Routers multiple protocols coexist without affecting each other

The reason why routers can run normally is the result of a lot of different protocols working together, why can the protocols coexist without affecting each other? This is what the following is about, the concepts of routing protocols running.

VRF: One of the security initiatives of routing protocols is routing isolation and information isolation, which is implemented through VPN routing forwarding and LSP in MPLS. On the PE router, there are multiple routing protocol concepts: VRF tables, which correspond to one or more of the child interfaces on the PE router, and are used to store the routing information of the VPN to which these sub-interfaces belong.

The VRF table contains only one VPN routing information, but when a sub-interface belongs to more than one VPN, its corresponding VRF table contains the routing information for all VPNs to which the sub-interface belongs. For each VRF table, there are two main properties of the routing specifier (ROUTEDISTINGUISHER:RD) and the routing target (ROUTETARGET:RT).

Second, the Rd:vpn IP address planning is made by the customer, it is possible that customers choose the private address defined in RFC1918 as their site address or different VPN use the same address domain, that is, the so-called address overlap phenomenon. One of the consequences of overlapping addresses is that BGP cannot differentiate between overlapping routes from different VPNs, causing a site to be unreachable.

1. In addition to using multiple VRF tables on the PE router, the routing protocol also introduces the concept of Rd. RD is globally unique by translating the 8-byte routing protocol concept two: Rd as an extension of the IPv4 address prefix to convert the IPV4 address to a unique Vpn-ipv4 address. The Vpn-ipv4 address is not visible to client devices and is used only for distribution of information on backbone network routing.

2, RD and VRF table established a one by one corresponding relationship. Typically, for a sub-interface belonging to the same VPN on a different PE router, assign the same rd to its corresponding VRF table, in other words, assign a unique rd to each VPN. However, for overlapping VPNs, where a site belongs to multiple VPNs, because a sub-interface on the PE router belongs to more than one VPN, the VRF table corresponding to that sub-interface can only be assigned an RD so that multiple VPNs share an RD.

The role of RT:RT is similar to the extended Community attribute in BGP for the distribution of routing information. It is divided into importrt and EXPORTRT, which are used for the import and export of routing information respectively. When you export a VPN route from the VRF table, you use EXPORTRT to mark the VPN route.

When you import VPN routes into the VRF table, only routes with the RT tag that match any of the IMPORTRT in the VRF table are imported into the VRF table. RT makes the PE router only contain the routing of VPN which is directly connected with it, instead of all VPN routing, thus saving the resources of PE router and improving the network expansion.

Routing protocols support routable protocols by sharing routing information between routers. Routing information is passed between neighboring routers, ensuring that all routers know the path to other routers.

In short, the routing protocol creates the routing table, describes the network topology, the routing protocol works with the router, performs the Routing and packet forwarding function.