In order to determine where a packet should be sent so that it can reach its desired destination, the router uses a static or dynamic routing protocol such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF), which both create and maintain a routing table.
If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.
Routers use the Border Gateway Protocol (BGP) to exchange reachability information with neighboring networks. BGP is a path-vector protocol, where routing decisions are made based on local policies. Inside a network, routers communicate using an Interior Gateway Protocol (IGP).
Packet-forwarding routers forward packets but do not run routing protocols. This type of router receives packets from one of its interfaces that is connected to a single network. These packets are then forwarded through another interface on the router to another local network.
Administrative distance is the feature that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols.
When a data packet comes in on one of the lines, the router reads the network address information in the packet header to determine the ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey.
With BGP on the other hand, the size of the routing table dictates the required device resources. Scale: BGP is more flexible and scalable than OSPF and it is also used on a larger network. Preferred path: OSPF is used to determine the fastest route while BGP puts emphasis on determining the best path.
The topmost criteria for BGP path selection is Administrative distance. So if a route is learnt over eBGP and iBGP at the same time, the routes learnt over eBGP take preference and make their way into the routing table, since it has a lower AD value of 20 as compared to the AD value of 200 for iBGP.
We prefer the path with the highest local preference. The default value is 100. To learn more, take a look at the BGP local preference attribute lesson.
The BGP nexthop attribute is the next hop IP address that is going to be used to reach a certain destination. For EBGP, the next hop is always the IP address of the neighbor specified in the neighbor command. For IBGP, the protocol states that the next hop advertised by EBGP should be carried into IBGP.
BGP Attributes list:
| Type Code value | Attribute Name | Attribute Type |
|---|
| 1 | ORIGIN | Well-known mandatory |
| 2 | AS_PATH | Well-known mandatory |
| 3 | NEXT_HOP | Well-known mandatory |
| 4 | MULTI_EXIT_DISC (MED) | Optional non-transitive |
The bgp bestpath as-path ignore command configures BGP to ignore the length of the autonomous system (AS) path when comparing routes. This behavior is disabled by default.
Basically if you have an eBGP route that is being introduced to an iBGP network, you must make sure that the next hop address is reachable in the routing table. If it is not, you would need to do something such as the configuring BGP next-hop-self to make sure that network gets added to the routing table.
BGP protocol: BGP (Border Gateway Protocol) is an Inter-AS routing protocol. The two most important attributes are AS-PATH and NEXT-HOP. The advertisement passed for the prefix values contains the AS's in the AS-PATH. The NEXT-HOP is the router interface that initiates the AS-PATH.
OSI Model Explained: The OSI 7 Layers
- Physical Layer.
- Data Link Layer.
- Network Layer.
- Transport Layer.
- Session Layer.
- Presentation Layer. The presentation layer prepares data for the application layer.
- Application Layer. The application layer is used by end-user software such as web browsers and email clients.
In the OSI reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
7 layers of the OSI reference modelThe layers 1 – 4 are the lower layers for the host, and 5 – 7 are the upper layers that contain application-level data are for networking. This seven-layer OSI model architecture indicates the protocols of its use to exchange data between users in a network.
Layer 3 of the OSI Model: Network Layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the transport layer (in contrast to the data link
1. Which is not a application layer protocol? Explanation: TCP is transport layer protocol. Explanation: For Application, Presentation and Session layers there is no data format for message.
1. What type of delivery uses data link layer addresses? If a device is sending frames to another device on the same local network, it uses ARP to determine the MAC address of the receiving device. The sending device then uses the Layer 2 addresses to send the frames.
