Interview Questions for Network Engineers

  1. What are the slots which can be used for the Supervisor Engine 720 for 6509

Ans – Slots 5 or 6

  1. What are the slots which can be used for the Supervisor Engine 720 for 6513

Ans – Slots 7 or 8

  1. What is the difference between Sup720 & Sup32


    1. Sup720 can forward upto 400 Mpps & Sup32 can forward upto 15 Mpps
    2. Sup720 supports Distributed Cisco Express Forwarding which is not possible with Sup32
  1. How many Cisco Catalyst 3750-X Series Switches can make up a Cisco StackPower stack?

           Ans – Up to four switches can become part of the same Cisco StackPower stack in a ring topology

  1. What are different OSPF LSA Types?

LSA 1 (Router LSA)

Generated by all routers in an area to describe their directly attached links (Intra-area routes). These do not leave the area.

LSA 2 (Network LSA)

Generated by the DR of a broadcast or nonbroadcast segment to describe the neighbors connected to the segment. These do not leave the area.

LSA 3 (Summary LSA)

Generated by the ABR to describe a route to neighbors outside the area. (Inter-area routes)

LSA 4 (Summary LSA)

Generated by the ABR to describe a route to an ASBR to neighbors outside the area.

LSA 5 (External LSA)

Generated by ASBR to describe routes redistributed into the area. These routes appear as E1 or E2 in the routing table. E2 (default) uses a static cost throughout the OSPF domain as it only takes the cost into account that is reported at redistribution. E1 uses a cumulative cost of the cost reported into the OSPF domain at redistribution plus the local cost to the ASBR.

LSA 6 (Multicast LSA)

Not supported on Cisco routers.

LSA 7 (NSSA External LSA)

Generated by an ASBR inside a NSSA to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA by the ABR. These routes appear as N1 or N2 in the IP routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost to the ASBR.

  1. What are the criteria or parameters which are checked for establishing OSPF neighbor relationship?
  • Subnet mask used on the subnet
  • Subnet number (as derived using the subnet mask and each router's interface IP address)
  • Hello interval
  • Dead interval
  • OSPF area ID
  • Must pass authentication checks (if used)
  • Value of the stub area flag
  1. Why does OSPF require all traffic between non-backbone areas to pass through a backbone area (area 0)? [ Ans Courtesy – Jeff Doyle]

The first concept is this:

Every link state router floods information about itself, its links, and its neighbors to every other router. From this flooded information each router builds an identical link state database. Each router then independently runs a shortest-path-first calculation on its database – a local calculation using distributed information – to derive a shortest-path tree. This tree is a sort of map of the shortest path to every other router.

One of the advantages of link state protocols is that the link state database provides a “view” of the entire network, preventing most routing loops. This is in contrast to distance vector protocols, in which route information is passed hop-by-hop through the network and a calculation is performed at each hop – a distributed calculation using local information. Each router along a route is dependent on the router before it to perform its calculations correctly and then correctly pass along the results. When a router advertises the prefixes it learns to its neighbors it’s basically saying, “I know how to reach these destinations.” And because each distance vector router knows only what its neighbors tell it, and has no “view” of the network beyond the neighbors, the protocol is vulnerable to loops.

The second concept is this:

When link state domains grow large, the flooding and the resulting size of the link state database becomes a scaling problem. The problem is remedied by breaking the routing domain into areas: That first concept is modified so that flooding occurs only within the boundaries of an area, and the resulting link state database contains only information from the routers in the area.  This, in turn, means that each router’s calculated shortest-path tree only describes the path to other routers within the area.

The third concept is this:

OSPF areas are connected by one or more Area Border Routers (the other main link state protocol, IS-IS, connects areas somewhat differently) which maintain a separate link state database and calculate a separate shortest-path tree for each of their connected areas. So an ABR by definition is a member of two or more areas. It advertises the prefixes it learns in one area to its other areas by flooding Type 3 LSAs into the areas that basically say, “I know how to reach these destinations.”

Wait a minute – what that last concept described is not link state, it’s distance vector. The routers in an area cannot “see” past the ABR, and rely on the ABR to correctly tell them what prefixes it can reach. The SPF calculation within an area derives a shortest-path tree that depicts all prefixes beyond the ABR as leaf subnets connected to the ABR at some specified cost.

And that leads us to the answer to the question:

Because inter-area OSPF is distance vector, it is vulnerable to routing loops. It avoids loops by mandating a loop-free inter-area topology, in which traffic from one area can only reach another area through area 0.

  1.  What is Cisco Express Forwarding?

Ans:  Cisco Express Forwarding (CEF) is a packet-switching technique that is the default for many of Cisco’s router lines over the last ten years. It provides the ability to switch packets through a device in a very quick efficient way while also keeping the load on the router’s processor low. This way the route process can be tasked with dealing with other duties that require larger amounts of processor time (Quality of Service, Encryption, etc.).

  1. What is the purpose of the Passive-Interface?

Ans:  Passive Interface command is to control the advertisement of routing information. The command enables the suppression of routing updates over some interfaces while it allows updates to be exchanged normally over other interfaces.

  1. What is FECN & BECN?

Ans:  FECN – Forward Explicit Congestion Notification.                    

         BECN – Backward Explicit Congestion Notification.


In a frame relay network FECN is a header bit transmitted by the source (sending) terminal requesting that the destination (receiving) terminal slow down its requests for data.

BECN is a header bit transmitted by the destination terminal requesting that the source terminal send data more slowly.

FECN and BECN are intended to minimize the possibility that packets will be discarded (and thus have to be resent) when more packets arrive than can be handled.

  1. What is the difference between HSRP & VRRP?

Ans: HSRP and VRRP both are the virtual routing protocols that overcome the problem of single gateway failure. Both works with many similarities, such as redundancy and load balancing, but with significant difference as below.

  • HSRP (Host Standby Router Protocol) is Cisco proprietary protocol whereas VRRP (Virtual Router Redundancy Protocol) is an open standards-based protocol.
  • HSRP use default hello timer of 3 second with a hold timer of 10 seconds whereas VRRP use default hello timer of 1 second with a hold timer of 3 seconds.
  • In HSRP, one router is active, one is standby and the rest are in listening state, if more than 3 routers are in the group. In VRRP the active router is called master router whereas all other routers in the group are in backup state.
  • VRRP supports default pre-emption where as HSRP needs it to configured.
  • In HSRP the highest interface address wins the election whereas in VRRP, if a router uses virtual IP as an interface IP, this router becomes the active or master, if the priorities are default.
  1. What is BGP synchronization?

Ans: The BGP synchronization rule states that if an AS provides transit service to another AS, BGP should not advertise a route until all of the routers within the AS have learned about the route via an IGP.