| 1:
| What was the original intent of MPLS? |
| A1:
| Answer: The original intent was to provide fast switching through an ATM network core by removing the Layer 3 lookup, using routing protocol control over the switched paths from the edge of the ATM cloud. |
| 2:
| What is the major use of MPLS today? Why has it changed? |
| A2:
| Answer: MPLS is used today primarily to tunnel customer traffic through a service provider network. The primary reasons it has changed are the new market demand for VPNs, which are being used in the place of dial and private line networks, and the increased switching speed of routers, especially through the use of ASICs. |
| 3:
| What are the three primary methods to route across an MPLS VPN through a service provider network? |
| A3:
| Answer: The three primary methods are overlay, peer-to-peer, and BGP/MPLS VPNs. Overlay overlays IGP routing on top of the MPLS VPN tunnels. Peer-to-peer redistributes routes into and out of the service provider BGP cloud. BGP/MPLS VPNs import and export routing information into and out of the service provider BGP cloud. |
| 4:
| What is the primary drawback of overlaying routing information on top of an MPLS VPN? |
| A4:
| Answer: Each site must create a set of MPLS tunnels, one to every other site it needs to communicate to, and then build an IGP adjacency across these tunnels. This method is difficult to deploy and maintain. |
| 5:
| What is the primary drawback in the peer-to-peer model of building a network on top of MPLS VPNs? |
| A5:
| Answer: Redistributing routes into and out of BGP at the service provider edge loses routing information and makes loops across backdoor links possible. |
| 6:
| How does a BGP/MPLS VPN carry IGP routing information through BGP? |
| A6:
| Answer: A BGP/MPLS VPN carries IGP routing information through BGP by attaching extended communities to the BGP routes. When a route is imported from IGP into BGP, the IGP routing information is placed in an extended community. When a route is exported, the IGP routing information is pulled from the extended communities and placed in the IGP routing table. |
| 7:
| How does EIGRP prevent backdoor routing loops when it is configured to run across a BGP/MPLS VPN? |
| A7:
| Answer: EIGRP prevents backdoor routing loops when it is configured to run across a BGP/MPLS VPN by attaching an SoO to the routes as they are imported from EIGRP into BGP. |
| 8:
| How does OSPF prevent routing loops through backdoor connections when it is running across a BGP/MPLS VPN? |
| A8:
| Answer: OSPF prevents routing loops by setting a down bit in routes when they are exported from BGP into OSPF at the provider edge. |
