Virtual Links Example 1 - D-Link NetDefend DFL-210 User Manual

4.5.2. OSPF
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Init
2-Way
ExStart
Exchange
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Aggregates
OSPF Aggregation is used to combine groups of routes with common addresses into a single entry
in the routing table. This is commonly used to minimize the routing table.
Virtual Links
Virtual links are used for:
• Linking an area that does not have a direct connection to the backbone.
• Linking the backbone in case of a partitioned backbone.
Areas without direct connection to the backbone
The backbone always need to be the center of all other areas. In some rare case where it is
impossible to have an area physically connected to the backbone, a virtual link is used. This virtual
link will provide that area with a logical path to the backbone area. This virtual link is established
between two ABRs that are on one common area, with one of the ABRs connected to the backbone
area. In the example below two routers are connected to the same area (Area 1) but just one of them,
fw1, is connected physically to the backbone area.
Figure 4.6. Virtual Links Example 1
This is the initial state of the neighbor relationship.
When a HELLO packet is received from a neighbor, but does NOT include the Router
ID of the firewall in it, the neighbor will be placed in Init state. As soon as the
neighbor in question receives a HELLO packet it will know the sending routers
Router ID and will send a HELLO packet with that included. The state of the
neighbors will change to 2-way state.
In this state the communication between the router and the neighbor is bi-directional.
On Point-to-Point and Point-to-Multipoint interfaces, the state will be changed to
Full. On Broadcast interfaces, only the DR/BDR will advance to Fullstate with their
neighbors, all the remaining neighbors will remain in the 2-Way state.
Preparing to build adjacency.
Routers are exchanging Data Descriptors.
Routers are exchanging LSAs.
This is the normal state of an adjacency between a router and the DR/BDR.
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Chapter 4. Routing