Multiple Instances for Label Distribution Protocol Overview
Versions of Junos OS earlier than Release 5.3 support multiple VPN routing and forwarding (VRF) instances of Border Gateway Protocol (BGP), Intermediate System-to-Intermediate System (IS-IS), Open Shortest Path First (OSPF), Protocol Independent Multicast (PIM), and Routing Information Protocol (RIP). Junos OS Release 5.4 and later adds support for multiple instances of the Label Distribution Protocol (LDP).
This support allows LDP to be used to advertise labels in a carrier-of-carriers scenario from a core provider edge (PE) router to a customer carrier edge (CE) router. This is especially useful when the carrier customer is a basic Internet service provider (ISP) and wants to restrict full Internet routes to its PE routers. By using LDP instead of BGP, the carrier customer shields its other internal routers from the Internet at large. Multiple-instance LDP is also useful when a carrier customer wants to provide Layer 2 VPN or Layer 3 VPN services to its customers.
Using multiple-instance LDP lets you circumvent one of the requirements of RFC 3107: the need to run full-mesh internal BGP (IBGP) within the carrier customer’s autonomous system (AS). When you use multiple-instance LDP, full-mesh IBGP is unnecessary.
In Figure 1, the customer carrier in AS 21 can configure one instance of LDP for all routers in AS 21 instead of using full-mesh IBGP.
Figure 1: Carrier-of-Carriers Example
In general, if there are a limited number of customer carrier sites and few internal routes in the customer carrier AS, it is simpler and quicker to use LDP than to configure a full IBGP mesh.
An instance of LDP operates essentially in the same way as a master instance. Each instance of LDP must be enabled on all the desired interfaces and a separate set of LDP data structures is maintained for each instance. Instance information includes a set of LDP interfaces, neighbors, sessions, and databases.
For more information about carrier-of-carriers VPNs, see the Junos VPNs Configuration Guide.
For more information about LDP, see the Junos MPLS Applications Configuration Guide.
