CCNA Networking Questions

The 'mpls ldp advertise-labels' command ensures that LDP advertises label bindings to its peers. If this is disabled (default is enabled, but can be overridden), LDP sessions may be up but no labels are distributed, resulting in missing labels in the LFIB. Re-enabling it forces LDP to send label mappings for all FECs, resolving the connectivity issue.

The Egress Tunnel Router (ETR) is the LISP device that registers its location (RLOC) to the mapping system (Map-Server) so that other devices can resolve the endpoint identifier (EID) to that RLOC. This registration process populates the mapping database, enabling remote Ingress Tunnel Routers (ITRs) to find and forward traffic to the ETR. Without ETR registration, the mapping system would have no location information for the EID.

SVI configuration requires creating the VLAN, then the interface VLAN, assigning an IP, and enabling it.

TLV 236 (IPv6 Reachability) carries IPv6 prefixes and associated metrics.

A stub area does not allow AS-external LSAs (type 5), reducing routing table size. NSSA allows limited external routes via type 7 LSAs.

A VPNv4 route is installed into a VRF table only if its Route Target (RT) matches an import statement in the VRF's route-target import list or import map. If the RT does not match, the PE router will not import the route into the VRF, even though the route is valid in the BGP VPNv4 table. This is the most common cause for a VPNv4 route being present in BGP but missing from the VRF.

The most likely cause is a mismatched MTU because OSPF uses the interface MTU to determine whether to form an adjacency. If the MTU on one side is smaller than the size of the Database Description (DBD) packet, the neighbor will reject the packet and remain in the EXSTART state, preventing full adjacency. This is a common issue when connecting interfaces with different MTU values, such as a default 1500 bytes on one side and a lower value on the other.

The Init state is the first step in OSPF neighbor formation after the Down state. A router enters Init when it receives a Hello packet from a neighbor but does not yet see its own Router ID in the neighbor's Hello packet, indicating that bidirectional communication is not yet confirmed.

In Segment Routing (SR-MPLS), the IGP (IS-IS or OSPF) is extended to advertise prefix Segment Identifiers (SIDs), which are used to build MPLS labels directly from the IGP database. This eliminates the need for a separate label distribution protocol like LDP, as the IGP itself carries the label information within its link-state advertisements.

ORIGINATOR_ID is set by the route reflector to identify the originator of the route. It is used to prevent loops in iBGP.

The BGP best-path selection algorithm considers attributes in a specific order before using the metric (MED). The sequence is: highest Weight, highest Local Preference, prefer locally originated routes, shortest AS_PATH, lowest Origin type, and then lowest metric. Therefore, all five options A, B, C, D, and E are evaluated before the metric step.

IS-IS TE extensions use sub-TLVs within the extended IS reachability TLV (type 22). Specifically, TLV 135 (extended IP reachability) carries wide metrics, but TE information is carried in the extended IS reachability TLV (type 22) with sub-TLVs. The question asks for the TLV that carries TE info; the correct answer is the extended IS reachability TLV (type 22).

BFD (Bidirectional Forwarding Detection) provides fast failure detection at the link layer, triggering routing protocol convergence.

BGP PIC uses backup paths that are pre-installed in the RIB/FIB. When the primary path fails, traffic is immediately switched to the backup path without waiting for BGP re-convergence.

BGP confederations divide an AS into sub-ASes. eBGP-like behavior is used between sub-ASes, but the AS_PATH length calculation treats the confederation as a single AS. The sub-AS number is appended to the AS_PATH with a special type code, but the AS_PATH length for best path selection ignores confederation segments.

6PE uses BGP multiprotocol extensions (MP-BGP) with the IPv6 Address Family Identifier (AFI) and subsequent AFI (SAFI) to carry IPv6 prefixes. The next-hop is encoded as an IPv4 address using the 'next-hop' attribute with an IPv4 address. This is done via MP_REACH_NLRI for IPv6 with an IPv4 next-hop.

BGP route selection starts with the highest weight (Cisco proprietary), then highest LOCAL_PREF, then locally originated routes.

ECMP with a hash algorithm based on Layer 3 and Layer 4 headers is the correct choice because it provides per-flow load balancing by computing a hash over source/destination IP addresses and TCP/UDP port numbers, ensuring that all packets belonging to the same flow are forwarded over the same equal-cost path. This preserves packet order while distributing different flows across multiple paths in an MPLS network.

Route summarization (A) reduces the number of BGP prefixes advertised, which inherently limits the impact of flapping because a single summary prefix represents many more-specific prefixes. If a specific subprefix flaps, the aggregate remains stable, preventing the flapping from propagating to BGP peers. This is a proactive approach to minimize the control-plane churn caused by unstable routes.

Totally stubby area blocks Type 4 (ASBR Summary) and Type 5 (External) LSAs, and also Type 3 (Summary) LSAs except for the default route.

QoS policies on PE routers with shaping and policing allow the service provider to enforce traffic contracts per customer, ensuring that one customer's traffic does not consume bandwidth allocated to another. This is achieved by applying ingress policing to limit customer traffic to the Committed Information Rate (CIR) and egress shaping to smooth bursts, preventing congestion in the MPLS core from affecting other customers.

BGP route reflectors use the cluster ID to detect and prevent routing loops. When a route reflector receives an update containing its own cluster ID in the cluster-list attribute, it discards the route, breaking the loop. This mechanism is defined in RFC 4456 and is essential for loop-free route propagation in non-full-mesh iBGP topologies.

6VPE uses the VPNv6 address family (or IPv6 labeled unicast with VPN extensions) to carry IPv6 prefixes with VPN-IPv6 addresses.

On a point-to-point OSPF link, the EXSTART state indicates that the routers have formed a bidirectional communication (2WAY) and are now negotiating the master/slave relationship for database description (DBD) packet exchange. A mismatched MTU between the two interfaces causes the DBD packets to be dropped or rejected, preventing the adjacency from progressing beyond EXSTART. This is a common issue on serial links where one side may have a different MTU configured.

The last byte '00' is the SEL (NSAP Selector), which is always '00' for IS-IS routing.

The NET (Network Entity Title) consists of AFI, Area ID, System ID, and SEL. The System ID uniquely identifies a router within an area.

A stub area blocks Type 5 LSAs, and a default route is injected for external destinations.

MAP-T (Mapping of Address and Port using Translation) uses IPv4-IPv6 translation (stateless NAT64-like) at the customer edge, not encapsulation.

OSPF uses different LSA types (e.g., Type 1 Router LSA, Type 2 Network LSA, Type 3 Summary LSA, Type 5 AS-External LSA) to advertise different categories of routing information, such as router links, network links, inter-area routes, and external routes. This LSA-type differentiation is fundamental to OSPF's link-state architecture, enabling efficient flooding and route computation within and across areas.

Route reflectors use cluster-id to detect loops and originator-id to prevent advertisement back to the originator. The non-client mesh requirement is eliminated with route reflectors.

Standard BGP communities include NO_EXPORT (0xFFFFFF01), NO_ADVERTISE (0xFFFFFF02), and LOCAL_AS (0xFFFFFF03). NO_EXPORT prevents advertisement to any eBGP peer; NO_ADVERTISE prevents advertisement to any peer; LOCAL_AS prevents advertisement outside the local AS.

LOCAL_PREF is used to influence the path selection for outbound traffic within the AS, with higher preference preferred.

Configuring backup tunnels with explicit paths that avoid high-latency links ensures that when MPLS TE Fast Reroute (FRR) activates, the backup path meets the latency requirements for real-time traffic like voice. By specifying explicit routes (e.g., via the 'path-option' command with explicit-path objects), the operator can exclude links known to have high propagation delay or congestion, thus preventing the poor voice quality experienced after the fiber cut.

MED (Multi-Exit Discriminator) is used to influence inbound traffic by suggesting to a neighbor AS which path to use.

The order: weight, local pref, originate (locally originated), AS_PATH length, ORIGIN, MED. So weight, local pref, and AS_PATH length are before MED.

Segment Routing via IGP (Option A) is a valid label distribution method because it uses the IGP (OSPF or IS-IS) to advertise prefix-SIDs, which are MPLS labels. The IGP distributes these labels as part of its link-state database, eliminating the need for a separate label distribution protocol like LDP. This is defined in RFC 8660 and RFC 8665 for OSPF and IS-IS respectively.

LLQ is correct because it combines a strict priority queue with CBWFQ, ensuring voice traffic (which is delay-sensitive) is always serviced first, thereby minimizing latency and jitter. The priority queue is policed to prevent starvation of other traffic, making it ideal for real-time applications like VoIP in service provider networks.

C is correct because a router always prefers the most specific prefix (longest prefix length) when forwarding packets, regardless of administrative distance or metric. This is a fundamental rule of IP routing: the route with the longest subnet mask match is chosen first, as it represents the most precise path to the destination.

Weight (D) is a Cisco-proprietary attribute that is checked first in the BGP best-path selection process, before AS-path length. Local Preference (E) is the second attribute evaluated, also preceding AS-path length. Both are considered before the AS-path length comparison in the default BGP decision process.

The BGP best path selection order is: weight, local pref, originated locally, AS_PATH length, ORIGIN, MED, etc. MED is considered after ORIGIN.

The correct matches are: PE connects to customer edge, P is MPLS core, ASBR connects to other ASes. Common confusions: mistaking P for CE or confusing Route Reflector with ASBR.

MED (Multi-Exit Discriminator) is the correct BGP attribute for influencing inbound traffic from a multihomed customer. When the customer advertises the same prefix to two PE routers in the same AS, setting a lower MED value on one PE will cause the customer's upstream AS to prefer that path for returning traffic. This allows the service provider to control which PE receives the ingress traffic without altering any outbound routing policies.

Weight, local preference, and AS_PATH length are considered before MED. ORIGIN is also before MED but AS_PATH is longer.

Setting the BGP 'no-export' community on routes from the undesired entry point prevents those routes from being advertised to the customer AS. Since the customer only learns routes from the desired entry point, they will prefer that entry point for inbound traffic. This is a standard technique for influencing inbound traffic without requiring customer-side configuration.

Confederation uses private AS numbers (from 64512 to 65535) for sub-ASs, and these are prepended to the AS_PATH but not shown to external peers.

OSPFv3 runs over IPv6, supports multiple instances per link, and uses link-local addresses. OSPFv2 does not have these.

Route reflectors allow iBGP speakers to advertise routes learned from other iBGP peers, reducing full mesh requirements, and use cluster-id and originator-id to prevent loops.

OSPFv3 runs per-link instead of per-subnet, uses link-local addresses for neighbor discovery, and separates the protocol from addressing by using LSAs with IPv6 prefix information.

L1/L2 routers perform both intra-area and inter-area routing, maintaining both L1 and L2 LSDBs.

Increasing the administrative distance of IS-IS to be higher than OSPF (e.g., from 115 to 120) makes OSPF routes preferred over IS-IS routes in the routing table. Since BGP uses the IGP route to resolve its next-hop, this ensures BGP always selects the OSPF path, preventing traffic drops when the next-hop is only reachable via OSPF. This is a simple, protocol-agnostic way to control IGP preference during a migration without altering route redistribution or tunneling.

BGP PIC uses precomputed backup paths (e.g., via BGP PIC edge or BGP add-path) to switch traffic upon failure without waiting for BGP convergence.

The OSPF network type is determined by the interface configuration. The 'ip ospf network point-to-point' command explicitly sets the interface to point-to-point mode, which eliminates the need for DR/BDR election and uses multicast hello packets (224.0.0.5) to neighbors. This is the correct answer because the exhibit shows this exact configuration.

OSPFv3 runs per-link, not per-subnet; it uses link-local addresses for adjacencies and floods LSA types specific to IPv6.

NSSA (Not-So-Stubby Area) allows the import of external routes as Type 7 LSAs, which are translated to Type 5 at the ABR.

Incremental SPF (iSPF) reduces SPF calculation time by recalculating only the affected portion of the Shortest Path Tree (SPT) after a topology change, rather than recomputing the entire tree. This is especially beneficial in large OSPF networks where full SPF runs are CPU-intensive and slow convergence.

BFD (Bidirectional Forwarding Detection) provides fast failure detection for routing protocols like OSPF. OSPF can be configured to use BFD for sub-second convergence.

VPLS (Virtual Private LAN Service) is the correct choice because it emulates an Ethernet bridge across an MPLS core, providing MAC address transparency and supporting multiple customer VLANs through a multipoint-to-multipoint topology. Unlike point-to-point services, VPLS learns MAC addresses and forwards frames based on them, enabling end-to-end Ethernet service across distributed sites.

Segment routing uses the IGP (IS-IS or OSPF) to distribute prefix SIDs. The IGP extensions for segment routing (RFC 8665 for OSPF, RFC 8667 for IS-IS) carry the prefix SID sub-TLV within the prefix reachability information, allowing each router to allocate and advertise the SID associated with a prefix. This is the native label allocation method for segment routing, as it leverages the existing IGP database without requiring a separate label distribution protocol.

6PE requires BGP to carry IPv6 prefixes with an IPv4 next-hop (MPLS label), and the core MPLS network must support IPv4 LSPs.

MPLS TE Fast Reroute (FRR) provides sub-50 ms protection against link or node failures by pre-computing and pre-signaling a backup tunnel that bypasses the protected element. This meets the requirement for bandwidth protection of voice traffic, which is sensitive to packet loss and delay during network convergence.

The last two octets of an IS-IS NET address represent the SEL (NSAP Selector), which is always 00 for a router.

IS-IS wide metrics (TLV 22 and 135) use 24-bit or 32-bit values, allowing higher link costs needed for TE. Narrow metrics are limited to 6 bits.

In MPLS L3VPN, customer routes learned via eBGP from the CE must be placed into the VRF's IPv4 unicast table first, and then the 'address-family vpnv4 unicast' under BGP is required to redistribute those VRF routes into the VPNv4 BGP table, which carries the route with its route distinguisher (RD) and extended community. This two-step process ensures the PE can advertise the route as a VPNv4 prefix to other PEs via MP-BGP.

Wide metrics support values up to 2^24-1 (16,777,215) and allow Traffic Engineering (TE) metrics, unlike narrow metrics which are limited to 6 bits (max 63).

The RPF (Reverse Path Forwarding) neighbor for an (S,G) entry is determined by the unicast routing table: the router expects to reach the source S via the interface and next-hop that the unicast route provides. If the unicast routing table does not have a route back to the source, the router cannot determine the correct RPF interface and neighbor, leading to an incorrect RPF neighbor in the 'show ip mroute' output. This is the most common cause of RPF failures in multicast troubleshooting.

The exhibit shows that the LDP session is operational (state = Operational), but no labels are being exchanged. In 'downstream on demand' mode, a router does not advertise labels unless explicitly requested by an upstream neighbor via a Label Request message. Since the session is up but no labels are exchanged, this mismatch in label advertisement mode is the most likely cause.

The IS-IS Router Capability TLV (242) carries sub-TLVs including the SR Capabilities sub-TLV that advertises the SRGB.

NAT64 uses a well-known prefix (64:ff9b::/96) to embed IPv4 addresses into IPv6 addresses. The IPv4 address is appended to the prefix, creating an IPv6 address that represents the IPv4 destination.

OSPF uses opaque LSAs to carry TE information. Type 10 (area-local) and Type 11 (AS-scoped) opaque LSAs are used for TE. Type 9 is link-local.

TLV 22 (Extended IS Reachability) is the mandatory TLV for IS-IS TE, defined in RFC 5305.

TLV 135 for IPv4 (extended IP reachability) and TLV 236 for IPv6 (IPv6 reachability) are used in IS-IS for prefix advertisement.

An L1 router only knows routes within its own area. It uses the closest L1/L2 router as a default route to reach other areas. The L1/L2 router leaks a default route into the L1 area.