Question

6. Consider the network below in which network W is a customer of ISP A, network Y is a custo mer of ISP B, and network X is a customer of both ISPs A and C. a. What BGP routes will B advertise to Y? b. What routes will B advertise to C? c. What routes will C advertise to X? d. What routes will X advertise to C? てB 7. What field in the IP header can be used to ensure that a packet is forwarded through no more than N routers?

Answer 1

Note: Done accordingly. Please comment for any problem. Please Uprate. Thanks.

a) B to Y

B will advertise that it can reach w and x, since y needs to know which networks its provider can reach. It may also advertise that it can reach A and C. However if A and C are only transit networks (that is, only providing service to/from their customer's networks), then B would not have to advertise A and C to y.

b) B to C

B will advertise that it can reach y. Note that since A and C is a peer network, B will only advertise its customers to C.

C will advertise that it can reach w and y, since x needs to know which networks its provider can reach. It may also advertise that it can reach A and B. However, if A and B are only transit networks (that is, only providing service to/from their customers networks), then C would not have to advertise A and B to x.

d) X to C

X will not advertise any routes to C, since otherwise C might try to route through x, and x is a customer network, not a transit network.

2)

TTL

An IP TTL is set initially by the system sending the packet. It can be set to any value between 1 and 255; different operating systems set different defaults. Each router that receives the packet subtracts at least 1 from the count; if the count remains greater than 0, the router forwards the packet, otherwise it discards it and sends an Internet Control Message Protocol (ICMP) message back to the originating host, which may trigger a resend.