When a BGP session is stuck in the Active state, it indicates the local router is trying to initiate a TCP connection but is failing. Since IP connectivity is confirmed, the most common cause is a filter (like an ACL, firewall, or security group) blocking the BGP TCP port (179). Mismatched AS numbers typically cause the session to flap between Idle and Active, not stick in Active.

A Feasible Successor is a backup path that meets the Feasibility Condition (reported distance less than the feasible distance). EIGRP pre-computes and stores this path in its topology table, allowing for instantaneous convergence to this backup route without needing to query neighbors when the successor (primary) path fails.

When OSPF priorities are equal on a multi-access network, the tie-breaker for DR/BDR election is the highest Router ID. The Router ID is determined by a manually configured router-id, the highest loopback IP address, or the highest active physical interface IP address, in that order.

In Cisco SD-Access, the Fabric Border Node is the specific role responsible for connecting the fabric domain to external networks (like WAN, Internet, or non-fabric parts of the enterprise). It runs BGP to exchange routes with external peers and redistributes routes between the fabric’s LISP-based control plane and the external BGP domain.

In IPv6, the Next Header field identifies the type of header immediately following the IPv6 header. The value 58 is assigned to ICMPv6, which is used for diagnostic functions (like ping), error reporting, and crucial control-plane functions such as Neighbor Discovery (ND) and Path MTU Discovery.

When connecting a Cisco switch to a device that does not participate in Dynamic Trunking Protocol (DTP), like most servers, hypervisors, or routers, it is best practice to disable DTP negotiation with the ‘switchport nonegotiate’ command. Without it, the switch port may be stuck in a dynamic auto or desirable state, waiting for a DTP response that will never come from the server. While ‘switchport trunk encapsulation dot1q’ is necessary on some older platforms, ‘nonegotiate’ is the key command for ensuring a static trunk establishes successfully with a non-DTP device.

Multicast-to-Unicast conversion is the recommended and most efficient method. The WLC takes a single multicast stream destined for a multicast group, replicates it, and sends it as multiple unicast frames over the CAPWAP tunnel to the AP. The AP then delivers these unicast frames to each client interested in the multicast group. This method preserves airtime efficiency as the AP transmits the data only once per client at the appropriate data rate, rather than once at the lowest basic rate for all clients as with native wireless multicast.

Traditional NetFlow v5 and v9 have fixed formats that do not include TCP flag information. Flexible NetFlow (FNF), an enhancement based on the v9 export format, allows you to define custom flow records. You can create a record that includes fields like ‘tcp flags,’ enabling detailed security and behavioral analysis to detect SYN floods, half-open connections, or other flag-based anomalies.

The crypto ACL (or access-list defining interesting traffic) is not applied directly to a router interface with an ‘ip access-group’ command. Instead, it is referenced within the crypto map configuration. The crypto map itself is then applied to the router’s physical outgoing interface facing the VPN peer. This tells the router to evaluate traffic exiting that interface against the crypto ACL; if it matches, the router will attempt to encrypt and send it via the IPsec tunnel.

When a BGP session is stuck in the Active state, it indicates the local router is trying to initiate a TCP connection but is failing. Since IP connectivity is confirmed, the most common cause is a filter (like an ACL, firewall, or security group) blocking the BGP TCP port (179). Mismatched AS numbers typically cause the session to flap between Idle and Active, not stick in Active.0