Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 550)

526 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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How MPLS VLL works
4
4. The VLL peer at the egress of the tunnel LSP examines the VC label. This VC label is mapped to
an endpoint for the VLL. The endpoint of a VLL specifies what happens to packets exiting the
VLL.
The endpoint can specify an untagged, dual-tagged, or single-tagged port.
• For untagged ports, the endpoint consists of an interface
• For single-tagged ports, the endpoint consists of an interface and a VLAN ID.
• For dual-tagged ports, the endpoint consists of an interface and dual (outer and inner) VLAN
IDs
The egress LER removes the VC label and forwards the packet out the interface specified as the
endpoint. When the endpoint is a single-tagged or dual-tagged port, the device transmits the
packet with the specified VLAN ID, or with the dual tags, forwarding it out the specified interface to
the CE device.
The two VLL peers advertise VC labels to each other using the Label Distribution Protocol (LDP).
Each PE router attempts to initiate an LDP session with its VLL peer. After the LDP session is
established, the locally assigned VC label, along with a VLL VC ID, is advertised to the VLL peer. In a
similar way, the PE also learns the remotely assigned VC label from the VLL peer. Alternatively, the
user can configure static local and remote VC labels manually on both VLL peers; in this case, LDP
is not used.
MPLS VLLs are not involved with spanning tree operations.
NOTE
When MTUs are mismatched on both sides of a VLL session, the session does not come up.
MPLS VLL packet encoding
When a packet is forwarded from the CE device, the PE router encapsulates it as an MPLS packet,
applying two labels. The resulting MPLS packet has the following structure.
FIGURE 58 Structure of a packet forwarded over an MPLS VLL
The S bit in the tunnel label is zero, indicating that it is not the bottom of the stack. The VC label is
significant only to the PE router at the other end of the VLL.
The Payload Ethernet header may be single-tagged or untagged.
Trunk load balancing of VLL traffic
Load balancing of VLL traffic across trunk ports behaves differently on Brocade NetIron XMR and
Brocade MLX series devices compared to Brocade NetIron CES and Brocade NetIron CER devices.
The following describes the difference:
• On Brocade NetIron XMR and Brocade MLX series devices, VLL traffic load balances across all
trunk ports.