The EIGRP 6PE/6VPE Site of Origin (SoO) functionality allows an Enhanced Interior Gateway Routing Protocol (EIGRP) network to support complex topologies, such as Multiprotocol Label Switching-VPN (MPLS-VPN) links between sites with backdoor links, customer-edge (CE) devices that are dual-homed to different provider-edge (PE) devices, and PEs supporting CEs from different sites within the same VPN routing and forwarding (VRF) instance. Path selection within the EIGRP network containing PE-CE links is based on route metrics that allow either the link through the VPN or the EIGRP backdoor to act as the primary (best) link or the backup link, if the primary link fails. EIGRP accomplishes this path selection by retrieving the Site of Origin (SoO) attribute from routes redistributed from the Border Gateway Protocol (BGP) network. This BGP/EIGRP interaction takes place through the use of the BGP Cost Community Extended Community attribute.
When routes are redistribued into EIGRP from a BGP network, BGP Cost Community Extended Community attributes are added to the routes. These attributes include the SoO attribute. The SoO attribute is used to identify the site of origin of a route and prevent advertisement of the route back to the source site. To enable the EIGRP SoO functionality, you must configure the ip vrf sitemap command on the PE interface that is connected to the CE device. This command enables SoO filtering on the interface. When EIGRP on the PE device receives CE routes on the interface that has a SoO value defined, EIGRP checks each route to determine whether there is an SoO value associated with the route that matches the interface SoO value. If the SoO values match, the route will be filtered. This filtering is done to stop routing loops.
When EIGRP on the PE receives a route that does not contain an SoO value or contains an SoO value that does not match the interface SoO value, the route will be accepted into the topology table so that it can be redistributed into BGP. When the PE redistributes an EIGRP route that does not contain an SoO value into BGP, the SoO value that is defined on the interface used to reach the next hop (CE) is included in the Extended Communities attribute associated with the route. If the EIGRP topology table entry already has an SoO value associated with the route, this SoO value, instead of the interface SoO value, will be included with the route when it is redistributed into the BGP table. Any BGP peer that receives these prefixes will also receive the SoO value associated with each prefix, identifying the site, where each prefix originated.
The EIGRP SoO functionality ensures that BGP does not follow its normal path-selection behavior, where locally derived routes (such as native EIGRP routes redistributed into BGP) are preferred over BGP-derived routes.
For more information on the Site of Origin functionality, see the “EIGRP MPLS VPN PE-CE Site of Origin” chapter in the IP Routing: EIGRP Configuration Guide.