Introduction

Label distribution deals with how to propagate the information of label bindings throughout the MPLS core and edges. There are few different modes that deal with how labels are distributed and protocols that define the exchange of labels.

Label Distribution Modes

• There are two label distribution modes. These relate to the way each LSR sends label bindings to its neighbors.
  • Unsolicited Downstream (UD)
  • Downstream on Demand (DoD)

Unsolicited Downstream (UD)

• LSR that operate in UD mode distributes all locally assigns label binding to all adjacent LSR routers in without neighbors requesting them
• UD is the default mode for Cisco routers on all interfaces except LC-ATM.

Downstream on Demand (DoD)

• LSR that operates in DoD mode, requests the next-hop label from adjacent LSR for each LSP. This is different than UD, where label bindings are unsolicited.

Label Distribution

• There are four methods that can be used to distribute MPLS labels bindings:
  • MPLS-LDP - Label Distribution Protocol which is used in MPLS Tunneling and MPLS VPNs.
  • MPLS-RSVP - Resource Reservation Protocol which is mostly used with MPLS Traffic Engineering.
  • MPLS-BGP-Label - BGP send-label feature which is used with MPLS VPN and Inter-AS MPLS.
  • MPLS-Static-Labels - Manual label assignment. Assigning a static label. Similar to a static route.

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