| 1 | In addition to the RIP configurations shown here, a subnet of 192.168.5.0 must be configured between RTE and RTF, using secondary addresses. Otherwise, subnets 192.168.5.192/27 and 192.168.5.96/27 are discontiguous. The RIP configurations are RTA: router rip network 192.168.2.0
RTB: router rip network 192.168.2.0
RTC: router rip network 192.168.2.0 network 192.168.3.0
RTD: interface ethernet 0 ip address 192.168.4.3 255.255.255.0 ip address 192.168.5.3 255.255.255.224 secondary router rip network 192.168.3.0 network 192.168.4.0
RTE: interface ethernet 0 ip address 192.168.4.1 255.255.255.0 ip address 192.168.5.1 255.255.255.224 secondary router rip network 192.168.4.0 network 192.168.5.0
RTF: interface ethernet 0 ip address 192.168.4.2 255.255.255.0 ip address 192.168.5.2 255.255.255.224 secondary router rip network 192.168.4.0 network 192.168.6.0
| | 2 | To unicast RIP updates between RTC and RTD, the configurations are RTC: router rip network 192.168.2.0 neighbor 192.168.3.2
RTD: router rip network 192.168.3.0 neighbor 192.168.3.1
| | 3 | The update time applies to the entire RIP process. If the update time is changed for the serial link, it will also be changed for the router's other links. That, in turn, means that the timers must be changed on the neighboring routers, which means those neighbors' neighbors must be changed, and so on. The cascade effect of changing the update timer on a single router means that the timers must be changed on every router in the RIP domain. The command to increase the RIP update period to two minutes, configured on every router, is timers basic 120 360 360 480
Because the update timer is changed, the invalid, holddown, and flush timers must also be changed. Setting the invalid and holddown timers to six times the update period, as the default timers are, would make the conversion time of the network extremely high. Therefore, the invalid and holddown timers are set to three times the update period. The flush timer must be longer than the holddown timer, so it is set to 60 seconds longer. | | 4 | Network 192.168.4.0 is two hops from RTA, so adding 14 to the metric will give the route an unreachable metric of 16. Remember that in Configuration Exercise 1 a subnet of 192.168.5.0 had to be configured on the same link as 192.168.4.0 using secondary addresses, so that the subnets of 192.168.5.0 are contiguous. Therefore, 192.168.5.0 is also two hops from RTB. Assuming the interfaces of RTA and RTB connected to RTC are E0 on both routers, the configurations are RTA: router rip offset-list 1 in 14 Ethernet0 network 192.168.2.0 ! access-list 1 permit 192.168.4.0 0.0.0.0
RTB: router rip offset-list 1 in 14 Ethernet0 network 192.168.2.0 ! access-list 1 permit 192.168.5.0 0.0.0.0
| | 5 | RTB, with its longer mask, can interpret all subnets correctly. The problem is at RTA. With a 27-bit mask, RTA interprets RTB's subnets 192.168.20.40/29 and 192.168.20.49/29 as 192.168.20.32/27the same subnet as its directly connected link. Therefore, RTA does not have a correct "view" of the network. However, packets can still be routed if Proxy ARP is enabled. For example, suppose RTA has a packet to forward with a destination address of 192.168.20.50. RTA incorrectly interprets this address as a member of its subnet 192.168.20.32/27, and ARPs for the MAC identifier of 192.168.20.50 on that subnet. RTB hears the ARP; it correctly interprets 192.168.20.50 as being a member of its subnet 192.168.20.48/29 and responds with the MAC identifier of its interface on 192.168.20.32/29. RTA then forwards the packet to RTB, and RTB forwards the packet to the correct destination. If Proxy ARP is disabled, packets will not be delivered correctly from RTA to RTB. |
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