Question 1.
What are the key function(s) of any relay, whether that device is called a router or a switch?

a) forwarding frames
b) forwarding packets
c) determining paths at an appropriate layer and forwarding the units of data at that layer and the layers below it.
d) understanding topology

Answer

Question 2.
How do you display the Routing Information Base?

a) show cache
b) show ip route
c) show rib
d) show route database

Answer

Question 3.
Which protocols, by default, consider hop count in their metric?

a) IP RIP, Apple RTMP
b) IP RIP, IPX RIP, Apple RTMP
c) IP RIP, IGRP
d) OSPF

Answer

Answers

Question 1.
What are the key function(s) of any relay, whether that device is called a router or a switch?

a) forwarding frames
b) forwarding packets
c) determining paths at an appropriate layer and forwarding the units of data at that layer and the layers below it.
d) understanding topology

Answer
c) determining paths at an appropriate layer and forwarding the units of data at that layer and the layers below it.

Explanation
While Cisco likes to talk about frames vs. packets vs. segments vs. messages, doing so is not correct OSI terminology. OSI formalism sometimes is very pedantic, but some of its terminology can be very precise and unambiguous.

OSI documents speak not of specifically named units at every layer (e.g., frame at layer 2), but of Protocol Data Units (PDU). At a specific layer, you speak of Transport PDUs or Data Link PDUs. Another useful concept, especially when dealing with protocol encapsulation, is that the layer above the current layer is called (N+1) while the layer below is (N-1). From the perspective of the Network Layer, it receives (N+1) PDUs from Transport, and sends out (N-1) PDUs to Data Link.

A relay, which is a term from OSI, is a device (or software function) with at least two interfaces. It receives PDUs on one interface and de-encapsulates them until it has the information on which it will make forwarding decisions. Ignoring devices such as multilayer switches, devices such as bridges and LAN and WAN switches accept physical layer bits, build them into Data Link PDUs, and make forwarding decisions on information at Data Link.

Routers receive bits, form frames, and extract Network PDUs from the Data Link PDUs. After examining Network Layer information, they internally forward Network PDUs to an outgoing interface, and then encapsulate these into Data Link PDUs and then Physical Layer information.

To make any of these forwarding decisions, the relay must first have an association between destination (and possibly other) information in the PDU at which it makes decisions, and information about the appropriate outgoing interface. The process of learning these associations is path determination. In bridges and LAN switches, path determination involves the spanning tree protocol, VLAN protocols, and source routing. In routers, path determination involves static and dynamic routing, as well as the up/down state of hardware interfaces.

So, choices a, b, and d are incorrect because they are only pieces of the entire problem.
[2292]

CCIE Topic(s): B
CCIE Objective(s): B2
CCNA Topic(s):
CCNA Objective(s):

Question 2.
How do you display the Routing Information Base?

a) show cache
b) show ip route
c) show rib
d) show route database

Answer
b) show ip route

Explanation
The routing information base is the general technical term for the place in which the results of the path determination process are stored. On Cisco routers, the routing table is the same as the RIB.

When the path for a particular destination goes through the process switching path, the RIB and Forwarding Information Base (FIB) are exactly the same table. In faster switching modes, the router has separate FIBs. Cisco's usage is that a cache has a lesser number of entries than in the full RIB, hopefully the most frequently used entries. Cisco Express Forwarding, however, uses a full FIB, which contains every entry in the RIB, but optimized for speed of lookup rather than for efficient updating by path determination.
[2293]

CCIE Topic(s): B
CCIE Objective(s): B2
CCNA Topic(s):
CCNA Objective(s):

Question 3.
Which protocols, by default, consider hop count in their metric?

a) IP RIP, Apple RTMP
b) IP RIP, IPX RIP, Apple RTMP
c) IP RIP, IGRP
d) OSPF

Answer
b) IP RIP, IPX RIP, Apple RTMP

Explanation
IP RIP and Apple RTMP use hop count as their only metric. IPX RIP has two factors in its metric, the primary one a delay-based component called ticks. In IPX RIP, hop count only comes into play as a tie-breaker between routes of equal tick count.

IGRP does consider hop count, but not as part of its metric. It uses hop count as a means of routing loop detection. IGRP metrics are principally based on bandwidth, usually consider delay, and may consider utilization and error rate. In practice, the latter two factors have not proven to be terribly useful.

OSPF route metrics, at the basic level, are a sum of interface costs. With Cisco and most other vendors, interface costs are derived from bandwidth. The Bay RS router operating system (which is the basis of Microsoft's RRAS) has a different and interesting default. Bay has a default interface cost of 1 for all interfaces, which is intended to help transition from RIP to OSPF. If you set the cost to 1 on all interfaces, in a Cisco or Bay environment, OSPF effectively will compute hop-count-based metrics. Bay does recommend changing manually to bandwidth-based metrics after RIP to OSPF conversion is complete.
[2294]

CCIE Topic(s): B,E,F
CCIE Objective(s): B2,E1,E5,F1,F2
CCNA Topic(s):
CCNA Objective(s):