Routing and Switching – Network Interview QnA

What is difference between PortFast, UplinkFast and BackboneFast?

Portfast: With conventional STP operation a laptop computer after booting have to wait at least 30 secs ( fifteen listening ,fifteen learning ) for transmit and or receive data . Portfast facilitate the laptop computer to be online going past the listening & learning state of switch port . To configure the PortFast in Switch [...]

Benefits of VLANs

The significant benefits of VLAN are listed below . • Broadcast Control : Broadcasts are needed for the regularly function of a network . Many protocols and application rely on broadcast communication to function properly . A layer 2 switched network is basically in a single broadcast domain and the broadcasts can reach the network [...]

What are different area types used in the OSPF?

Area Types -Standard area -Backbone area (area 0) -Stub area -Totally stubby area -Not-so-stubby area (NSSA) -Totally NSSA Stub Area (area stub) -It contain type 1, 2, and 3 LSAs -No LSA type 4 and 5 (E1 or E2) is allowed -Routers can connect to the External routes via the default route that is injected [...]

What are the link aggregation protocols in Cisco Switches, what are their modes required to bundle a link?

Dynamic Configuration Cisco switches support two dynamic aggregation protocols: PAgP (Port Aggregation Protocol) – Cisco proprietary aggregating protocol. LACP (Link Aggregation Control Protocol) – IEEE standardized aggregation protocol, originally defined in 802.3ad. PAgP and LACP are not compatible – both sides of an Etherchannel must use the same aggregation protocol. EthernChannel – PAgP It supports [...]

In Cisco Switches for VTP, what frame tagging protocol they support?

Cisco switches support two frame tagging protocols: Inter-Switch Link (ISL) IEEE 802.1Q Inter-Switch Link (ISL) It is Cisco’s proprietary frame tagging protocol. It encapsulates a frame with an additional header (26 bytes) and trailer (4bytes). It increases the size of a frame by 30 bytes. The header contains several fields, including a 15-bit VLAN ID. [...]

What are Switching modes?

Switching Modes 1. Store and Forward Switching 2. Cut-through Switching 3. Fragment-Free Switching 1. Store and Forward Switching Switch copies each complete frame into the switch memory and do a Cyclic Redundancy Check (CRC) for errors. If an error is found, the frame is dropped and if there is no error, the switch forwards the [...]

Are OSPF routing protocol exchanges authenticated?

Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication may be through simple passwords or through MD5 cryptographic checksums. To configure simple password authentication for an area, use the command ip ospf authentication-key to assign a password of up to eight octets to each interface attached to the area. Then, issue the area x [...]

What is the difference in Route Distinguisher vs Route Target?

Route Distinguisher We know VRFs allow IP address space to be reused among isolated routing domains. For example, assume you have to connect to three customer sites, all of which are using 192.168.10.0/24 as their local network. We can assign each customer its own VRF so that the overlapping networks are kept isolated from one [...]

Useful BGP commands on Cisco Routers

When BGP is not behaving correctly, a “trick” to temporarily stop peering with a neighbor is to use the following command: router bgp 194 neighbor<ipaddress> password xxx Since the other router doesn’t have the same password, the two routers will stop talking to one another, without you having to do anything else. Later, when the [...]

What is the difference between F1 & F2 modules in Nexus 7000 Switch?

The F2 still provide all the built-in features of F1 line card. It actually take the operation further to provide better bandwidth. Currently, the F2 card will give wire-rate speed. This means that 48 port populated with 10G links will run in non-blocking architecture. Something new that was not possible in the older line cards, [...]

What is the difference between M1 & M2 module on Nexus 7000 series ?

The main difference is that the M1 cards will do major and all Layer 3 related features and operations. The M2 cards will give the data center the expansion to run 40G and 100G infrastructure. The initial series of line cards launched by cisco for Nexus 7000 series switches were M1 and F1. M1 series [...]

How does an arp attack work?

we already know that the purpose of ARP requests in the network is to give the device an appropriate mapping of MAC address to IP address. So, How does an ARP attack works? ARP attack is done through ARP spoofing, where it is done by modifying the ARP tables which are small databases linking to [...]

Common switch chassis errors & troubleshooting

How Switch Chassis looks like? Voltage Failure Voltage Termination (VTT) module terminates signals on the Catalyst switching bus. There are three voltage regulators (VTT) in the backplane. If one VTT module fails, it is a minor alarm. If two fail, there is a major problem and the corrective action shuts down the system. Example Command [...]

What command displays a summary list of OSPF interfaces that includes a column for the cost of each interface?

R1#show ip ospf interface brief

BGP Quick notes

A transit AS is an AS that routes traffic from one external AS to another external AS The “show ip bgp” command is used to display entries in the BGP routing table. The AS-PATH attribute is used to prevent BGP routing loops. When receiving an BGP advertisement, the router checks the AS-PATH attribute, [...]

Passive interface (RIP,OSPF,EIGRP)

Passive-interface command is used in all routing protocols to disable sending updates out from a specific interface. However the command behavior varies from one protocol to another. RIP: In RIP this command will disable sending multicast updates via a specific interface but will allow listening to incoming updates from other RIP enabled neighbors.This simply means [...]

Quick notes for EIGRP

EIGRP is a Cisco proprietary protocol which means it will work only on Cisco routers. EIGRP is also called advanced distance vector or Hybrid routing protocol. Multicast or unicast is used for exchange of information. Multiple network layer protocols are supported. 100% loop-free. By default,EIGRP will limit itself to use no more than 50% of [...]

Memorising the BGP decision making process

1. Weight (Bigger is better) 2. Local preference (Bigger is better) 3. Self originated (Locally injected is better than iBGP/eBGP learned) 4. AS-Path (Smaller is better) 5. Origin 6. MED (Smaller is better) 7. External (Prefer eBGP over iBGP) 8. IGP cost (Smaller is better) 9. EBGP Peering (Older is better) 10. Router- ID

What is Multi VLAN port ?

The multi-VLAN port is a feature which allows switch for configuring a single port for two or more vlans. This feature allows users from different VLANs to access a server or router without implementing InterVLAN routing capability. A multi-VLAN port performs normal switching functions in all its assigned VLANs. Multi-VLAN port will not work when [...]

OSPF Neighbourship conditions

How to troubleshoot if OSPF neighbors are stuck in the two-way state

When Open Shortest Path First (OSPF) is enabled on a router or when a router configured for OSPF is powered up, it tries to discover its OSPF neighbors and synchronize its database with them. Routers are said to be OSPF neighbors when they see their router ID in the received hello packet and the status [...]

What is PBR and How to configure it?

Introduction: Policy-Based Routing (PBR) provides a method to forward packets by overriding the information available in the IP routing table. By using PBR, customers can implement policies that selectively cause packets to take different paths. Traditional IP routing forwards packets based only on the destination IP address in the packet. PBR can be configured to [...]

How configuration rollback feature works in Nexus Switches

The rollback feature allows you to take a snapshot, or user checkpoint, of the Cisco NX-OS configuration and then reapply that configuration to your device at any point without having to reload the device. This checkpoint can be extremely useful when a new change is being tested and want immediate return to an original/stable configuration [...]

Can two different VTP Domain communicate with each other. If yes then how

Two different VTP domains cannot exchange VLAN database information. In fact, splitting a switched network into more VTP domains is one of the few ways how to make one part of the network independent from another with respect to VTP. If two switches are supposed to synchronise their VLAN databases via VTP, they must be [...]

What needs to be done if changes in BGP are not taking effect

Routing policies for a peer include all the configurations such as route-map, distribute-list, prefix-list, & filter-list that may impact inbound or outbound routing table updates. Whenever there is a change in the routing policy, the BGP session must be cleared, or reset, for the new policy to take effect. There are two types of reset, [...]

Is it EIGRP support secondary ip address

Yes the Enhanced Interior Gateway Routing Protocol (EIGRP) supports secondary addresses. However, because EIGRP supply data packets from the primary address, all the routers should be configured with the primary addresses that belong on the same subnet. Be sure that the primary address on the interface is configured for EIGRP by issuing the network command [...]

How to use Cisco Router as DNS Server?

The DNS protocol is used to resolve FQDN (Fully Qualified Domain Names) to IP addresses worldwide. This allows us to successfully find and connect to Internet websites and services no matter where they are. Its usefulness, however, doesn’t stop there: local company and private networks also rely on DNS to operate efficiently and correctly. In [...]

What is BGP backdoor?

In BGP it is a component that is used to modify the admin distance of eBGP in order for an interior portal routing protocol (IGP) to take precedence over an eBGP route. By default,External BGP (eBGP) has an admin distance value of 20. Administrative distance is the first criterion that a router uses to determine [...]

How to configure EIGRP bandwidth usage over Frame Relay and low speed links ?

By default EIGRP limits itself to using no more than 50% of the interface bandwidth. Although, this is not very significance on high speeds like on Ethernet, it becomes critical on low speed links, especially on Nonbroadcast Multiaccess Address (NBMA) networks like Frame Relay. If the interface bandwidth configuration cannot be changed because of routing [...]

With which network type will OSPF establish router adjacencies but not perform the DR/BDR election process?

A- Point-to-point B- Backbone area 0 C- Broadcast multi-access D- Non-broadcast multi-access Answer is:A

What is error disable state? How to recover from error disable port?

The ErrDisable feature is implemented to handle critical situations where the switch detected excessive or late collisions on a port, port duplex misconfiguration, Ether Channel misconfiguration, Bridge Protocol Data Unit (BPDU) port-guard violation, UniDirectional Link Detection (UDLD), and other causes. The error-disable function let the switch to shut down a port when it encounters physical, [...]

What you will do if OSPF neighbor flaps and receives the OSPF-5-ADJCHG error message

The Open Shortest Path First (OSPF) neighbors are established by exchanging hello packets at multicast address 224.0.0.5. If these packets fail due to any Layer 2 (L2) issue, OSPF neighbors flap. This results in the error message ‘OSPF-5-ADJCHG’ To resolve this issue, ensure that the underlying L2 between OSPF neighbors is working properly by issuing [...]

How to use the embedded wireshark in Cisco 3850 switches to capture the traffic

On the 3850 switches has embedded wireshark that can be used to packet capture during the troubleshooting this negate the need of SPAN to capture the traffic. -Define your source monitor capture mycap interface GigabitEthernet1/0/1 both -Set your match statement monitor capture mycap access-list myacl monitor capture mycap match ipv4 any any -Set your destination [...]

Is it possible to use distribute-list command to filter routes in OSPF?

Configuring distribute-list commands that utilize route maps will not achieve desired Link State Advertisement (LSA) filtering. Open Shortest Path First Protocol (OSPF) routes cannot be filtered from entering the OSPF database using distribute-list. The distribute-list in command only filters routes from entering the routing table, and it does not prevent LSA packets from being propagated. [...]

What does ‘passive interface’ command does in OSPF ? How to configure it?

In router you can choose to manually set the ‘passive-interface’ command on the interfaces where adjacency is not desired. The interface address that you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified device interface. Passive interface can be configured in [...]

What is VSS?

The Virtual Switching System (VSS) is a clustering technology that combines two Cisco Catalyst 4500 or 6500 Series into a single virtual switch. In a VSS, the data plane of both clustered switches is active at the same time in both chassis. VSS members are connected by virtual switch links (VSLs) using standard Gigabit or [...]

What is the use of null0 interface? How to configure it?

If a static route is configured for the network to the null0 interface, the packets will be dropped without the ICMP message or without having to configure an access list to block/deny traffic. This interface is always up and can never forward or receive traffic and any encapsulation will always fail. The null interface provides [...]

How to configure GLBP?

Gateway Load Balancing Protocol (GLBP) provides redundancy for IP networks, ensuring that user traffic immediately and transparently recovers from first hop router failures, while allowing packet load sharing between a group of redundant routers. GLBP provides load balancing over multiple routers (gateways) using a single virtual IP address and multiple virtual MAC addresses. Each host [...]

How to enable authentication in OSPF?

There are two ways of authentication in OSPF. 1) Clear text authentication 2) MD5 authentication 1) Clear text authentication Area based authentication Cisco_Router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Cisco_Router(config)#interface f0/0 Cisco_Router(config-if)#ip ospf authentication-key cisco@123 Cisco_Router(config-if)#exit Cisco_Router(config)#router ospf 100 Cisco_Router(config-router)#area 2 authentication Cisco_Router(config-router)#exit Interface based authentication Cisco_Router(config)#int fa0/0 Cisco_Router(config-if)#ip ospf authentication [...]

Suppose if you have two internet links if one gets down, how you will set up redundancy?

To check the connectivity we can use the IP SLA feature in Cisco IOS which will be constantly pinging the internet address as soon as the icmp check will fail it will change the priority of the route so the traffic will get out through the other link. First we create our IP SLA cinfig. [...]