About VPC and VPC

The full name of the network Protocol is Enhance Interio Gateway Routing Protocol. It can be seen from the literal that it is an enhanced IGRP, that is, it is improved again by IGRP to become an ivernet) and Link-State) to accelerate convergence. The method used is DUALDiffusing Update Aigorithm). When the path is changed, DUAL transmits the changed part instead of the entire path table, both the Router and the Router store the neighboring path tables. When the path changes, the Router can quickly respond, and does not periodically send change messages to reduce bandwidth usage, in addition, it is worth noting that the network-layer protocols supported by the network-layer protocol are supported by the network-layer protocol, such as the IP-layer protocol, the IPX-layer protocol, and the RTMP-to-RTMP protocol of the AppleTalk protocol, for example:

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Integrated with the Integrated network. It provides three protocols: IP, AppleTalk, and IPX.

VPN is the most typical balanced Hybrid Routing Protocol. It integrates the advantages of the Distance Vector and link status routing selection protocol and uses the scattering update algorithm (DUAL ), high routing performance is achieved.

The following describes the features of the Protocol.

The router that runs the VPN gateway on the vroeigrp to form a neighbor relationship and exchange route information. The neighboring routers maintain the relationship between each other by sending and receiving Hello packets. The default sending interval of the Hello packet is 5s.

● The vroeigrp running the VPN gateway uses all route table information of its adjacent vrouters to quickly adapt to route changes;
● If no suitable route exists, the system queries the adjacent routers to find the routes that can be replaced.
● Use irregular updates, that is, only some update information is sent when the router changes the metering standard or the topology changes.
● Supports variable-length Subnet Mask (VLSM) and non-consecutive subnets, and allows you to set the automatic route summary function.
● Supports multiple network layer protocols. In addition to the IP protocol, it also supports IPX, AppleTalk, and other protocols.
● An adjacent router table, a topology table, and a route table are arranged within the router that runs the VPN gateway.
● The DUAL algorithm has good routing convergence characteristics.
● Routes are exchanged between the routers and the IGRP with the same autonomous system number.

The configuration of the VPN protocol is similar to that of the IGRP Protocol. However, because of its support for VLSM and many other features, it differs from IGRP in Advanced Configuration and viewing and monitoring commands, this should be noted in the experiment of configuring the MongoDB.

VPN Overview
The enhanced Internal Gateway Routing Protocol (hereinafter referred to as "VPN") is a distance vector routing protocol developed by Cisco and supports multiple network layer protocols such as IP and IPX. As TCP/IP is the most commonly used protocol in today's networks, this article only discusses the VPN protocol in the IP network environment.
It is a term created by Cisco, a balanced Hybrid Routing Protocol. It has both the characteristics of the traditional distance vector protocol: route information is advertised by neighboring routers and follows horizontal routing segmentation and reverse routing poisoning rules, routing is automatically summarized, easy to configure, and has the characteristics of the traditional link status Routing Protocol: there is no limit on the number of Route hops. When the route information changes, it is incrementally updated, retain the knowledge of all possible routing network topologies, and support variable-length subnet mask and manual route induction. The Protocol also has its own unique characteristics: it supports load balancing on non-cost routes, and uses the differential update algorithm DUAL) to quickly converge without a routing loop. Therefore, it is suitable for medium and large networks.
Concepts and terminologies
1. There are five types of data packets in the MongoDB:

HELLO: it is sent in multicast mode. It is used to discover the neighbor router and maintain the neighbor relationship.
Update): When the router receives the first HELLO packet from a neighboring router, an update packet containing the route information it knows is sent back in single-point transmission mode. When the route information changes, an update packet containing only the changed information is sent as a multicast packet. Note that the content of the two update packages is different.
Query): When a link fails and the Router performs route computing again, but there is no feasible route in the topology table, the router sends a query packet to its neighbor in multicast mode to check whether they have a feasible route to the destination.
Reply to reply): return the data packet to the query side in a single point to respond to the query data packet.
Confirm ACK): it is transmitted in a single point to confirm update, query, and reply data packets to ensure the reliability of update, query, and reply transmission.
2. feasible distance: Measure of the shortest route to a destination.
3. successor: The successor is a directly connected neighbor router with the shortest route to the destination. Forward the package to the destination through the successor router.
4. advertised distance from advertise distance): The Metric value of the shortest route advertised by the neighboring router to reach a specific destination.
5. feasible successor: a feasible successor is a neighbor router which can be used to reach the destination. If this router is not used, the measurement value of the route through which it reaches the destination is higher than that of other routers, however, its advertised distance is smaller than the feasible distance, so it is saved in the topology table and used as the alternative route.
6. feasible conditon: The four terms above constitute a feasible condition, which is the basis for the update of the route table and Topology table of the VPN Router. Feasible Conditions can effectively block routing loops and achieve fast convergence of routes.
7. active state: When the router loses the route to a destination and no successor is available, the route enters the active state, which is an unavailable route. When a route is active, the router sends a query to all neighbors to find another route to reach the destination.
8. passive state: When a router loses a route and then has a feasible route, or finds another route, the route enters the passive state, which is an available route.
Run the MongoDB instance.
The routers that initially run the VPN gateway on the network must discover the neighbors, understand the network, and select routes. In this process, three independent tables are created simultaneously: lists the neighboring tables of the adjacent routers, the topology that describes the network structure, and the route table. These three tables are updated when the network in the Operation changes.
1. Establish adjacent relationships
From the very beginning, the vroeigrp that runs the VPN gateway on the vroeigrp that uses the multicast address to send HELLO packets from the interfaces involved in the VPN gateway. When the router receives the first HELLO packet from a neighboring router, it returns an update packet through single-point transmission. After receiving confirmation from the other router about the update packet, the two sides establish a neighbor relationship.
2. Find the network topology and select the shortest route
When a router dynamically discovers a new neighbor, it also obtains the route information advertised by the new neighbor. The router first compares the route update information obtained with the information recorded in the topology table, A route that meets the feasible conditions is placed in the topology table, and then added to the route table through the route table of the successor router, if the route of the feasible successor router is within the configured range of non-equal cost routing load balancing, the route table is also added. Otherwise, the route table is saved in the topology table as the alternative route. If the router learns multiple routes to the same destination through different routing protocols, the Management Distance of the routes is compared, and the routes with the minimum management distance are the optimal routes.
3. Route query and update
When the route information does not change, the OSPF neighbor only sends a HELLO packet to maintain the neighbor relationship to reduce the usage of network bandwidth. When a neighbor is lost or a link is unavailable, the system immediately searches for a feasible successor Router from the topology table and enables the alternative route. If no successor router exists in the topology table, because it relies on its neighbor to provide route information, after the route is set to active, it sends query data packets to all neighbors.
If a neighbor has a route entry to the destination, it will reply to the query and no longer spread the query. Otherwise, it will further query each of its neighbors, only after all the queries have a response, the route is recalculated by the network and a new router is selected.
Verify the run of the MongoDB instance
In the network topology shown in, the vroeigrp performs the basic configuration of the VPN gateway. All vrouters belong to the Autonomous System 1 of the VPN gateway and no other routing protocols are configured, we use the information obtained by running the relevant commands of VPN to verify the running of the network.

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Let's take vror2 R2 as an example to verify how to understand the network and select a route.
For Destination 192.168.1.0 and 172.16.1.4, vror2 R2 receives the route advertised by its two neighboring routers R310.1.1.3) and r00000000.1.2 respectively. The shortest route to the destination 192.168.1.0 is through R3, and the feasible distance is 20563200, but the advertised distance of R4 is 281600) smaller than the feasible distance, which meets the feasible conditions. Therefore, R4 is the feasible successor router of the route. The shortest route to the destination 172.16.1.4 is through R3, the feasible distance is 20537600, And the advertised distance through R4 is 20537600) equal to note: not less than) the feasible distance, does not meet the feasible conditions, therefore, R4.
By default, it is the Server Load balancer on the same cost route. Therefore, the route to the destination 192.168.1.0 in the routing table is only through the router R310.1.1.3) save it in the topology table. Because it is a route learned through the internal MongoDB, the Management Distance of the route is 90. If non-cost load balancing is configured, the Standby route is also added to the route table.
At last, we need to emphasize that, because VPN is a proprietary route protocol of Cisco, all the content discussed in this article is based on the vro of Cisco.