How does LDP interact with BGP and IGP in MPLS VPN scenarios?

1 What is LDP?

LDP stands for Label Distribution Protocol, and it is responsible for assigning and exchanging labels between MPLS routers. Labels are short identifiers that are attached to packets to indicate their forwarding path across the MPLS network. LDP uses TCP or UDP to communicate with its peers and exchange label information. LDP can operate in two modes: downstream unsolicited (DU) and downstream on demand (DoD). In DU mode, a router sends labels to its neighbors without waiting for a request, while in DoD mode, a router sends labels only when requested by a neighbor.

2 What is BGP?

BGP stands for Border Gateway Protocol, and it is the routing protocol that carries the VPN information between MPLS routers. BGP can create VPNs by using extended attributes, such as route distinguishers (RDs) and route targets (RTs), to differentiate and control the VPN traffic. BGP can also use multiprotocol extensions to carry IPv4, IPv6, or other types of VPN traffic. BGP uses TCP to establish and maintain sessions with its peers and exchange VPN routes.

3 What is IGP?

IGP stands for Interior Gateway Protocol, and it is the routing protocol that runs within an MPLS network to provide reachability and connectivity between MPLS routers. IGP can be any protocol, such as OSPF, IS-IS, EIGRP, or RIP, that supports MPLS extensions. IGP advertises the loopback addresses and the interface addresses of the MPLS routers, as well as the label switching capabilities and the traffic engineering parameters. IGP also creates the shortest path tree (SPT) that determines the best path for each packet across the MPLS network.

4 How does LDP interact with IGP?

LDP interacts with IGP in two ways: label binding and label switching. Label binding is the process of assigning a label to an IGP prefix, such as a loopback address or an interface address. Label switching is the process of forwarding a packet based on its label value, rather than its destination IP address. LDP relies on IGP to provide the topology and the routing information of the MPLS network, and uses the SPT to bind labels to the IGP prefixes. LDP also uses IGP to discover its peers and establish adjacencies.

5 How does LDP interact with BGP?

LDP interacts with BGP in two ways: label allocation and label imposition. Label allocation is the process of assigning a label to a BGP prefix, such as a VPN route. Label imposition is the process of adding a label stack to a packet that carries a BGP prefix. LDP relies on BGP to provide the VPN information and the policies of the MPLS network, and uses the RDs and RTs to allocate labels to the BGP prefixes. LDP also uses BGP to determine the ingress and egress routers of the MPLS network, and imposes labels according to the BGP next-hop.

6 What are the benefits and challenges of LDP?

LDP has some benefits and challenges when used in MPLS VPN scenarios. For instance, LDP is simple to configure and troubleshoot, as it follows the IGP routing and does not require complex tunneling or mapping mechanisms. It is also scalable and flexible, as it can support different types of VPN traffic and services. Furthermore, LDP is efficient and fast, reducing the packet header size and the routing lookup overhead. However, it is not secure, as it does not encrypt or authenticate the labels or the packets. Additionally, it is not interoperable, as it does not support other label distribution protocols. Lastly, it is not optimal, as it does not consider the traffic load or the bandwidth availability of the MPLS network.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?