VIRTUALRACK for Network Engineers: LAB: OSPF Path Selection with non-backbone Transit Areas

- Configure R4’s Frame Relay and point-to-point Serial links connecting to R5 to have an OSPF cost of 1.
- Modify the SPF calculation in the OSPF domain so that R6 cannot use R4 to reach area 0 by transiting area
- Verify this by ensuring that traffic from R6 going to the Loopback0 network of SW2 is still first sent to R1, and then directly to R5.

"capability transit" is enabled by default in cisco ios. this means none-backbone areas can be used for inter-area transit if a shorter path can be found through them.

there is a virtual-link between R6 and R1 to area 0. normally it would assume that R6 must route via the virtual-link path to R1 to reach the inter-Area destination advertised by area0. But in the case below R6 chooses R4 as a next-hop to reach 150.1.8.0/24 network.

to determine why R6 chooses the path via R4, we can verify how R6 is learning the inter-area LSA 3 route.

output below shows that R5 is the originating ABR for 150.1.8.0

below is to verify how R6 reaches R5, it shows that R5 is adjacent to R1 via frame-relay and R4 via point-to-point serial connection.

to verify what is the metric R1 uses to reach R5 via frame-relay. from the below output it uses metric of 3000.

to verify what is the metric R4 uses to reach R5 via frame-relay and point-to-point serial connection. both connections has a metric of 1.

R6 has reachability to R1 via the virtual-link with metric of 300, R6 has a reachability to R4 via the DR of VLAN 146 in area 1 with metric of 300.

so the total metric

R6 via R1 = 300(R6toR1) + 3000(R1toR5) + 6(R5to150.1.8.0/24network) = 3306
R6 via R4 = 300(R6toR4) + 1(R4toR5) + 6(R5to150.1.8.0/24network) = 307

Under normal conditions R6 should choose to route R1 since this is the route through area 0, since the capability transit is enabled by default, this can choose shorter path through area 1 to R4 so the result is R4 as a next-hop router are being installed in the routing table to reach 150.1.8.8

issuing the command at R6:

R6

router ospf 1

no capability transit

after issuing the command above, the resulting next-hop is R1 in the routing table.

but when R6 sends the packet to R1, R1 redirect the packets back to R4 based on the above traceroute output. to verify lets check R1. it shows that R1 has a metric of 3000 via the frame-relay interface to R5, while R4 has a metric of 300 to reach R5. so the path via R4 is preferred by R1.