Dell 6.2 Understanding OSPF v2 by Example using a Branch Office S cenario

O 10.1.1.0/24 [1/0] via 4.1.1.1
O 12.1.1.0/24 [1/0] via 4.1.1.1
C 4.1.1.0 is directly connected, VLAN4
Below is the routing table for Router 2:
(router2) #show ip route
Codes: C - connected, O - OSPF, R - RIP, S - static
M - mgmt, U - route usable, * - candidate default
O 10.1.1.0/24 [2/0] via 5.1.1.1
O 12.1.1.0/24 [2/0] via 5.1.1.1
C 5.1.1.0 is directly connected, VLAN5
Understanding OSPF v2 by Example using a Branch Office S cenario
The branchoffice scenario has a numberof remote branch offices with D ellcontrollers talking to a central office via
concentrator/controllerusing site-to-site VPN tunnels or master-localI Psec tunnels.T he centraloffice c ontrolleris i n
turnt alkingto upstreamrouters (see Figure 41). In this scenario the default route is normally pointed to the uplink
router;in many cases the ISP. Configure the area as stub so that inter-area routes are also advertised enablingthe
branchoffice co ntrollerto reach the corporate subnets.
Branch Office T opology
All the OSPFcontrol packets exchanged between the Branch office andt heCentral office Dell controllers undergo
GRE encapsulation beforeentering the IPsec t unnels.The Dell controllers in the branchoffices advertise all the user
subnet addressest o the Central office controlleras stub addresses in router LSA. The Centralo ffice controllerin t urn
forwardsthose routerLSAs to the upstream routers.
Figure 41: BranchOffice OSPF Topology
All the branchoffice Dell controllers, the Centraloffice co ntroller,and the upstream routersare part of a stub area.
Since the OSPFpackets follow GRE encapsulation over IPsec tunnels,the Central office controller can be a
controlleror any vendor’s VPN concentrator.R egardless,the controller in the branch office will interoperate with
other vendorss eamlessly.
In Figure 41, the branchoffice controller is configured usingVLA N 14 and VLAN 15. Layer 3 GRE tunneli s
configuredwit h IP address 20.1.1.1/24 and OSPF is enabled on the tunnel interface.
In the Central office controller,OSPF is enabled on VLAN i nterfaces4, 5 , and,the Layer 3 GRE tunnel interface
(configuredwit h IP address 20.1.1.2/24). OSPF interface cost on VLAN 4 is configured lower than VLAN 5.
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