BFD provides sub-second link failure detection and can be integrated with OSPF. This allows OSPF to react to a BFD session going down almost immediately, far faster than waiting for the dead timer to expire, which is the most effective solution for fast convergence. Simply reducing OSPF timers can lead to instability and increased control-plane traffic.

When an EIGRP router has a default route from another source (like an ISP), it will, by default, advertise this learned default route to its EIGRP peers. To prevent this potentially undesirable behavior, a common practice is to configure a distribute-list to filter the 0.0.0.0/0 route from being advertised into the EIGRP domain. This filter would stop all routes advertised out that interface if not configured correctly, causing the observed issue.

The PE-CE routing protocol (or static route) is responsible for the exchange of routes between the customer edge and provider edge routers within a specific VRF. This includes the route to the PE router’s interface facing the CE. If this protocol is down or misconfigured, the CE will not have a route to the PE’s IP address, making a ping impossible, regardless of the state of the core MPLS or MP-BGP infrastructure.

Using route tags is a fundamental and scalable method for loop prevention during mutual redistribution. When redistributing routes from EIGRP into OSPF, you tag them (e.g., tag 100). Then, when configuring redistribution from OSPF into EIGRP, you use a route-map to deny routes carrying that specific tag (100). This effectively blocks the OSPF-learned versions of the original EIGRP routes from being injected back into EIGRP.

In ISIS, the command ‘default-information originate’ configured under the ISIS router process is what instructs a Level-1-2 router to set the ATTACHED bit in its Level-1 LSP. This bit signals to Level-1 routers that this router has connectivity to another area. Level-1 routers, upon seeing the ATTACHED bit set, automatically install a default route pointing to the nearest Level-1-2 router that advertised it. The command itself generates the advertisement that triggers this behavior.

LOCAL_PREF is a well-known discretionary BGP attribute that is exchanged only among iBGP peers within the same Autonomous System. Its primary purpose is to influence the outbound traffic path from the local AS. A higher LOCAL_PREF value makes a route more preferred. When an edge router sets a high LOCAL_PREF for routes learned from Provider A and advertises them via iBGP, all other routers in the AS will see those routes as best and will forward outbound traffic destined for the internet to the edge router connected to Provider A.

The command ‘show ip bgp neighbors ‘ displays detailed information about the BGP session with the specified neighbor. In its output, you will find a line for ‘Connection state’ and, critically, a line indicating ‘BGP state = Established’. More importantly for this question, if MD5 authentication is configured and successful, the output will explicitly state ‘MD5 authenticated’ or show the connection as ‘Established’ without authentication errors. If authentication fails, the session will not reach the Established state, which this command would also reveal.

The OSPF DR/BDR election process uses two values in sequence: first, the highest OSPF interface priority (default 1). If priorities are equal, which is common in multi-vendor environments where defaults are used, the tie is broken by selecting the router with the highest Router ID. The Router ID is typically the highest loopback IP address or, if no loopback is configured, the highest active interface IP address. Configuring loopback interfaces is a best practice to ensure a stable, predictable Router ID.

When you configure an EIGRP summary address (e.g., ‘ip summary-address eigrp 100 10.10.0.0 255.255.0.0’), the router automatically creates a local discard (null0) route for that summary to prevent routing loops. If the summary mask is too broad (e.g., 255.255.0.0 when you only have 10.10.1.0/24 to 10.10.10.0/24), the null0 route will match destinations for subnets that don’t actually exist (like 10.10.20.0/24). Traffic to those non-existent subnets within the summary range will be sent to the summarizing router and then dropped, causing the observed intermittent failures for specific destinations.

BFD provides sub-second link failure detection and can be integrated with OSPF. This allows OSPF to react to a BFD session going down almost immediately, far faster than waiting for the dead timer to expire, which is the most effective solution for fast convergence. Simply reducing OSPF timers can lead to instability and increased control-plane traffic.0