Question

Test Your Understanding 10. a) Distinguish between Step 1 and Step 2 in the routing process. b) If any row other than the default row matches an IPv4 address, why will the router never choose the default row? c) Which rows in Figure 8-11 match 128.171.17.13? (Don't forgst the default row.) Show your calculations for rows that match. d) Which of these is the best-match row? Justify your answer. e) What rows match 172.40.17.6? Show your calculations for rows that match. f) Which of these is the best-match row? Justify your answer. g) Which rows match 172.30.12.47? Show your calculations for rows that match. h) Which of these is the best-match row? Justify your answert 5) How would your previous answer change if the metric had been reliability? I Packet arrives with destination IP address 128.171.17.13 Decide how to send the packet back on Select the best-match row Find all row matches Metric Interface Cost) Ne Mask (/Prefix) Row Destination Router Network or Subnet 128.171.0.0 172.30.33.0 60.168.6.0 123.0.0.0 47 255.255.0.0 (/16) 255.255.255.0 (24) 255.255.255.0 (24) 255.0.0.0 (/8) 255.255.255.0 (24) 255.255.255.0 (/24) 255.255.255.0 (/24) 255.255.255.0 /24) 255.255.255.0 (/24) 255.255.255.0 24) 255.255.255.0 /24) 255.255.0.0 /16) 0.0.0.0 V0) 12 34 5 172.29.8.0 47 172.40.6.0 128.171.17.0 172.29.8.0 172.12.6.0 172.30.12.0 172.30.12.0 60.168.0.0 0.0.0.0 20 23 10 16 12 13 Routing Table FIGURE 8-11

Answer 1

A. The 1st step in routing process consists of finding the row matching the destination IP address in the routing table which is done by masking the IP address of arriving packet with the mask value in the corresponding row of the table.Then, the result of mask operation is matched with the value in the destination column of that row in the routing table.

The 2nd step in routing process involves selecting the best-match row which is the process by which the router decides the best route on which the packet should be forwarded among all the row-matches.It is decided on the basis of longest prefix match rule(It involves finding the entry which has the longest prefix matching with the incoming packet's destination IP address).If there are multiple prefix matches, then the metric column rule to find the lowest cost metric is used to find the best route to send the packet through.

B. As the name suggests a default row is used when there are no other specific choices.So, if any row other than the default row matches an IP address, the router never chooses the default row since it is made for the purpose of ensuring that at least one row will match the destination IP address of every arriving packet.As long as there is an alternative to the default row, it is never used.And also, the default row will always have the shortest length match which is another reason for not selecting this row, since the row with the longest length match is selected as the best-match row.

C. There are three rows matching the incoming packet with the IP address 128.171.17.13.The 1st one is the 1st row with the destination IP address as 128.171.0.0 and mask value 255.255.0.0.The 2nd row match is with the 7th row having 128.171.17.0 and 255.255.255.0 as the destination IP and prefix value respectively.The 3rd and final row-match is with the default row which is the 13th row and as we know, the default row matches with every incoming packet.

D. The 2nd step is to find the best-match row.The best-match row among all the matches in the given case is row 7 since it has the longest match with the incoming packet's destination IP address.Row 1 has a mask of 255.255.0.0, which shows that it has a 16-bit match.7th row has the prefix of /24,which indicates that it has a 24-bit match.Row 13 has a prefix of 0/ since it is the default row and we know that the default row has the shortest length match and it is not selected as the best-match row whenever there is another row match.As we can see here, the 7th row has the longest length of match, so the router selects this as the best-match row.In case of multiple matches, The best- match row is found based on the lowest cost metric which is not needed in this case.