The proper functioning of a router relies on the collaborative effort of many different protocols. How can these protocols coexist without interfering with each other? This brings us to some key concepts behind how routing protocols operate.
1. VRF: One of the security measures for routing protocols is route isolation and information isolation, achieved through VPN Routing and Forwarding tables and LSPs in MPLS. On a PE router, multiple instances of the first routing protocol concept, VRF tables, exist. These VRF tables correspond to one or more sub-interfaces on the PE router and store routing information for the VPN to which those sub-interfaces belong.
A VRF table typically contains routing information for only one VPN. However, when a sub-interface belongs to multiple VPNs, its corresponding VRF table contains the routing information for all the VPNs that sub-interface belongs to. Every VRF table has two key attributes: a Route Distinguisher (RD) and a Route Target (RT).
2. RD: IP address planning within a VPN is determined independently by the customer. This can lead to customers choosing private addresses defined in RFC 1918 as their site addresses, or different VPNs using the same address space鈥攁 phenomenon known as address overlap. One consequence of address overlap is that BGP cannot distinguish overlapping routes from different VPNs, potentially making a site unreachable.
1) Besides using multiple VRF tables on the PE router, the routing protocol introduces the concept of the RD. An RD is globally unique. By using the 8-byte routing protocol concept two: RD as an extension of the IPv4 address prefix, non-unique IPv4 addresses are transformed into unique VPN-IPv4 addresses. VPN-IPv4 addresses are invisible to customer edge devices and are used only for routing information distribution across the backbone network.
2) A one-to-one correspondence is established between RDs and VRF tables. Typically, for sub-interfaces belonging to the same VPN on different PE routers, the same RD is assigned to their corresponding VRF tables. In other words, each VPN is assigned a unique RD. However, in overlapping VPN scenarios, where a site belongs to multiple VPNs, a sub-interface on the PE router belongs to those multiple VPNs. In this case, the corresponding VRF table can only be assigned one RD, meaning multiple VPNs share a single RD.
3. RT: The function of RT is similar to the BGP extended community attribute and is used for routing information distribution. It is divided into Import RT and Export RT, used for route import and export policies, respectively. When exporting VPN routes from a VRF table, the routes are tagged with the Export RT.
When importing VPN routes into a VRF table, only routes whose RT tag matches at least one Import RT in the VRF table are imported. RT ensures that a PE router only