Chapter 6 - Routing IP Packets

Question 1

Administrative distance (AD)

Answer 1

A routing protocol’s index of believability. Routing protocols with a smaller AD are considered more believable than routing protocols with a higher AD.

Question 2

ARP command

Answer 2

Can be used in either the Microsoft Windows or the UNIX environment to see what a Layer 2 MAC address corresponds to in a Layer 3 IP address.

Question 3

Border Gateway Protocol (BGP)

Answer 3

The only EGP in widespread use today. In fact, BGP is considered to be the routing protocol that runs the Internet, which is an interconnection of multiple autonomous systems. BGP is a path-vector routing protocol, meaning that it can use as its metric the number of autonomous system hops that must be transited to reach a destination network, as opposed to the number of required router hops.

Question 4

Default static route

Answer 4

A default static route is an administratively configured entry in a router’s routing table that specifies where traffic for all unknown networks should be sent.

Question 5

Distance vector

Answer 5

A category of routing protocol that sends a full copy of its routing table to its directly attached neighbors.

Question 6

Dynamic NAT (DNAT)

Answer 6

A variant of NAT in which inside local addresses are automatically assigned an inside global address from a pool of available addresses.

Question 7

Enhanced Interior Gateway Routing Protocol (EIGRP)

Answer 7

A Cisco proprietary protocol. So, although EIGRP is popular in Cisco-only networks, it is less popular in mixed-vendor networks. Like OSPF, EIGRP is an IGP with very fast convergence and high scalability. EIGRP is considered to be an advanced distance vector or a hybrid routing protocol.

Question 8

Exterior Gateway Protocol (EGP)

Answer 8

A routing protocol that operates between autonomous systems, which are networks under different administrative control. Border Gateway Protocol (BGP) is the only EGP in widespread use today.

Question 9

Hold-down timers

Answer 9

Can speed the convergence process of a routing protocol. After a router makes a change to a route entry, the hold-down timer prevents subsequent updates for a specified period of time. This approach can help stop flapping routes (which are routes that oscillate between being available and unavailable) from preventing convergence.

Question 10

Interior Gateway Protocol (IGP)

Answer 10

A routing protocol that operates within an autonomous system, which is a network under a single administrative control. OSPF and EIGRP are popular examples of IGPs.

Question 11

Intermediate System-to-Intermediate System (IS-IS)

Answer 11

A link-state routing protocol similar in its operation to OSPF. IS-IS uses a configurable, yet dimensionless, metric associated with an interface and runs Dijkstra’s shortest path first algorithm. Although using IS-IS as an IGP offers the scalability, fast convergence, and vendor-interoperability benefits of OSPF, it has not been deployed as widely as OSPF.

Question 12

Internet Group Management Protocol (IGMP)

Answer 12

A multicast protocol used between clients and routers to let routers know which of their interfaces has a multicast receiver attached.

Question 13

Link-state

Answer 13

A category of routing protocol that maintains a topology of a network and uses an algorithm to determine the shortest path to a destination network.

Question 14

Link-state advertisement (LSA)

Answer 14

Sent by routers in a network to advertise the networks the routers know how to reach. Routers use those LSAs to construct a topological map of a network. The algorithm run against this topological map is Dijkstra’s shortest path first algorithm.

Question 15

Metric

Answer 15

A value assigned to a route. Lower metrics are preferred over higher metrics.

Question 16

Network Address Translation (NAT)

Answer 16

Allows private IP addresses (as defined in RFC 1918) to be translated into Internet-routable IP addresses (public IP addresses).

Question 17

Next-hop

Answer 17

An IP address on the next router to which traffic should be forwarded.

Question 18

Open Shortest Path First (OSPF)

Answer 18

A link-state routing protocol that uses a metric of cost, which is based on the link speed between two routers. OSPF is a popular IGP because of its scalability, fast convergence, and vendor interoperability.

Question 19

Poison reverse

Answer 19

This feature of a distance-vector routing protocol causes a route received on one interface to be advertised back out of that same interface with a metric considered to be infinite.

Question 20

Port Address Translation (PAT)

Answer 20

A variant of NAT in which multiple inside local IP addresses share a single inside global IP address. PAT can distinguish between different flows based on port numbers.

Question 21

Protocol Independent Multicast (PIM)

Answer 21

A multicast protocol used between multicast-enabled routers to construct a multicast distribution tree.

Question 22

Route redistribution

Answer 22

Allows routes learned by one routing protocol to be injected into the routing process of another routing protocol.

Question 23

Routed protocol

Answer 23

A protocol with an addressing scheme (for example, IP) that defines different network addresses.

Question 24

Routing Information Protocol (RIP)

Answer 24

A distance-vector routing protocol that uses a metric of hop count. The maximum number of hops between two routers in an RIP-based network is 15. Therefore, a hop count of 16 is considered to be infinite. RIP is considered to be an IGP.

Question 25

Routing protocol

Answer 25

A routing protocol (for example, RIP, OSPF, or EIGRP) that advertises route information between routers, which describes how to reach specified destination networks.

Question 26

Split horizon

Answer 26

This feature of a distance-vector routing protocol prevents a route learned on one interface from being advertised back out of that same interface.

Question 27

Static NAT (SNAT)

Answer 27

A variant of NAT in which an inside local IP address is statically mapped to an inside global IP address. SNAT is useful for servers inside a network that need to be accessible from an outside network.

Question 28

Time To Live (TTL)

Answer 28

The TTL field in an IP header is decremented once for each router hop. Therefore, if the value in a TTL field is reduced to 0, a router discards the frame and sends a time exceeded ICMP message back to the source.