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Monday, October 18, 2010

OSPF GRE instead of Virtual-link

As you all know each area in an OSPF AS should be connected to the backbone area - area 0.
Basically in order to do that we are using virtual-link to interconnect between areas but there is another way to do that using GRE.

Following Narbik Advanced CCIE R&S Workbook 2.0 here is OSPF LAB 6:

In order to connect area 3 (loopback interface on R1) to area 0, we need to create a link between R2 and R1.
We can use virtual link and configure the following on the routers:
R1(config-router)#area 1 virtual-link
R2(config-router)#area 1 virtual-link
Where and represent Router-ID for each router.

or we can configure GRE tunnel between both routers, take note that GRE does pass broadcast and multicast and often use with IPsec in order to provide this abilities which IPsec doesn't support.

So configuration goes as follows:
R1(config-router)#network area 0
R1(config)#interface tunnel 1
R1(config-if)#ip address
R1(config-if)#tunnel source
R1(config-if)#tunnel destination
R2(config-router)#network area 0
R2(config)#interface tunnel 1
R2(config-if)#ip address
R2(config-if)#tunnel source
R2(config-if)#tunnel destination

pay attention that we should advertise the tunnel ip in area 0 else OSPF won't advertise this link and wouldn't be able to exchange routing information.

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