Router
Name
Features
Responsibility
ASBR
Connect with the
routers outside the
OSPF AS by other
routing protocol
Maintain independent routing tables for different routing
protocols, import the routing information learned by other
routing protocol to OSPF domain through a certain
standard, and then establish a uniform routing table.
Table 10-2 Router Types
2.
Virtual Link
In practice, some physical restrictions might keep ABR of some areas from directly connecting
to the backbone area, which can be solved by configuring an OSPF virtual link. Virtual link
sketch is shown as below.
Figure 10-38 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.
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