Exam 300-320 topic 1 question 223 discussion

It is typically undesirable for traffic from a distribution device to use a remote device as a transit path. A typical connection from a distribution device to a remote device would have much less bandwidth than a connection at the network core. Attempting to use a remote device with a limited bandwidth connection as a transit path would generally produce excessive congestion at the remote device. The EIGRP stub routing feature can prevent this problem by preventing the remote device from advertising core routes back to the distribution devices. In the above example, routes learned by the remote device from distribution Device 1 will not be advertised to distribution Device 2. Therefore, distribution Device 2 will not use the remote device as a transit for traffic destined to the network core.

i dont know about anybody else but i sill stare mesmorized at all these answers wanting to choose all .... C sounds better but actually unable to work this one out

C is correct. Page 66 CCDP ARCH Foundation Guide. Following figure 2-14 in the book and below output, routes learned from spoke where stub configured are not advertised through the links (transit links) to other spokes from hub perspective. With the EIGRP stub routing feature, the routers (typically the spokes) configured as a stub will send a special peer information packet to all neighboring devices to report its status as a stub router. In turn, any EIGRP neighbor that receives a packet informing it of the stub status will not query the stub device for any routes (see Figure 2-14), and a router that has a stub peer will not query that peer. Therefore, the stub device will depend on the hub router(s) to send proper updates to all other spokes. Also, when an EIGRP stub is configured at the spokes routers, they will not be used as transit routers by the other spokes or hub routers.

D is correct

@adamconda is right - Answer is D Transit routes are filtered from stub networks to the network hub Default configuration of EIGRP stub only advertise directly connected and summery routes but it does not advertise learned routes. So if a stud router is being used as a transit path - then traffic will drop example from the ARCH book Figure 2-16 EIGRP Over Dual Hubs and Spokes with a Backdoor Link Router RTR-X receives the 10.1.1.0/24 route from router RTR-Y, but it does not adver-tise it to router RTR-A because stub routers do not advertise learned routes. Network 10.1.1.0/24 is now not reachable from the hub, even though one of the redundant connec-tions between the hub and the spoke is alive. A similar problem occurs in the opposite direction. Router RTR-X is a stub, so it does not advertise the summary route of the core network (10.0.0.0/16) to router RTR-Y. As a result, router RTR-Y has no connectivity with the other hub; therefore, RTR-Y will be isolated, and it can only reach RTR-X subnets.

I think A is the only correct Answer here. In ARCH 300-320 book we have: Because the hub is the only point through which spokes can reach the other networks, advertise only the default route from the hub to the spokes. As discussed earlier in this chapter, it is recommended that you summarize spoke net- works on the hub toward the core; this way, you minimize the number of routes and create a logical layer (boundary) to limit EIGRP queries propagation from the remote sites to the core to achieve more stable routing design. In addition, when building topology with point-to-point links, consider using /31 subnets for the links to conserve address space when possible. Address the links out of the address space that is available on the spoke to allow for simple summarization. If this action is not possible, consider using distribute- lists to filter link subnets from being advertised back into the core of the network.

Answer D for me: With EIGRP, it can be desirable to configure EIGRP stub networks. This informs central routers that they should not use a remote site as a transit network From: http://www.ciscopress.com/articles/article.asp?p=1763921&seqNum=3

Answer C states "It prevents the hub router from advertising networks learned from the spokes" but it is actually the opposite. Stub routing prevents the spoke to advertise routes learned from one hub to another hub. Without stub the spoke would be deamed as a transit device. But I have to say none of the other answers satisfy either...

I am sure C is correct as per https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_eigrp/configuration/15-mt/ire-15-mt-book/ire-eigrp-stub-rtg.html

I think it's B