Reference Guide

664 | Open Shortest Path First (OSPFv2 and OSPFv3)
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Each router link is defined as one of four types: type 1, 2, 3, or 4. The LSA includes a link ID field that
identifies, by the network number and mask, the object this link connects to.
Depending on the type, the link ID has different meanings.
• 1: point-to-point connection to another router neighboring router
• 2: connection to a transit network IP address of Designated Router
• 3: connection to a stub network IP network/subnet number
• 4: virtual link neighboring router ID
Virtual Links
In the case in which an area cannot be directly connected to Area 0, you must configure a virtual link
between that area and Area 0. The two endpoints of a virtual link are ABRs, and the virtual link must be
configured in both routers. The common non-backbone area to which the two routers belong is called a
transit area. A virtual link specifies the transit area and the router ID of the other virtual endpoint (the other
ABR).
A Virtual Link cannot be configured through a Stub Area or NSSA.
Router Priority
Router priority is the method used by FTOS to “rate” the routers. Priority is a numbered rating 0-255. The
higher the number, the higher the priority. By default, the OSPF router with the highest priority is selected
as the DR. The OSPF router with the second highest priority is selected as the BDR.
For example, in Figure 35-3, Router 1 with priority 200 is the DR and Router 2 with priority 180 is the
BDR. If Router 1 fails, Router 2 would be elected DR and Router 4 would become the BDR.
Figure 35-3. Priority and Costs Example
Router 1
Priority 200
Cost 21
Router 2
Priority 180
Cost 50
Router 3
Priority 100
Cost 25
Router 4
Priority 150
Cost 20
Router 1 selected by the system as DR.
Router 2 selected by the system as BDR.
If R1 fails, the system subtracts 21 from R1s priority
number. R1s new priority is 179.
R2 as both the selected BDR and the now-highest
priority, becomes the DR.
If R3 fails, the system subtracts 50 from its priority.
R2s new priority is 130.
R4 is now the highest priority and becomes the DR.