Connecting a Non Backbone Area Through a Stub Area

Area 0

Generic Routing Encapsulation (GRE) allows you to connect a discontiguous area to the backbone through a stub area

* GRE will cause extra packet overhead due to tunnel header information

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Cisco CCIE Prep v1.0—Mod

In this example, Area 1 has been configured as a stub area. This prevents the use of a virtual link, as virtual links are not allowed across stub areas. To provide Area 2 with connectivity to the backbone area, you could alternatively build a Generic Routing Encapsulation (GRE) tunnel between R6 and R1 and put the tunnel interfaces in area 0.

Table 6-19: GRE Tunnel Commands



interface tunnel number

Creates a virtual tunnel interface on the router.

tunnel source interface / ip address

Specifies the source of the point-to-point GRE tunnel. Can be specified by either the IP address or physical interface on the router.

tunnel destination ip address

Specifies the destination of the point-to-point GRE tunnel. This is the IP address the router on the other side of the tunnel.

The main differences between using a GRE tunnel or a virtual link to connect a discontiguous area to Area 0 are described in the following table:

Table 6-21: GRE Tunnel vs. Virtual Link

GRE Tunnel

Virtual Link

All traffic in the tunnel is encapsulated and decapsulated by the tunnel endpoints.

The routing updates are tunneled, but the data traffic is sent normally.

Tunnel headers in every packet cause overhead.

Data traffic is not subject to any tunnel overhead.

The tunnel can go through a stub area.

The transit area for a virtual link cannot be a stub area, because routers in the stub area will not have routes for external destinations.

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