The routing protocols can coexist without mutual influence.

The reason why routers can run normally is mainly the result of the joint efforts of many different protocols. Why can protocols coexist without affecting each other? This is what we will talk about and some concepts about the operation of the routing protocol.

1. VRF: One of the security measures of the routing protocol is routing isolation and information isolation. It is implemented through VPN route forwarding table and LSP in MPLS. On a PE router, there are multiple routing protocols. Concept 1: VRF table. These VRF tables correspond to one or more sub-interfaces on the PE router, used to store the route information of the VPN to which these subinterfaces belong.

The VRF table only contains the route information of one VPN. However, when the sub-interface belongs to multiple VPNs, the corresponding VRF table contains the route information of all the VPNs to which the sub-interface belongs. Each VRF table has two attributes: RouteDistinguisher: RD and RouteTarget: RT.

2. RD: the IP address planning in VPN is developed by the customer, therefore, the customer may choose the private address defined in RFC1918 as their site address or use the same address domain for different VPNs, that is, address overlap. One of the consequences of address overlapping is that BGP cannot distinguish overlapping routes from different VPNs, resulting in a site being inaccessible.

1. In addition to using multiple VRF tables on a PE router, the routing protocol also introduces the concept of RD. RD is globally unique. By extending 8-byte routing protocol concept 2: RD as an IPv4 address prefix, it converts an ununique IPv4 address to a unique VPN-IPv4 address. The VPN-IPv4 address is invisible to client devices and is only used for distribution of routing information on backbone networks.

2. There is a one-to-one correspondence between RD and VRF tables. Generally, for sub-interfaces of the same VPN on different PE routers, allocate the same RD to the corresponding VRF table. In other words, is to assign a unique RD for each VPN. However, for overlapping VPNs, that is, when a site belongs to multiple VPNs, because a sub-interface on the PE router belongs to multiple VPNs, the VRF table corresponding to this sub-interface can only be allocated with one RD, so that multiple VPNs share one RD.

Iii. RT: RT is used to expand group attributes in BGP for routing information distribution. It is divided into ImportRT and ExportRT for the import and export policies of route information respectively. When exporting a VPN route from the VRF table, use ExportRT to mark the VPN route.

When you import a VPN route to the VRF table, only the route marked with RT that matches any ImportRT in the vrf table will be imported to the VRF table. RT allows the PE router to only include the VPN route directly connected to it, instead of all the VPN routes of the whole network, thus saving the resources of the PE router and improving the network scalability.

The routing protocol supports the routing protocol by sharing routing information between routers. Route information is transmitted between adjacent routers to ensure that all routers know the path to other routers.

In short, the routing protocol creates a route table and describes the network topology. The routing protocol works with the router to implement routing selection and packet forwarding.