When a BGP session is stuck in the Active state, it indicates the local router is trying to initiate a TCP connection but failing. After verifying IP reachability and AS numbers, mismatched hold timers are a common culprit. The routers must agree on the hold time value for the session to transition to Established.

VPLS creates a Layer 2 broadcast domain that emulates an Ethernet switch. It forwards traffic based on MAC address learning, which meets the requirement of forwarding based on destination MAC address without IP route lookups. IES and VPRN are Layer 3 services, and an epipe is a point-to-point Layer 2 service, not a multi-point LAN.

While the RD is essential for uniqueness, the vrf-target (Route Target) community is the mechanism that controls the import and export of routes into the VRF table. If not configured or mismatched, routes learned from a CE will not be distributed to the remote PE, even if the VPRN BGP session between PEs is established.

An export policy on a BGP route reflector is applied to routes before they are reflected to other clients. This allows the network administrator to manipulate path attributes like LOCAL_PREF, MED, or communities for all routes sent to the clients, providing a central point of control.

The command ‘show service sap-using sap ‘ displays detailed information about a specific SAP, including the encapsulation (e.g., dot1q) and the VLAN ID configured on it. This is the most direct way to check for a VLAN tag mismatch between the two SAPs of the epipe service.

The split-horizon rule in VPLS states that a frame received from a mesh pseudowire (SDP-binding) must not be forwarded to another mesh pseudowire on the same PE. This is crucial because the full mesh of pseudowires replicates the behavior of a physical switch; without split-horizon, loops could form between the pseudowires on a single PE.

In an IES service, a unique interface name (e.g., ‘ies-100’) is specifically required when creating a Routed VLAN Interface (RVI). This RVI is the Layer 3 logical interface that receives traffic from one or more SAPs (Layer 2 access points) for routing. A plain SAP attached directly to the IES does not need this separate named interface.

The standard BGP decision process, after preferring the path with the highest LOCAL_PREF, will prefer the path with the shortest AS_PATH length. ORIGIN is checked later, and MED is only compared if the paths are from the same neighboring AS.

Path MTU Discovery (PMTUD) is the standard method for determining the maximum transmission unit (MTU) size along a network path. It works by sending packets with the Don’t Fragment (DF) bit set. If a link in the path has a smaller MTU, the device will send an ICMP ‘Fragmentation Needed’ message back, allowing the source to adjust the packet size accordingly.

When a BGP session is stuck in the Active state, it indicates the local router is trying to initiate a TCP connection but failing. After verifying IP reachability and AS numbers, mismatched hold timers are a common culprit. The routers must agree on the hold time value for the session to transition to Established.0