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Cisco CCNA Routing Protocols and WAN

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Question 1

What is the purpose of a "null 0" (Null0) interface in Cisco routing?

Answer choices

  • A. A loopback interface for router management

  • B. A "bit-bucket" interface used to drop packets that match a route, preventing routing loops when using summary routes (Correct)

  • C. An interface that connects to the ISP

  • D. An interface used for out-of-band management access

Explanation

Null0 is a virtual interface that simply discards any packets sent to it. It is used with summary routes as a "discard route" to prevent routing loops. Example: if a router summarizes 192.168.0.0/22 and sends this to other routers, but internally has no route to 192.168.3.0/24 (it doesn't exist yet), packets for that subnet could bounce between routers. Installing a route `ip route 192.168.0.0 255.255.252.0 Null0` ensures that packets for undefined subnets within the summary are locally discarded instead of looping. EIGRP and OSPF automatically create Null0 routes for summary addresses.

Question 2

To configure OSPF on a Cisco router and advertise network 192.168.1.0/24 in Area 0 (backbone):

```

Router(config)# router ospf [[blank1]] (process ID — locally significant)

Router(config-router)# router-id [[blank2]] (manually set router ID for stability)

Router(config-router)# network 192.168.1.0 [[blank3]] area [[blank4]]

```

The wildcard mask for /24 (255.255.255.0) is [[blank5]].

Explanation

OSPF `network` command uses wildcard masks (inverse of subnet mask). /24 mask = 255.255.255.0 → wildcard = 0.0.0.255. Wildcard: 0 bits = must match, 255 bits = any value. `router ospf 1` — process ID (1–65535) is locally significant — does NOT need to match the neighbor's process ID. Router ID: manually configured for stability; if not set, OSPF uses the highest active loopback IP, then highest active physical interface IP. Area 0 is the backbone area — all other areas must connect to Area 0.

Question 3

What does "route summarization" (also called "route aggregation") accomplish?

Answer choices

  • A. It creates redundant paths between routers

  • B. It combines multiple specific routes into a single less-specific route, reducing routing table size and update traffic (Correct)

  • C. It encrypts routing table entries for security

  • D. It automatically backs up routing table entries to a TFTP server

Explanation

Route summarization (aggregation, supernetting) combines multiple contiguous networks into one summary route. Example: 192.168.1.0/24, 192.168.2.0/24, 192.168.3.0/24, 192.168.4.0/24 can be summarized as 192.168.0.0/22 (or partial summaries). Benefits: smaller routing tables = faster lookup, fewer CPU cycles, less memory; fewer routing updates across WAN links; instability (flapping routes) in one area doesn't affect others. Downside: less specific routing information — may cause suboptimal routing if not carefully designed. Summary routes reduce LSA flooding in OSPF.

Question 4

Which of the following describe WAN technologies used for enterprise connectivity? (Select THREE)

Answer choices

  • A. MPLS (Multiprotocol Label Switching) — provider-managed, uses labels for fast forwarding (Correct)

  • B. Spanning Tree Protocol — creates redundant WAN paths

  • C. Metro Ethernet — carrier Ethernet service extending LAN technologies to WAN distances (Correct)

  • D. SD-WAN (Software-Defined WAN) — centrally managed virtual WAN using multiple transport types (Correct)

  • E. VTP (VLAN Trunking Protocol) — extends VLANs across WAN links

Explanation

WAN technologies: (A) MPLS: carrier-provided, uses label switching (not IP lookup) for forwarding — connects multiple sites with QoS guarantees; common for enterprise multisite. (C) Metro Ethernet: carrier Ethernet extending native Ethernet services (E-Line, E-LAN, E-Tree) to WAN — familiar technology, no protocol conversion. (D) SD-WAN: uses policy-based routing over multiple transports (broadband internet, LTE, MPLS) with centralized management — increasingly replacing traditional WAN. (B) STP is Layer 2 LAN technology. (E) VTP is VLAN management — not a WAN protocol.

Question 5

EIGRP uses the Dijkstra Shortest Path First (SPF) algorithm to calculate routes.

Answer choices

  • A. True

  • B. False (Correct)

Explanation

EIGRP uses the DUAL (Diffusing Update Algorithm) — not Dijkstra SPF. DUAL guarantees loop-free paths and fast convergence by maintaining a Feasibility Condition. EIGRP selects a Successor route (best path to destination, stored in routing table) and a Feasible Successor (backup path that meets the feasibility condition — guaranteed loop-free). If the successor fails, EIGRP instantly uses the feasible successor without recomputing (sub-second convergence). OSPF uses Dijkstra SPF. EIGRP's DUAL is one of its key competitive advantages.