user VLAN. Qualified learning can result in large Forwarding Information Base (FIB) table sizes, because the logical MAC address is now a VLAN tag + MAC address.

Unqualified

A PE learns only the MAC addresses of Ethernet frames. In this mode, all user VLANs share the same broadcast domain and MAC address space. The MAC address of each user VLAN must be unique.

If an AC interface is associated with multiple user VLANs, this AC interface must be a physical interface bound to a unique VSI.
NOTE:

At present, the device supports only MAC address learning in unqualified mode.

Flooding

Packets with unknown addresses are broadcast in Ethernet. Therefore, in VPLS, the received packets with unknown unicast addresses, broadcast addresses, or multicast addresses are flooded to all the other interfaces. If these packets need to be forwarded in multicast mode, PEs use other methods such as Internet Group Management Protocol (IGMP) snooping.

Implementation

Table 6-5 describes the MAC address learning process.

Table 6-5 MAC address learning process

MAC Address Learning Process

Description

Learning MAC addresses from user-side packets

After receiving packets from a CE, a PE maps their source MAC addresses to AC interfaces. Figure 6-8 shows a mapping example with Port1.

Learning MAC addresses from PW-side packets

A PW consists of a pair of MPLS Virtual Circuits (VCs) transmitting in opposite directions. A PW will go Up only after the two MPLS VCs are established. After a PE receives a packet with an unknown source MAC address from a PW, the PE maps the source MAC address to the PW receiving the packet.

Figure 6-8 shows the process of MAC address learning and flooding on a PE. PC1 and PC2 both belong to VLAN10. When PC1 pings IP address 10.1.1.2, PC1 does not know the MAC address corresponding to this IP address and advertises an Address Resolution Protocol (ARP) Request packet.

Figure 6-8 MAC address learning and flooding process

  1. After receiving the ARP Request packet sent by PC1 from Port1 that connects to CE1, PE1 adds the MAC address of PC1 to its own MAC address table, as shown in the blue section of the MAC entry.
  2. PE1 floods the ARP Request packet (the blue dotted line on PE1) to other interfaces (PW1 and PW2 are regarded as interfaces at this time).
  3. After receiving the ARP Request packet from PW1, PE2 adds the MAC address of PC1 to its own MAC address table, as shown in the blue section of the MAC entry.
  4. Based on split horizon, PE2 sends the ARP Request packet to only the interface connecting to CE2 (as indicated by the blue dashed line), but not to PW1. This ensures that only PC2 receives the ARP Request packet. VPLS split horizon ensures that packets received from public network PWs are forwarded to only private networks, not to other public network PWs.
  5. After PC2 receives the ARP Request packet and finds that it is the destination of this packet, PC2 sends an ARP Reply packet to PC1 (as indicated by the orange dashed line).
  6. After receiving the ARP Reply packet from PC2, PE2 adds the MAC address of PC2 to its own MAC address table, as indicated by the orange section of the MAC entry. The destination MAC address of the ARP Reply packet is the MAC address of PC1 (MAC A). After searching its MAC address table, PE2 sends the ARP Reply packet to PE1 over PW1.
  7. After receiving the ARP Reply packet from PE2, PE1 adds the MAC address of PC2 to its own MAC address table, as shown in the orange section of the MAC entry. After searching its MAC address table, PE1 sends the ARP Reply packet to PC1 through Port1.
  8. After receiving the ARP Reply packet from PC2, PC1 completes MAC address learning.
  9. While advertising the ARP Request packet to PW1, PE1 also advertises the ARP Request packet to PE3 over PW2. After receiving the ARP Request packet, PE3 adds the MAC address of PC1 to its MAC address table, as shown in the blue section of the MAC entry. Based on split horizon, PE3 sends the ARP Request packet to only PC3. Because PC3 is not the destination of the ARP Request packet, PC3 does not send any ARP Reply packet.

MAC Address Withdrawal

Dynamic MAC addresses need to be updated and relearned. The VPLS draft defines a MAC Withdraw message with an optional MAC type-length-value (TLV) to remove or relearn the MAC address list.

When the topology changes, MAC Withdraw messages enable devices to delete matching MAC addresses quickly. MAC Withdraw messages are classified into two types:

  • Messages with a MAC address list

  • Messages without a MAC address list

When a backup link (AC link or VC link) becomes Up, a PE that detects the link status change receives a MAC Withdraw message carrying a list of MAC addresses to be relearned. After receiving the message, the PE updates the MAC address entries in the forward information base (FIB) table of the corresponding VSI, and sends the message to PEs directly connected to it through Label Distribution Protocol (LDP) sessions. If the MAC Withdraw message contains an empty MAC address list TLV, the PE deletes all the MAC addresses in the specified VSI except the MAC address learned from the PE that sends the message.

MAC Address Aging

An aging mechanism removes MAC entries that a PE no longer needs. If a MAC entry is not updated within a specified period of time, this entry will be aged.

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Updated: 2019-04-03

Document ID: EDOC1100075353

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