BGPFree Core

When the IP network of a service provider must forward traffic, each router must look up the destination IP address of the packet. If the packets are sent to destinations that are external to the service provider network, those external IP prefixes must be present in the routing table of each router. BGP carries external prefixes, such as the customer prefixes or the Internet prefixes. This means that all routers in the service provider network must run BGP.

MPLS, however, enables the forwarding of packets based on a label lookup rather than a lookup of the IP addresses. MPLS enables a label to be associated with an egress router rather than with the destination IP address of the packet. The label is the information attached to the packet that tells every intermediate router to which egress edge router it must be forwarded. The core routers no longer need to have the information to forward the packets based on the destination IP address. Thus, the core routers in the service provider network no longer need to run BGP.

The router at the edge of the MPLS network still needs to look at the destination IP address of the packet and hence still needs to run BGP. Each BGP prefix on the ingress MPLS routers has a BGP next-hop IP address associated with it. This BGP next-hop IP address is an IP address of an egress MPLS router. The label that is associated with an IP packet is the label that is associated with this BGP next-hop IP address. Because every core router forwards a packet based on the attached MPLS label that is associated with the BGP next-hop IP address, each BGP next-hop IP address of an egress MPLS router must be known to all core routers. Any interior gateway routing protocol, such as OSPF or ISIS, can accomplish this task.

Figure 1-1 shows the MPLS network with BGP on the edge routers only.

Figure 1-1 BGP-Free MPLS Network

Edge MPLS Router

Edge MPLS Router

An Internet service provider (ISP) that has 200 routers in its core network needs to have BGP running on all 200 routers. If MPLS is implemented on the network, only the edge routers—which might be 50 or so routers—need to run BGP.

All routers in the core of the network are now forwarding labeled packets, without doing an IP lookup, so they are now relieved from the burden of running BGP. Because the full Internet routing table is well above 150,000 routes, not having to run BGP on all routers is a serious consideration. Routers without the full Internet routing table need a lot less memory. You can run the core routers without the complexity of having to run BGP on them.

Micro Expression Master

Micro Expression Master

If You Could Read Everyone Life A Book You Can Have Better Career, Great Relationships And Become Successful. This Book Is One Of The Most Valuable Resources In The World When It Comes To Reading the smallest and tiniest body Language and know what people are thinking about.

Get My Free Ebook


Post a comment