IP Routing

Virtual Links

TraderMary is planning to establish a new office in Paris with an area ID of 2. The first router in area 2 will be called Paris2. A direct circuit needs to be established from NewYork2 (the ABR) to Paris2, since all OSPF areas must connect directly to the backbone (area 0). This international circuit has a long installation time. And, since a physical path is already available to area 2 via area 1, you may ask if OSPF provides some mechanism to activate area 2 before the NewYork2 → Paris2 circuit can be installed. The answer is yes. OSPF defines virtual links (VLs) which can extend the backbone area. Area 2 will directly attach to the backbone via the VL. A VL may be viewed as a point-to-point link belonging to area 0. The endpoints of a VL must be ABRs.

In our example in Figure 6-11, a virtual link may be defined from NewYork2 to Paris2 through area 1.

Figure 6-11. Virtual link to area 2

The syntax for configuring a virtual link is as follows:

area area-id virtual-link router-id [hello-interval seconds] [retransmit-interval  
seconds] [transmit-delay seconds] [dead-interval seconds] [[authentication-key key] | 
[message-digest-key keyid md5 key]]

where area-id specifies the transit area and router-id specifies the ABR with which the neighbor relationship is to be established. The four timers refer to the time between hello packets (default is 10 s), the time between LSA ...