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Versatile Routing and Services with BGP: Understanding and Implementing BGP in SR-OS by Colin Bookham, Alcatel-Lucent

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Chapter 2

BGP/MPLS IP-VPN

The framework for building BGP/Multi-Protocol Label Switching (BGP/MPLS) based IP Virtual Private Networks (IP-VPNs) relies on Multi-Protocol BGP (RFC 4760) and the optional-transitive BGP Extended Communities (RFC 4360) attribute “Route Target.”

Multi-Protocol BGP is used for advertising of VPN-IPv4/VPN-IPv6 prefixes, and, because both are labeled prefixes, they follow the encoding of labeled BGP (RFC 3107), where the prefix is constructed of an 8-byte Route-Distinguisher followed by a 4-byte IPv4 prefix or 16-byte IPv6 prefix. The purpose of the RD is to allow the concatenation of RD and IPv4/IPv6 prefixes to create a unique VPN-IPv4/VPN-IPv6 prefix.

For VPN-IPv4 the AFI is 1 (IPv4), and for VPN-IPv6 the AFI is 2 (IPv6). Both VPN-IPv4 and VPN-IPv6 use a SAFI of 128 (MPLS-labeled VPN address).

Figure 2-1 VPN-IPv4/IPv6 NLRI Encoding

image

When a route is redistributed into VPN-IPv4, a Route Target Extended Community is appended to the prefix. The Route Target Extended Community is a transitive attribute (RFC 4360) used to define the set of sites belonging to a given VPN. When a VPN-IPv4 prefix is received at a Provider Edge (PE) router, it parses the Route Target value and checks whether any locally configured VRFs have an import policy that matches that value. If it does, the route is imported into that VPRN. If it doesn't, the route is not imported into any ...

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