TP-Link T2700G-28TQ T2700G-28TQ User Guide V1 - Page 196
Stub Area and NSSA Area, Virtual Link Sketch
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Figure 10-36 Virtual Link Sketch As in Figure 10-36, ABR of Area 2 has no physical link to connect directly with the backbone area, in which case Area 2 could not communicate with others without configuring a virtual link. Then a virtual link between ABR1 and ABR2, passing through Area 1, could provide a logical link for Area 2 to connect with the backbone area. A virtual link is a point-to-point connection between two ABRs. Hence, simply configuring the virtual link parameters on two ordinary router interfaces makes two ends of the virtual link. Two ABR directly forward the OSPF packets to each other's interface IP address, while the OSPF routers between them transmit these packets as regular IP packets. In general, configuring a virtual link is a temporary means to fix the problems of network topology, which usually would to certain degree complicate the network. Therefore, when networking in reality, a virtual link should be avoided if possible. 3. Route Types OSPF prioritize routes into four levels: 1) Intra-area route 2) Inter-area route 3) Type-1 external route: It has high credibility and its cost is comparable with the cost of an OSPF internal route. The cost from a router to the destination of the Type-1 external route equals to the cost from the router to the corresponding ASBR plus that from the ASBR to the destination of the external route. 4) Type-2 external route: It has low credibility, so OSPF considers the cost from the ASBR to the destination of the Type-2 external route is much bigger than the cost from the ASBR to an OSPF internal router. Therefore, the cost from the internal router to the destination of the Type-2 external route equals to that from the ASBR to the destination of the Type-2 external route. If two routes to the same destination have the same cost, then take the cost from the router to the ASBR into consideration. Intra-area route and inter-area route describe the internal network structure of the autonomous system, while the external routes tell how to select the route to the destination outside the autonomous system. 4. Stub Area and NSSA Area An area that can connect to the autonomous system and forward the communication data to external areas only through ABR could be set as Stub Area. Once an area is set to be Stub Area, ABR would no longer flood the external routing information described by the AS-External LSA to it, and meanwhile a default route with a target network 0.0.0.0 would be generated. This default routing would be announced to the other routers in the area. All the packets forwarded to external areas would be sent to ABR and then be forwarded outwards through it. Since there is no need to 184