CCIE SP Quickly understand the working principle of VPLS
VPLS (Virtual Private LAN Service) is a point-to-multipoint L2VPN service provided in public networks. VPLS enables geographically isolated user sites to be connected through MAN (Metropolitan Area Network) or WAN (Wide Area Network), and makes the connection between each site as if it were in a LAN.
VPLS technology is a layer 2 packet bearing technology, which is essentially a L2 layer 2 virtual private network technology based on IP/MPLS and Ethernet technology. The core idea is to use signaling protocols to establish and maintain PW (Pseudo Wire) between PE (operator edge router) nodes in the VPLS instance, encapsulate the Layer 2 protocol frame and transmit and exchange on the PW to make the wide area range Multiple local area networks are integrated into a network at the data link level to provide users with virtual Ethernet services. VPLS technology effectively combines the characteristics of multiple technologies such as IP/MPLS and L2 Layer 2 virtual private network Ethernet switching. It supports point-to-point, point-to-multipoint, and multipoint-to-multipoint service types, and can be used in larger networks. Support carrier-grade Ethernet services at scale.
The VPLS network mainly includes the following important components:
AC: The connection between the user and the service provider, that is, the link connecting the CE and the PE. The corresponding interface can only be an Ethernet interface.
PW: A bidirectional virtual connection between VSIs on two PE devices. It consists of a pair of unidirectional MPLS VC (Virtual Circuit, virtual circuit) in opposite directions, also called emulation circuit.
Tunnel: A tunnel is used to carry PWs. One tunnel can carry multiple PWs. The tunnel is a direct connection channel between the local PE and the peer PE, which completes the transparent transmission of data between the PEs, which can be an MPLS or GRE tunnel.
PW Signaling: PW signaling protocol, the basis of VPLS implementation, used to create and maintain PW. The PW signaling protocol can also be used to automatically discover the peer PE equipment of the VSI. Currently, PW signaling protocols mainly include LDP and BGP.
VSI: Virtual switching instance, an VPLS instance is an Ethernet bridge function entity on a PE device, and performs Layer 2 packet forwarding based on MAC address and VLAN TAG.
VPLS provides reachability through MAC address learning. Each PE device maintains a bridge MAC address table.
1. Learning the source MAC address
Remote MAC address learning associated with PW:
PW consists of a pair of unidirectional VC LSPs (only when both VC LSPs are up are PWs up). After learning an original unknown MAC address on the inbound VC LSP, the PW is required to form a mapping relationship between this MAC address and the outbound VC LSP.
Local MAC address learning:
For the packets transmitted on the CE, the source MAC address in the packets needs to be learned to the corresponding port of the VSI.
2. MAC address recovery
The dynamically learned MAC address must have a refresh and re-learning mechanism. A dynamic learning method is provided in the VPLS-related draft, that is, the use of address recovery messages. The address recovery message carries the MAC TLV, and the device that receives this message deletes the MAC addresses or re-learns these MAC addresses according to the parameters specified in the TLV. If the MAC address specified in the TLV is NULL, all MAC addresses under this VSI are deleted, but the MAC addresses learned on the PW that received this message are not deleted.
In order to quickly remove the MAC address when the topology changes, you can use the address recovery message. Address recovery messages are divided into two categories: those with MAC address lists and those without MAC address lists.
3. MAC address aging
The remote MAC addresses learned by the PE that are related to the VC label but are no longer used need to be removed by an aging mechanism. The aging mechanism uses the aging timer corresponding to the MAC address. When receiving and processing the message, according to the source MAC address in the message, if this source address starts the corresponding aging timer, the PE resets the aging timer.
To avoid loops, general Layer 2 networks require STP (Spanning Tree Protocol, Spanning Tree) protocol to be enabled. However, for users using VPLS, they will not perceive the ISP network. Therefore, when STP is enabled on the private network side, the ISP network cannot be considered. In VPLS, full connection and split horizon forwarding are used to avoid using the STP protocol on the VPLS private network side of the ISP.
The method to avoid VPLS loop is as follows:
The PEs are logically fully connected (PW fully connected), that is, each PE must create a tree for each VPLS forwarding instance to all other PEs under the instance.
Each PE device must support the split horizon strategy to avoid loops, that is, the PE cannot forward packets between PWs with the same VSI (because each PE is directly connected in the same VSI), that is, from the public network side The data packets received by the PW are no longer forwarded to other PWs, and can only be forwarded to the private network side.
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