Chapter 3. Fundamentals of WANs and IP Routing Flashcards
- Which of the following fields in the HDLC header used by Cisco routers does Cisco add, beyond the ISO standard HDLC?
a) .Flag
b) .Type
c) .Address
d) .FCS
1.B. The standard HDLC header does not include a Type field, which identifies the type of packet encapsulated inside the HDLC frame
- Two routers, R1 and R2, connect using an Ethernet over MPLS service. The service provides point-to-point service between these two routers only, as a Layer 2 Ethernet service. Which of the following are the most likely to be true about this WAN? (Choose two answers.)
a) .R1 will connect to a physical Ethernet link, with the other end of the cable con-nected to R2.
b) .R1 will connect to a physical Ethernet link, with the other end of the cable con-nected to a device at the WAN service provider point of presence.
c) .R1 will forward data-link frames to R2 using an HDLC header/trailer.
d) .R1 will forward data-link frames to R2 using an Ethernet header/trailer.
2.B and D. The physical installation uses a model in which each router uses a physical Ethernet link to connect to some SP device in an SP facility called a point of presence (PoP). The Ethernet link does not span from each customer device to the other. From a data-link perspective, both routers use the same Ethernet standard header and trailer used on LANs; HDLC does not matter on these Ethernet WAN links.
- Imagine a network with two routers that are connected with a point-to-point HDLC serial link. Each router has an Ethernet, with PC1 sharing the Ethernet with Router1 and PC2 sharing the Ethernet with Router2. When PC1 sends data to PC2, which of the following is true?
a) .Router1 strips the Ethernet header and trailer off the frame received from PC1, never to be used again.
b) .Router1 encapsulates the Ethernet frame inside an HDLC header and sends the frame to Router2, which extracts the Ethernet frame for forwarding to PC2.
c) .Router1 strips the Ethernet header and trailer off the frame received from PC1, which is exactly re-created by Router2 before forwarding data to PC2.
d) .Router1 removes the Ethernet, IP, and TCP headers and rebuilds the appropriate headers before forwarding the packet to Router2.
3.A. PC1 will send an Ethernet frame to Router 1, with PC1’s MAC address as the source address and Router 1’s MAC address as the destination address. Router 1 will remove the encapsulated IP packet from that Ethernet frame, discarding the frame header and trailer. Router 1 will forward the IP packet by first encapsulating it inside an HDLC frame, but Router 1 will not encapsulate the Ethernet frame in the HDLC frame but rather the IP packet. Router 2 will de-encapsulate the IP packet from the HDLC frame and forward it onto the Ethernet LAN, adding a new Ethernet header and trailer, but this header will differ. It will list Router 2’s MAC address as the source address and PC2’s MAC address as the destination address.
- Which of the following does a router normally use when making a decision about routing TCP/IP packets?
a) .Destination MAC address
b) .Source MAC address
c) .Destination IP address
d) .Source IP address
e) .Destination MAC and IP addresses
4.C. Routers compare the packet’s destination IP address to the router’s IP routing table, making a match and using the forwarding instructions in the matched route to forward the IP packet.
- Which of the following are true about a LAN-connected TCP/IP host and its IP rout-ing (forwarding) choices?
a) .The host always sends packets to its default gateway.
b) .The host never sends packets to its default gateway.
c) .The host sends packets to its default gateway if the destination IP address is in a different subnet than the host.
d) .The host sends packets to its default gateway if the destination IP address is in the same subnet as the host.
5.C. IPv4 hosts generally use basic two-branch logic. To send an IP packet to another host on the same IP network or subnet that is on the same LAN, the sender sends the IP packet directly to that host. Otherwise, the sender sends the packet to its default router (also called the default gateway).
- Which of the following are functions of a routing protocol? (Choose two answers.)
a) .Advertising known routes to neighboring routers
b) .Learning routes for subnets directly connected to the router
c) .Learning routes and putting those routes into the routing table for routes adver-tised to the router by its neighboring routers
d) .Forwarding IP packets based on a packet’s destination IP address
6.A and C. Routers do all the actions listed in all four answers; however, the routing protocol does the functions in the two listed answers. Independent of the routing pro-tocol, a router learns routes for IP subnets and IP networks directly connected to its interfaces. Routers also forward (route) IP packets, but that process is called IP routing, or IP forwarding, and is an independent process compared to the work of a routing protocol.
- A company implements a TCP/IP network, with PC1 sitting on an Ethernet LAN. Which of the following protocols and features requires PC1 to learn information from some other server device?
a) .ARP
b) .ping
c) .DNS
d) .None of these answers is correct.
7.C. Address Resolution Protocol (ARP) does allow PC1 to learn information, but the information is not stored on a server. The ping command does let the user at PC1 learn whether packets can flow in the network, but it again does not use a server. With the Domain Name System (DNS), PC1 acts as a DNS client, relying on a DNS server to respond with information about the IP addresses that match a given hostname.
