A BGP session in the Active state indicates the router is trying to initiate a TCP connection but cannot complete the three-way handshake. While mismatched timers (B) cause issues after establishment, and TTL (C) or version (D) mismatches typically cause other errors, a blocked TCP port 179 is a classic reason the TCP connection itself fails, leaving the session perpetually trying to connect (Active).

DS-TE extends standard MPLS TE by allowing the creation of separate bandwidth pools (Class-Types) for different traffic classes. This is ideal for the scenario because it enables the operator to reserve a specific amount of bandwidth end-to-end for the premium customer’s traffic class (mapped from their IP range), guaranteeing it across the network while other traffic uses remaining capacity.

While all options can cause issues, an excessively low LSP generation interval means routers are generating and flooding new Link State PDUs (LSPs) very frequently, even for minor changes or no real change. This forces all routers in the area to run the SPF algorithm each time they receive a “new” LSP, leading to constant high CPU usage despite a stable physical topology.

PIM-SSM (Source-Specific Multicast) with Inclusive Trees is the method described. In this model, BUM traffic from a PE is sent to a single core multicast group address. Other PEs interested in BUM traffic join this specific (S,G) tree. This is far more efficient than Ingress Replication (A), which sends unicast copies to every PE, but it does require PIM to be enabled in the core network.

In Cisco-style route display notation, the first number in brackets ([110/50]) is the Administrative Distance (AD). The AD is a measure of the trustworthiness of a routing source. ‘110’ is the default AD for OSPF routes. The second number (’50’) is the metric specific to the protocol (in this case, the OSPF cost).

The SRGB is a local label range on each router. A node’s Prefix-SID (Segment ID) is an index. The actual MPLS label used in the data plane is calculated by adding the Prefix-SID index to the base of the local SRGB. This ensures global meaning (the index) is translated into a local label for forwarding, provided all routers use a consistent SRGB range (e.g., 16000-23999).

A /30 subnet provides 2 usable host addresses (perfect for two router interfaces) but wastes the network and broadcast addresses. A /31 subnet, defined in RFC 3021 for point-to-point links, provides exactly 2 total addresses, both usable as host addresses. This effectively doubles the utilization of your address block for P2P links, offering significant conservation of IPv4 space.

OSPF reacts quickly to topology changes. When an interface goes down, the router immediately originates a new version of its Router LSA (or Network LSA if it was the DR) that omits the failed link. It does not wait for the periodic update. This updated LSA is packaged in a Link State Update (LSU) packet and sent as a multicast (or unicast on some network types) to relevant neighbors to trigger a fast, partial SPF calculation.

While the Route Target (RT) community controls VRF route exchange, a VRF Route Import/Export Map provides an additional layer of security and policy. It is an access-list or prefix-list applied to the import/export process for a VRF. Even if a route has the correct RT to be imported, the import map can filter it based on prefix, providing a final check to prevent accidental leakage between VRFs on the same PE.

A BGP session in the Active state indicates the router is trying to initiate a TCP connection but cannot complete the three-way handshake. While mismatched timers (B) cause issues after establishment, and TTL (C) or version (D) mismatches typically cause other errors, a blocked TCP port 179 is a classic reason the TCP connection itself fails, leaving the session perpetually trying to connect (Active).0