CCNA Interview Questions and Answers -15


1. Which protocols are used to configure trunking on a switch?

DTP [Dynamic Trunking Protocol

2. Explain difference between 802.1Q and ISL?

single communication link called trunk is used between devices to carry traffic which may belong to multiple VLANS. We can configure the device to allow or deny particular VLAN through the trunk by its VLAN identifier.

VLAN identifier is a special tag that is encapsulated in a Ethernet frame. There are two main types of encapsulation protocols called ISL (Inter Switch Link) which is Cisco proprietary protocol and 802.1q which is an IEEE Standard.

 ISL is an Cisco proprietary protocol.
 Supports up to 1000 Vlans
 Original frame is encapsulated and a new header is inserted during encapsulation process.
 A 26 byte header and a 4 byte FCS (frame check sequence) are inserted. Hence a total of 30
Bytes of overhead.
 ISL tags frames from native Vlans.
 ISL is less preferred in networks because of its high overhead value which is added to each Ethernet frame.

– It is an IEEE Standard.
– 802.1q supports 4096 Vlans.
– IN 802.1q encapsulation process, a 4 byte tag is inserted into original frame and FCS (Frame Check Sequence) is re-calculated.
– 802.1q does not tag frames from native Vlans.

3. What is a Native VLAN and What type of traffic will go through Native VLAN?

The native VLAN is the only VLAN which is not tagged in a trunk, in other words, native VLAN frames are transmitted unchanged. Per default the native VLAN is VLAN 1 but you can change

4. What is Inter-Vlan Routing?

Layer 2 switches cannot forward traffic between VLANs without the assistance of a router. InterVLAN routing is a process for forwarding network traffic from one VLAN to another, using a router.

5. Two Switches are in VTP Server mode with different VLAN’s. Revision Number of both the switches is 0. Which switch will overwrite his own VLAN Database onto other, if both switches are in the same VTP Domain?

The one where you make a change first; Both switches do nothing until you add or remove any vlans so that revision number goes up; then that switch will push vlans onto the other one.

6. If a Switch is in Client mode, Can we enter into the configuration mode?


7. What is VTP?

VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol used by Cisco switches to exchange VLAN information. VTP is used to synchronize VLAN information (Example:-VLAN ID or VLAN Name) with switches inside the same VTP domain.

8. What are different VTP modes?

VTP Server mode – By default every switch is in server mode. Switch in VTP Server Mode can create, delete VLANs and will propagate VLAN changes.

VTP Client mode – Switch in VTP client mode cannot create or delete VLANs. VLAN Trunking Protocol (VTP) client mode switches listen to VTP advertisements from other switches and modify their VLAN configurations accordingly. It listens and forwards updates.

VTP Transparent mode – Switch in VTP Transparent mode does not share its VLAN database but it forwards received VTP advertisements. we can create and delete VLANs on a VTP transparent switch but these changes are not sent to other switches.

9. What are the requirements to exchange VTP messages between two switches?

– Switch should be configured as either a VTP server or VTP client.
– .VTP domain name must be same on both switches.
– VTP versions must match.
– link between the switches should be a trunk link.

10. What is VTP Pruning?

VLAN Trunking Protocol (VTP) pruning is a feature in Cisco switches, which stops VLAN update information traffic from being sent down trunk links if the updates are not needed. Broadcast frames, multicast frames or unicast frames for which the destination MAC address is unknown are forwarded over a trunk link only if the switch on the receiving end of the trunk link has ports in the source VLAN. This avoids unnecessary flooding. VLAN 1 can never prune because it’s an administrative VLAN.

11. What is The Difference between PAP & Chap?

Password Authentication Protocol –
PAPauthentication involves a two-way handshake where the username and password are sent across the link in clear text; hence, PAP authentication does not provide any protection against playback and line sniffing.

Challenge Handshake Protocol –
CHAP authentication, on the other hand, periodically verifies the identity of the remote node using a three-way handshake. After the PPP link is established, the host sends a “challenge” message to the remote node. The remote node responds with a value calculated using a one-way hash function. The host checks the response against its own calculation of the expected hash value. If the values match, the authentication is acknowledged; otherwise, the connection is terminated.

12. What is Virtual Circuit?

As a WAN protocol, Frame Relay is most commonly implemented at Layer 2 (data link layer) of the Open Systems Interconnection (OSI) seven layer model. Two types of circuits exist: permanent virtual circuits (PVCs) which are used to form logical end-to-end links mapped over a physical network, and switched virtual circuits (SVCs). The latter are analogous to the circuit-switching concepts of the public switched telephone network (PSTN), the global phone network.

13. Can I use IP unnumbered with Frame Relay?

If you do not have the IP address space to use many subinterfaces, you can use IP unnumbered on each subinterface. You need to use static routes or dynamic routing for your traffic to get routed. And you must use point-to-point subinterfaces. For more information, refer to the Unnumbered IP over a Point-to-Point Subinterface Example section of Configuring Frame Relay.

14. Can I configure a Cisco router to act as a Frame Relay switch?

Yes. You can configure Cisco routers to function as Frame Relay data communication equipment (DCE) or network-to-network interface (NNI) devices (Frame Relay switches). A router can also be configured to support hybrid data terminal equipment/data communication equipment/permanent virtual circuit (DTE/DCE/PVC) switching. . For more information, refer to the Configuring Frame Relay section of the Cisco IOS Wide-Area Networking Configuration Guide, Release 12.1.

15. Can I bridge traffic over a Frame Relay link?

Yes. On multipoint interfaces, Frame Relay map statements must be configured using the framerelay map bridge command to identify permanent virtual circuits (PVCs) for bridged traffic. Spanning(remove hyphen)Tree Protocol (STP) Bridge Protocol Data Units (BPDUs) are passed at regular intervals depending on the bridging protocol configured.

16. Is a special configuration necessary to connect Cisco routers to other vendor devices over Frame Relay?

Cisco routers use proprietary Frame Relay encapsulation by default. The Internet Engineering Task Force (IETF) encapsulation format must be specified to interact with other vendor devices. The IETF encapsulation can be specified on an interface or per data-link connection identifier (DLCI) basis. For more information, refer to the Frame Relay Configuration Examples section of Configuring Frame Relay, in the Cisco IOS Wide-Area Networking Configuration Guide, Release 12.1.

17. What is Frame Relay AutoInstall and how does it work? Is an additional configuration required?

AutoInstall allows you to configure a new router automatically and dynamically. The AutoInstall procedure involves connecting a new router to a network in which an existing router is preconfigured, turning on the new router, and enabling it with a configuration file that is downloaded from a TFTP server. For more information, refer to Using Configuration Tools.

To support AutoInstall over a link on which the existing router is configured with a point-to-point subinterface, the frame-relay interface-dlci command requires additions. The additional information provided with the frame-relay interface-dlci command is used to respond to the Bootstrap Protocol (BOOTP) request of the remote router. The addition of protocol ipip-address to the command indicates the IP address of the main interface of a new router or access server onto which a router configuration file is to be installed over a Frame Relay network. Use this option only when the device acts as the BOOTP server for automatic installation over Frame Relay.

To support AutoInstall over a link on which the existing router is configured with a multipoint (sub) interface, the frame-relay map command should be configured on the existing router, mapping the IP address of the new router to the local data-link connection identifier (DLCI) used for connecting to the new router.

Apart from this, the Frame Relay (sub) interface of the existing router should be configured with the ip helper-address command pointing to the IP address of the TFTP server.

18. Is Frame Relay Inverse Address Resolution Protocol (IARP) on by default? The inversearp command does not show up in the configuration.


19. Can Frame Relay Inverse Address Resolution Protocol (IARP) work without Local Management Interface (LMI)?

No. It uses LMI to determine which permanent virtual circuits (PVCs) to map.

20. Under what Local Management Interface (LMI) conditions does a Cisco router not send packets over the data-link connection identifier (DLCI)?

When the permanent virtual circuit (PVC) is listed as inactive or deleted.

21. Will a Cisco router process and map an Inverse Address Resolution Protocol (IARP) if it comes across while a data-link connection identifier (DLCI) is down?

Yes, but the router will not use it until the DLCI is active.

22. When implementing a show frame map command, data-link connection identifiers (DLCIs) are defined and active. This can occur when the DLCIs are not working. What does defined and active mean?

The message defined and active tells you that the DLCI can carry data and that the router at the far end is active.

23. Can I change subinterfaces from point-to-point to multipoint or the reverse?

No, after a specific type of subinterface is created, it cannot be changed without a reload. For example, you cannot create a multipoint subinterface Serial0.2, and change it to point-to-point. To change it, delete the existing subinterface and reload the router or create another subinterface. When a subinterface is configured, an interface descriptor block (IDB) is defined by the Cisco IOS® Software. IDBs defined for subinterfaces cannot be changed without a reload. Subinterfaces that are deleted with the no interface command are shown as deleted by issuing the show ip interface brief command.

24. What does illegal serial line type xxx mean?

This message is displayed if the encapsulation for the interface is Frame Relay (or High-Level Data Link Control [HDLC]) and the router attempts to send a packet containing an unknown packet type.

25. What are Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) packets? How do they affect performance?

This congestion notification is accomplished by changing a bit in the address field of a frame as it traverses the Frame Relay network. Network DCE devices (switches) change the value of the FECN bit to one on packets traveling in the same direction as the data flow. This notifies an interface device (DTE) that congestion avoidance procedures should be initiated by the receiving device. BECN bits are set in frames that travel the opposite direction of the data flow to inform the transmitting DTE device of network congestion.

Frame Relay DTE devices may choose to ignore FECN and BECN information or may modify their traffic rates based on FECN and BECN packets received. The frame-relay adaptive-shaping command is used when Frame Relay traffic shaping is configured to allow the router to react to

BECN packets. For information on how the router adjusts traffic rates in response to BECNs, refer to Traffic Shaping.

26. How can I improve performance over a slow Frame Relay link?

Poor performance over a Frame Relay link is generally caused by congestion on the Frame Relay network and from packets that are discarded while in transit. Many service providers only provide best effort delivery on traffic that exceeds the guaranteed rate. This means that when the network becomes congested, it discards traffic over the guaranteed rate. That action can cause poor performance.

Frame Relay traffic shaping allows traffic to be shaped to the available bandwidth. Traffic shaping is frequently used to avoid performance degradation caused by congestion packet loss. For a description of Frame Relay traffic shaping and configuration examples, refer to Frame Relay Traffic
Shaping or the Frame Relay Traffic Shaping section of the Comprehensive Guide to Configuring and Troubleshooting Frame Relay.

To improve performance, refer to theConfiguring Payload Compression or Configuring TCP/IP Header Compression sections of Comprehensive Guide to Configuring and Troubleshooting Frame Relay.

27. What is Enhanced Local Management Interface (ELMI) and how is it used for dynamic traffic shaping?

ELMI enables automated exchange of Frame Relay Quality of Service (QoS) parameter information between the Cisco router and the Cisco switch. Routers can base congestion management and prioritization decisions on known QoS values such as committed information rate (CIR), committed burst (Bc), and excess burst (Be). The router reads QoS values from the switch and can be configured to use those values in shaping traffic. This enhancement works between Cisco routers and Cisco switches (BPX/MGX and IGX platforms). Enable ELMI support on the router by issuing the frame-relay qos-autosense command. For information and configuration examples, refer to the Enabling Enhanced Local Management Interface section of the Configuring Frame Relay and Frame Relay Traffic Shaping.

28. Can I reserve bandwidth for certain applications?

A recently developed Cisco feature called Class-Based Weighted Fair Queuing (CBWFQ) allows reserved bandwidth for different applications of flows depending on Access Control List (ACL) or incoming interfaces. For configuration details, refer to Configuring Weighted Fair Queueing.

29. Can I use priority queuing with Transmission Control Protocol (TCP) header compression over Frame Relay?

For the TCP header compression algorithm to function, packets must arrive in order. If packets arrive out of order, the reconstruction will appear to create regular TCP/IP packets but the packets will not match the original. Because priority queuing changes the order in which packets are transmitted, enabling priority queuing on the interface is not recommended.

30. Can Frame Relay prioritize voice traffic carried in IP packets over non-voice packets?

Yes. The Frame Relay IP RTP Priority feature provides a strict priority queueing scheme on a Frame Relay private virtual circuit (PVC) for delay-sensitive data, such as voice, which is identified by its Real-Time Transport Protocol (RTP) port numbers. This feature makes sure that voice traffic is given strict priority over other non-voice traffic.