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Miscellaneous Questions

July 8th, 2017 in ROUTE 300-101 Go to comments

Question 1


The command “clear ip route” clears one or more routes from both the unicast RIB (IP routing table) and all the module Forwarding Information Bases (FIBs).

Question 2


The prefix-list “ip prefix-list name permit ge 24 le 24” means
+ Check the first 16 bits of the prefix. It must be 10.8
+ The subnet mask must be greater or equal 24
+ The subnet mask must be less than or equal 24

-> The subnet mask must be exactly 24

Therefore the suitable prefix that is matched by above ip prefix-list should be 10.8.x.x/24

Question 3


This is a new user (client) that has not been configured to accept SSL VPN connection. So that user must open a web browser, enter the URL and login successfully to be authenticated. A small software will also be downloaded and installed on the client computer for the first time. Next time the user can access file shares on that network normally.

Question 4


Fragmentation and Path Maximum Transmission Unit Discovery (PMTUD) is a standardized technique to determine the maximum transmission unit (MTU) size on the network path between two hosts, usually with the goal of avoiding IP fragmentation. PMTUD was originally intended for routers in IPv4. However, all modern operating systems use it on endpoints.

Note: IP fragmentation involves breaking a datagram into a number of pieces that can be reassembled later.

Question 5


Bandwidth-delay product (BDP) is the maximum amount of data “in-transit” at any point in time, between two endpoints. In other words, it is the amount of data “in flight” needed to saturate the link. You can think the link between two devices as a pipe. The cross section of the pipe represents the bandwidth and the length of the pipe represents the delay (the propagation delay due to the length of the pipe).

Therefore the Volume of the pipe = Bandwidth x Delay. The volume of the pipe is also the BDP.


Return to our question, the formula to calculate BDP is:

BDP (bits) = total available bandwidth (bits/sec) * round trip time (sec) = 64,000 * 3 = 192,000 bits

-> BDP (bytes) = 192,000 / 8 = 24,000 bytes

Therefore we need 24KB to fulfill this link.

For your information, BDP is very important in TCP communication as it optimizes the use of bandwidth on a link. As you know, a disadvantage of TCP is it has to wait for an acknowledgment from the receiver before sending another data. The waiting time may be very long and we may not utilize full bandwidth of the link for the transmission.


Based on BDP, the sending host can increase the number of data sent on a link (usually by increasing the window size). In other words, the sending host can fill the whole pipe with data and no bandwidth is wasted.Bandwidth-delay_Product_Optimized.jpg

Question 6

Question 7


Asymmetric routing is the scenario in which outing packet is through a path, returning packet is through another path. VRRP can cause asymmetric routing occur, for example:

R1 and R2 are the two routers in the local internal LAN network that are running VRRP. R1 is the master router and R2 is the backup router.

These two routers are connected to an ISP gateway router, by using BGP. This topology provides two possible outgoing and incoming paths for the traffic.

Suppose the outgoing traffic is sent through R1 but VRRP failover occurs, R2 becomes the new master router -> traffic passing through R2 instead -> asymmetric routing occurs.

Question 8

Question 9

  1. most specific
    August 22nd, 2017

    Qu 9 Reasons:

    Economic reasons[edit]
    When a site is multihomed, announcing non-local routes to a neighbour different from the one it was learned from amounts to advertising the willingness to serve for transit, which is undesirable unless suitable agreements are in place. Applying output filtering on these routes avoids this issue.

    Security reasons[edit]
    An ISP will typically perform input filtering on routes learned from a customer to restrict them to the addresses actually assigned to that customer. Doing so makes address hijacking more difficult.

    Similarly, an ISP will perform input filtering on routes learned from other ISPs to protect its customers from address hijacking.

    Technical reasons[edit]
    In some cases, routers have insufficient amounts of main memory to hold the full global BGP table. A simple work-around is to perform input filtering, thus limiting the local route database to a subset of the global table.[1] This can be done by filtering on prefix length (eliminating all routes for prefixes longer than a given value), on AS count, or on some combination of the two; security is the most important point for this.

    However, this practice is not recommended, as it can cause suboptimal routing[2] or even communication failures with small networks[citation needed], and frustrate the traffic-engineering efforts of one’s peers.

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