The ingress LER, upon receiving an IP packet, performs a label push operation. This adds an MPLS label stack to the packet, allowing subsequent core (LSR) routers to forward the packet using only label switching (label swap) without inspecting the IP header. This is the fundamental action that enables MPLS’s fast, label-based forwarding through the core.

The administrative state (‘up’) is controlled by the configuration. The operational state (‘down’) reflects the physical/link layer status. A discrepancy where admin is up but oper is down almost always indicates a Layer 1 problem, such as a disconnected cable, faulty port, or no signal from the far-end device. Configuration errors like IP address or routing protocols would not prevent the link from coming up operationally.

LDP session establishment follows a state machine. After TCP connection and session initialization messages are exchanged successfully, the session reaches the ‘Operational’ state. Only in this state can the LDP peers begin exchanging label mapping messages for Forwarding Equivalence Classes (FECs). ‘Initialization’ and ‘OpenSent’ are transitional states, and ‘Non Existent’ is the starting state before session establishment begins.

The ‘indirect’ parameter, when set, instructs the router to install the static route in the Routing Table Base (RTB) even if the next-hop address is not resolved to a directly connected interface. This is useful for floating static routes or scenarios where the next-hop is reachable via an alternate path. Without ‘indirect’, the router requires the next-hop to be directly reachable for the route to be active.

A fundamental difference is that OSPF elects a Designated Router (DR) and Backup Designated Router (BDR) on broadcast multi-access networks (like Ethernet) to optimize adjacency formation and LSA flooding. IS-IS does not have a DR/BDR concept; all routers on a LAN form adjacencies with all others, and one router is elected as a pseudo-node to represent the LAN for SPF calculations, but this is not equivalent to OSPF’s DR role.

The ‘system’ interface represents the router itself. Its IP address serves as a stable, always-up identifier for the router. It is automatically used as the Router ID for OSPF and other protocols if not explicitly configured otherwise. It also becomes the source IP address for originated protocol packets (like BGP updates, ICMP echoes) and is crucial for tasks like iBGP peering and network management.

RSVP-TE’s key differentiator is support for Traffic Engineering. Its explicit path signaling allows the ingress LER to specify the exact path (strict or loose hops) an LSP must take through the network, bypassing the IGP shortest path. This enables critical network engineering functions like load balancing across non-congruent paths, avoiding congested links, and meeting specific bandwidth or latency requirements for certain traffic flows.

In BGP, address families define the type of Network Layer Reachability Information (NLRI) exchanged. The ‘ipv4’ address family specifically refers to IPv4 unicast routes. Other address families like ‘ipv6’, ‘mcast-ipv4’, or ‘vpn-ipv4’ are used for IPv6 unicast, IPv4 multicast, and MPLS VPN routes, respectively. They must be explicitly configured on the session to exchange those route types.

A key benefit of LAG is resilience and load distribution. When a member link fails or is shut down, the LAG’s control protocol (like LACP) detects the change. The aggregate bandwidth is reduced, and the hash algorithm used for load balancing is reapplied to redistribute flows from the failed link onto the remaining active member links. Existing sessions (e.g., TCP flows) should not be dropped as their traffic is simply redirected to a different physical link.

The ingress LER, upon receiving an IP packet, performs a label push operation. This adds an MPLS label stack to the packet, allowing subsequent core (LSR) routers to forward the packet using only label switching (label swap) without inspecting the IP header. This is the fundamental action that enables MPLS’s fast, label-based forwarding through the core.0