CS-GY 6843 Spring 2020 Final Exam
Name:____Tianbo Tan___________________ N#:__11552331___________________
1) IP Multicast
a) (5 pts) What is IP Multicast? What is a Multicast group and how is it identified?
IP multicast is to send IP datagrams to a group of receivers that requested it in one copy.
A multicast group is a group of devices that waits for receiving addressed traffic to a shared IP multicast address. The IP address is specific.
b) (5 pts) What is IGMP and what are IGMP messages?
The IGMP is Internet Group Management Protocol, a communication protocol that is used by hosts and adjacent routers to establish multicast group membership.
IGMP messages are used by hosts to join and leave multicast groups. Hosts will identify multicast group membership by sending IGMP messages to other local multicast devices. Devices will also listen to IGMP messages to periodically send queries to determine groups are active or not on a specific subnet.
c) (5 pts) What is IGMP snooping? When is it used?
IGMP snooping will check IGMP packets that are transmitted in the network, take out the group registration, and configure multicasting if needed. More specifically, IGMP snooping will help prevent the local hosts from receiving traffic for a multicast group they didn’t join.
We use snooping when we have multicast. Without snooping, multicast messages will be treated like broadcast messages and be sent to all ports on the network. While IGMP Snooping will help multicast traffic to forward packets to only the ports that are members of a multicast group.
d) (5 pts) Describe an advantage of using IP Multicast over unicast, broadcast or anycast.
Multicast is more efficient than broadcast. A broadcast message must be received by every device on the local link and accepted and sent to the CPU to process even if it is unimportant. In multicast, the NIC does not need to listen to multicast packets unless they are told to do so.
e) (5 pts) Explain what a Multicast tree is and how it is created from routing protocols.
A multicast tree represents the shortest path tree from the sender to a group of listeners.
In PIM routing protocol, it will first build a group shared tree that has one tree for one multicast group. For each tree, it will have a core router that is responsible to shape the shortest path to all receivers having the core router as the root of the tree.
2) MPLS
a) (5 pts) Describe a Label Switched Path (LSP)?
A LSP is a unidirectional path between a pair of routers, routed across an MPLS network. In a MLPS, if there is a LSP between router1 and router 2, all traffic from router 1 to router 2 will take the same deterministic path.
b) (5 pts) What is Label Edge Router (LER) and what role does in play in MPLS?
A LER is the router that first encapsulates a packet in a MPLS LSP. it makes the initial path selection in MPLS.
c) (5 pts) What is a Label Switching Router (LSR)?
A LSR is the middle router of an LSP that does MPLS switching
d) (5 pts) Compare/contrast Label Switching to IP Routing.
In IP routing: the path from source to destination is determined by destination address alone.
In MPLS: the path from source to destination can be based on source and destination address.
e) (5 pts) What role does RSVP play in an MPLS network and how does it work?
RSVP reserves bandwidth across the network in MPLS.
RSVP associates the LSP with a bandwidth value since the LSP is a tunnel between two nodes in the network. Then RSVP will look for the shortest path with enough bandwidth available to carry the LSP. Once found, the LSP is signalled across a set of links and this bandwidth is removed from the available bandwidth pool. Other LSP will be associated depending on whether there is enough bandwidth left.
f) (5 pts) What is MPLS Fast Reroute? Describe it.
The MPLS Fast Reroute will pre-calculate the backup path for possible link or node failures in the original path. Since it is pre-calculated, it will be also pre-programmed in the FIB router and activated once a failure happens.
3) BGP
a) (5 pts) Describe an Autonomous System? What is an ASN? How is it used?
An AS is a group of routers that share similar routing policies and operate within a single administrative domain.
AN ASN is a SD number that is uniquely assigned to an AS that is connected to the public internet using an exterior gateway protocol like BGP.
b) (5 pts) What is an AS path and how is it used?
An AS Path is the AS that routing information passed through to get to a specific router. It indicated the origin of the route and prevented routing loops in BGP.
4) (10 pts) Given the following DNS zone file. For rows i-x describe each of the fields in the rows. In your descriptions make sure you use the fully qualified domain names when describing a field.
i. base name used in domain name substitution is example.com
ii. Time to live for the zone is 1 hour
iii. Start of authority has primary name servers ns.example.com. , username.example.com. Has serial number 20071207, refresh time = 1day, refresh retry = 2h; expiry = 4weeks; ttl = 1hour
iv. The domain ns is authoritative for the domain example.com.
V. The domain ns.somewhere is authoritative for the domain example.com.
Vi. Mail Exchange. the mail.example.com is the mail server to the domain example.com. With priority 10
Vii. the domain example.com. Is pointed to the IP address 192.0.2.1 Viii. the domain ns Is pointed to the IP address 192.0.2.2
Ix. the domain www is pointed to another hostname example.com. X. the domain mail is pointed to the IP address 192.0.2.3
5) OSPF
a) (5 pts) In OSPF how is a default route specified and what is it used for?
A default route is always on an area border router indicating itself as the next hop to the external networks.
b) (5 pts) Why would someone mark an interface as passive in OSPF?
We want to control the advertisement of routing information. By setting an interface passive, that interface does not send or receive OSPF route updates.
6) Wireless
The Ethernet addresses of the AP1, AP2, Router and Host are respectively Mac1, Mac2, MacR and MacH. The IP address of the router is IPR
a) (5 pts) What values are in the ARP table of the host when it is associated with AP1?
MacR and IPR
b) (5 pts) What is in the ARP table of the host after it moves and is associated with AP2?
MacR and IPR
c) (5 pts) 802.3 is able to perform frame collisions while 802.11 is not. Why?
802.3 is an ethernet and it uses CSMA/CD and it is wired. The physical collision-detection is more effective than those of 802.11. 802.3 has separate TX and RX pairs that the NIC can detect frame collision.
802.11 is wireless and it cannot simultaneously transmit and receive. So it uses the CSMA/CA (collision avoidance)