OSPF capability transit

capability transit

If we want to get from router on the left to the router on the far right, the cost is only 100 through area 1; however, OSPF says we need to go through area 0 to go to another area UNLESS capability transit flag is ON.

On cisco routers it is ON by default, so we actually pick the lower cost path even though it is in area 1.

Step 1: create a virtual link to the first router in area 1 (see further below)

Step 2: issue show ip ospf topology-info

 

R4#show ip ospf topology-info

OSPF Router with ID (10.1.4.4) (Process ID 1)
Base Topology (MTID 0)

Topology priority is 64
Router is not originating router-LSAs with maximum metric
Number of areas transit capable is 2
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Area BACKBONE(0)
SPF algorithm last executed 00:00:26.936 ago
SPF algorithm executed 113 times
Area ranges are
Area 14
This area has transit capability: Virtual Link Endpoint
SPF algorithm last executed 00:09:24.535 ago
SPF algorithm executed 5 times
Area ranges are
Area 45
This area has transit capability: Virtual Link Endpoint
SPF algorithm last executed 00:09:24.535 ago
SPF algorithm executed 7 times
Area ranges are
Area 47
SPF algorithm last executed 00:09:24.535 ago
SPF algorithm executed 4 times
Area ranges are

Important: you can only route in this way if there is a shorter Inter-Area pathvia a non-area 0 router. So if the router in area 1 at the bottom suddenly gained an additional link in area 0, it would no longer be possible to route through area 1. ***Unless we create a virtual link from the router on the left to the bottom router.

Just having capability transit ON does NOT mean that if, for example, we will simply go through the path of lower cost (e.g. the path through area 0 = 50000 and path through a different area is 1000).

If we look at RFC, it says that we need to enable a virtual link to a router in this area! see https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/212607-ospf-virtual-links-transit-capability.html for details.

TransitCapability is set to TRUE
        if and only if there are one or more fully adjacent virtual
        links using the area as Transit area), and is used as an input
        to a subsequent step of the routing table build process (see
        Section 16.3). When an area's TransitCapability is set to TRUE,
        the area is said to be a "transit area"

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