When the CE-PE routing protocol is OSPF, we used down bit/domain tag to prevent possibility of routing loop.
In BGP, when we use a backdoor link in addition to MPLS network, there are possibilities of routing loop.
1) As we use “as-override” option in the PE-router, it replaces the customer AS with its own AS and distribute to customer-running BGP(site2). CE router which receives this route might again advertise to CE router(in site1) connected via back-door link, as it is unaware that those prefix are originated from site1.
2) Even when back door link was not used and when a CE router is connected to 2 different PE routers, SoO will help us to prevent the loop.
SoO is the extended community represented by As:nn or IP address:nn.
The concept is;
- configure a SoO for a customer site. And apply it to Customer eBGP neighbor. (can be done using neighbor statement-“neighbor x.x.x.x soo <>”/peer-policy template/peer-group)
- when this PE redistribute this BGP prefix, it adds the SoO with the vpnv4 prefix. Same Soo has to be configured