Huband Spoke

Often, customers do not want their sites to have full interconnectivity. This means they do not want or need the sites to be fully meshed. A typical scenario involves one main site at a company with many remote sites. The remote sites or spokes need connectivity to the main or hub site, but they do not need to communicate between them directly. Perhaps the connectivity is possible but not wanted for security reasons. This scenario is commonly referred to as the hub-and-spoke scenario. It can also be achieved across MPLS VPN, but care must be taken. The following is needed:

■ The spoke sites can communicate only with the hub site.

■ Spoke-to-spoke traffic needs to be sent to the hub site first.

To achieve this, adhere to the following needs:

■ Two different RTs

■ Different RDs

First, choose the RTs carefully. You need two different RTs: one is attached to the spoke routes when the spoke site sends them to the hub site, and one is attached to the hub routes when the hub site sends them to the spoke sites. At the same time, when a spoke site sends the spoke routes to the other spoke sites, the routes should be rejected because the VRF on the spoke sites does not import the RTs. However, the same PE router might use them for another site, or the automatic inbound filtering might be turned off if this is an ASBR router in the case of Inter-Autonomous MPLS VPN. (See the section on Inter-Autonomous MPLS VPN in the online supplement for how to turn off the automatic inbound filtering.) In those cases, the PE router accepts the route, but the VRF does not import the route because the RT is not configured for import. However, it might be selected as the best vpnv4 route, which means that the PE will not import the other vpnv4 route with the correct RT. Therefore, it is strongly advised that you use one unique RD per spoke site. You might get away with using the same RD for all spoke sites most of the time, but that does not work in all scenarios. You might have two spoke CE routers connected to the same PE router. In that case, the only thing that prevents the spoke routes from being sent directly to the other spoke CE router that is connected to the same PE router is the different RD for each spoke CE router.

Figure 7-29 shows an example of a hub-and-spoke network.

Figure 7-29 Hub-and-Spoke Scenario

Spoke-to-hub RT: 100:100 Hub-to-spoke RT: 100:101


Spoke-to-hub RT: 100:100 Hub-to-spoke RT: 100:101


Virtual Experimentation Process

The spoke PE routers advertise the routes with RT=100:100 while the hub PE router imports this RT. The hub PE router advertises the routes from the hub CE router with RT=100:101. The spoke PE routers import RT 100:101, which ensures that spoke-to-spoke traffic goes through the hub site.

SOO uniquely identifies the site that originates a route. It is a BGP extended community that prevents routing loops or suboptimal routing, specifically when a backdoor is present between VPN sites. SOO provides loop prevention in networks with dual-homed sites (sites that are connected to two or more PE routers). You can use it when an IGP is the PE-CE routing protocol. You can also use it when BGP is used between PE and CE, when the as-path loop prevention cannot be trusted anymore. This happens when BGP uses as-override or allowas-in. If the SOO is configured for a CE router and a vpnv4 route is learned with the same SOO, the route must not be put in the VRF routing table on the PE and advertised to the CE. In Figure 7-30, the vpnv4 prefix is advertised from PE-2 toward the other PE routers with SOO 1:100. This vpnv4 route can be advertised back to the same site and received on PE-3 via MP-BGP. When PE-3 notices the same SOO on the vpnv4 route as the SOO in the configuration, it does not install the prefix in the VRF routing table.

Figure 7-30 SOO Preventing Routing Loops

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