Unicast Routing Protocols (RIP, OSPF, and BGP)

Transcription

1 CHAPTER 4 Unicast Routing Protocols (RIP, OSPF, and BGP) Exercises. RIP is an intradomain routing protocol that enables routers to update their routing tables within an autonomous system.. A RIP message is used by a router to request and receive routing information about an autonomous system or to periodically share its knowledge with its neighbors. 3. The expiration timer is 6 times that of the periodic timer to allow for some missed communication between routers. 4. The hop count limit helps RIP instability by limiting the number of times a message can be sent through the same router, thereby limiting the back and forth updating that may occur if part of a network goes down. 5. The two major shortcomings are two-node instability and three-node instability. For the former, infinity can be re-defined as a number such as 0. Another solution is the split horizon strategy or split horizon combined with poison reverse. These methods do not work for three-node instability. 6. The basis for classification of networks in OSPF is the number of routers connected to the network. 7. In distance vector routing each router sends all of its knowledge about an autonomous system to all of the routers on its neighboring networks at regular intervals. It uses a fairly simple algorithm to update the routing tables but results in a lot of unneeded network traffic. In link state routing a router floods an autonomous system with information about changes in a network only when changes occur. It uses less network resources than distance vector routing in that it sends less traffic over the network but it uses the much more complex Dijkstra Algorithm to calculate routing tables from the link state database.

2 SECTION 8. See Figure 4.. Figure 4. Exercise 8 Start More Destination? false true Increment hop count false Already in table? true false Nest hop the same or count smaller? true Add to table Replace old with new Stop 9. OSPF messages are propagated immediately because a router using OSPF will immediately flood the network with news of any changes to its neighborhood. RIP messages are distributed slowly because a network using RIP relies on the periodic updates that occur every 30 seconds to carry any news from one router to the next and to the next. This process may take a lot of time. 0. The general formula can be given as follows: Number of bytes in the message = 4 + (0 N) N is the number of advertised networks. A RIP message that advertises a single network (N = ) would be 4 bytes.. One periodic timer is needed.. 0 expiration timers are needed, one for each entry garbage collection timers are needed, one for each invalid route.

3 SECTION 3 4. We assume that router C is hop away. Then the modified table from C is: Table 4. Network Hops Net 3 Net Net3 4 Net4 8 Comparing this to the old table, we get the following: Table 4. Network Hops Net 3 C Net C Net3 F Net4 5 G 5. + (0 N) = Empty bytes in a message advertising N networks 6. See Figure 4.. Figure 4. Exercise 6 Com: Version Family: Family: Family: Family: net 4 net net 3 net See Figure 4.3.

4 SECTION 4 Figure 4.3 Exercise E B IP address for designated router of N Router address 5 Interface Number 8 8. See Figure 4.4. Figure 4.4 Exercise Age: 0 0 Fletcher's checksum Length: 60 E B IP address for designated router of N3 Router address Interface Number 8

5 SECTION 5 9. See Figure 4.5. Figure 4.5 Exercise Fletcher's checksum Length: 76 E B 3 IP address of router B Interface number Network address for N4 4 Network mask for N4 3 IP address of designated router for N3 Router address 5 0. See Figure 4.6. Figure 4.6 Exercise IP address of designated router for N 0 IP address of designated router (C) IP address of router C 8 Network mask for N IP address of router C. See Figure See Figure See Figure 4.9.

6 SECTION 6 Figure 4.7 Exercise 4 56 IP address of designated router for N4 0 IP address of designated router E 8 Network mask for N4 Figure 4.8 Exercise 4 56 IP address of designated router for N5 0 IP address of designated router (F) IP address of router F 8 Network mask for N5 IP address of router F Figure 4.9 Exercise Age: Network mask for N IP address of router B IP address of router C 4. See Figure 4.0.

7 SECTION 7 Figure 4.0 Exercise Network mask for N3 IP address of router F 5. See Figure 4.. Figure 4. Exercise A F N N N3 N C B D E N See Figure 4.. We assume that the router C has does not know any neighbor yet. Therefore, we did not include any neighbor IP addresses.. Figure 4. Exercise 6 From router C through Hello interval 0 0 Priority Dead interval Designated router for Backup designated router for From router C through Hello interval 0 Dead interval Priority

8 SECTION 8 7. See Figure 4.3. Figure 4.3 Exercise 7 Length: variable Message sequence number LSA header for each entry in the database 8. See Figure 4.4. We have shown the case that the router is looking for one LSA. It needs to repeated if the router needs more LSAs.. Figure 4.4 Exercise 8 This part can be repeated. 3 Length: variable See Figure 4.5. Figure 4.5 Exercise 9 N3 R5 N R N7 R4 N6 R6 N4 R R7 N R3 N8 R8 N5

9 SECTION See Figure 4.6. Figure 4.6 Exercise 30 N N R R R3 N7 N8 R4 R8 N3 N6 N5 R5 R7 R6 N4 3. Transient networks: N, N, N5, and N6. Stub networks: N3 and N4 3. See Figure 4.7. Figure 4.7 Exercise 3 Marker Option length Length Type: Version: 4 AS Hold time Options (variable length) 33. See Figure 4.8. Figure 4.8 Exercise 33 URL: Unfeasible Route Length Marker URL Path attributes length Network reachability length Length Type: Withdrawn routes (variable length) Path attributes (variable length) Netid of AS URL

10 SECTION See Figure 4.9. Figure 4.9 Exercise 34 Marker Length Type: See Figure 4.0. Figure 4.0 Exercise 35 Marker Error subcode Length Type: 4 Error code Error data (variable length)