code 128 font for excel Figure 26-34: Area 1 qualifies as an NSSA, but areas 2 and 3 do not in Objective-C

Drawer PDF 417 in Objective-C Figure 26-34: Area 1 qualifies as an NSSA, but areas 2 and 3 do not

Figure 26-34: Area 1 qualifies as an NSSA, but areas 2 and 3 do not
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Area 1 cannot be a stubby area because it has an ASBR in the area, but it has only one path to reach other external destinations (RIP), through the ABR In addition, IGRP doesn't need to know any of the networks associated with OSPF; a default route to Area 1 will work fine (because it has no other links to any other routing domain) So you don't need the full functionality of a transit network, in which all ASBR-generated routes are advertised to out internal routers; you just need to be able to redistribute the IGRP routes into OSPF If you make Area 1 an NSSA, routes from IGRP will be distributed to routers in Area 1 as a Type 7 LSA You can then choose to have the ABR either drop all Type 7 LSAs (not allowing other areas to learn of the IGRP routes), or convert the Type 7 LSAs into Type 5 LSAs for distribution to other areas (because only an NSSA can accept Type 7 LSAs) by telling the ASBR to set or clear the P bit in the LSA If the P bit is set to 1, the ABR automatically translates the Type 7 LSA into a Type 5 LSA for distribution to Area 0 Otherwise, the Type 7 LSA is dropped, and no other areas will learn of the routes through IGRP On the other hand, Areas 2 and 3 should really not be NSSAs Both areas have an ASBR for the RIP domain, meaning that RIP has more than one path to get to OSPF networks Now, RIP is pretty horrible with multiple paths in the first place but if you told RIP it had two default routes, it would make some pretty poor choices about which one to use at times (Remember, load balancing is not performed over default routes) So, you should really redistribute all OSPF routes back into RIP (except those that would cause RIP to loop), meaning that Type 5 LSAs from Area 1 should really be available to Areas 2 and 3 Virtual Links Virtual links are the bane of OSPF Don't ever use them See, wasn't that easy Okay, so that was a bit harsh Sometimes use of a temporary virtual link is acceptable; but in general, you should avoid them A virtual link is a pretend connection to the backbone Virtual links are used when there is absolutely no way for an area to be directly attached to the backbone, or when the backbone has been split due to a failure Figures 26-35 and 26-36 show both of these cases
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Figure 26-35: A virtual link used to establish connectivity to the backbone for a remote area In Figure 26-35, there's an area (Area 1) that cannot have a direct connection to the backbone To temporarily resolve this problem, you can add a virtual link between the ABR of Area 1 and the ABR of Area 2 Your virtual link acts like an unnumbered (a link without an IP address) point-to-point connection to Area 0, with the cost of the link equaling the cost of the path between the two ABRs Once you add the virtual link, the two ABRs establish an
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adjacency with each other and transfer Types 3, 4, and 5 LSAs just as if the ABR of Area 1 were connected to the backbone Figure 26-36 shows the other situation that may require a virtual link In this example, a link within the backbone fails, which would partition the backbone into two Area 0's that are unable to communicate with each other In this case, once the backbone was partitioned, Area 4 would not be able to communicate with any other areas besides Area 1 To resolve this problem, you can create a virtual link between the two ABRs for Area 1, which patches Area 0 back together again
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Figure 26-36: A virtual link established to patch a partitioned backbone One way or another, however, you should attempt to solve your problems without using virtual links Use them only temporarily because they add complexity to any network When trying to troubleshoot problems in an OSPF network that uses virtual links, you must troubleshoot each path between ABRs that host virtual links as well, which can become a nightmare Therefore, designers typically view virtual links in an OSPF network as an indicator of a poor routing design If you need to establish a virtual link, keep in mind that you must do so through a transit area That is, virtual links cannot be built through any type of stub area
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