{"id":4087,"date":"2025-11-13T00:00:00","date_gmt":"2025-11-13T00:00:00","guid":{"rendered":"https:\/\/lp.szlogic.cn\/knowledge-center\/lsr-and-ler-mpls-network-guide\/"},"modified":"2026-06-22T05:09:15","modified_gmt":"2026-06-22T05:09:15","slug":"lsr-and-ler-mpls-network-guide","status":"publish","type":"post","link":"https:\/\/resources.l-p.com\/pt\/knowledge-center\/lsr-and-ler-mpls-network-guide","title":{"rendered":"Inside MPLS: Label Switch Routers and Label Edge Routers"},"content":{"rendered":"

In modern MPLS (Multiprotocol Label Switching)<\/strong><\/a> networks, Label Switch Routers (LSRs)<\/strong> and Label Edge Routers (LERs)<\/strong> form the backbone of efficient, high-speed data transport. Understanding their roles is essential for network engineers, integrators, and data center designers looking to optimize traffic flow, enforce quality of service, and ensure reliable connectivity.<\/p>\n\n\n\n

✅ What Are LSRs and LERs?<\/h2>\n\n\n\n
\"Label<\/figure>\n\n\n\n

\u25b7 Label Switch Router (LSR)<\/h3>\n\n\n\n

An LSR<\/strong> is a core MPLS router that forwards packets based solely on labels<\/strong>, rather than IP addresses. By using a Label Forwarding Information Base (LFIB)<\/strong>, LSRs can swap incoming labels for outgoing ones and send packets to the next hop at wire speed<\/strong>.<\/p>\n\n\n\n

LSRs are the intermediate nodes<\/strong> in a Label Switched Path (LSP)<\/a>, enabling fast, deterministic, and scalable packet forwarding<\/strong>.<\/p>\n\n\n\n

\u25b7 Label Edge Router (LER)<\/h3>\n\n\n\n

An LER<\/strong> operates at the edge of an MPLS network, handling traffic entering and exiting the network<\/strong>. Its primary functions are:<\/p>\n\n\n\n

    \n

  1. Ingress LER:<\/strong> Assigns MPLS labels to incoming IP packets (Label Imposition).<\/p><\/li>

  2. Egress LER:<\/strong> Removes labels before forwarding packets to the destination network (Label Disposition).<\/p><\/li>\n<\/ol>\n\n\n\n

    LERs also maintain routing information to map IP prefixes to labels and communicate with LSRs via protocols like LDP (Label Distribution Protocol)<\/strong><\/a>.<\/p>\n\n\n\n

    ✅ How LSRs and LERs Work Together<\/h2>\n\n\n\n
    \"Label<\/figure>\n\n\n\n

    MPLS networks typically involve three stages of packet forwarding:<\/p>\n\n\n\n

    \n\n<\/colgroup>

    Device<\/p><\/th>

    Role<\/p><\/th>

    Function<\/p><\/th><\/tr>

    Ingress LER<\/strong><\/p><\/td>

    Entry point<\/p><\/td>

    Adds labels to IP packets<\/p><\/td><\/tr>

    Core LSR<\/strong><\/p><\/td>

    Transit node<\/p><\/td>

    Swaps labels and forwards packets based on LFIB<\/p><\/td><\/tr>

    Egress LER<\/strong><\/p><\/td>

    Exit point<\/p><\/td>

    Removes labels and forwards to the destination IP network<\/p><\/td><\/tr><\/tbody>\n<\/table>\n<\/figure>\n\n\n\n

    This label-based forwarding allows substantially faster packet processing<\/strong> compared to traditional IP routing, while enabling traffic engineering<\/strong> and QoS differentiation<\/strong>.<\/p>\n\n\n\n

    ✅ Applications of LSRs and LERs<\/h2>\n\n\n\n