Information Technology Reference
In-Depth Information
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Multiprotocol BGP and
MPLS VPN
Multiprotocol Label Switching (MPLS)
is a signaling and forwarding technology that uses
labels to make forwarding decisions. MPLS virtual private networks (VPNs) deliver private
network services over a shared MPLS infrastructure. BGP is extended to provide
multiprotocol support, which allows BGP to carry VPN-IPv4 reachability information. An
MPLS VPN built in such a manner is called a
Layer 3 VPN
.
This chapter discusses BGP multiprotocol support for MPLS VPN. The focus of this MPLS
VPN discussion is BGP-based Layer 3 VPN.
BGP Multiprotocol Extension for MPLS VPN
The capability of a BGP speaker to support multiprotocol extensions for MPLS VPN is
advertised to its peer during session setup. As discussed in Chapter 2, “Understanding BGP
Building Blocks,” the Capabilities Code 1 is for multiprotocol extension. Support for MPLS
VPN is indicated with an address family identifier (AFI) of 1 (IPv4) and a subsequent
address family identifier (SAFI) of 128.
The next sections describe the prefix format and attributes of VPN-IPv4.
Route Distinguisher and VPN-IPv4 Address
A VPN-IPv4 (VPNv4 for short) address has two components: an eight-octet Route Distin-
guisher (RD) and a four-octet IPv4 address. The purpose of an RD is to distinguish multiple
VPN routes that have an identical IPv4 prefix. The prepending of an RD to an IPv4 address
makes the same IPv4 address unique for different VPNs.
Although the format of an RD is a structured string consisting of a 2-byte Type field and a
6-byte Value field. As defined in RFC 2547bis, it has no mandatory semantics. In fact, when
BGP compares two RDs, it ignores the structure and compares the entire 8-byte values. One
common way of defining RD is to split it into two components: an AS number (4 bytes) and
an assigned number (2 bytes). An example of an RD is 65000:1001, where 65000 represents
an AS number and 1001 is a locally assigned number.
