Tag - router

What is the difference between a router and hub or switch?

Question: What is the difference between a router and hub or switch?
Answer: A¬†router is a more sophisticated network device than either a¬†switch or a¬†hub. Like hubs and switches, network routers are typically small, box-like pieces of equipment that multiple computers can connect. Each features a number of “ports” the front or back that provide the connection points for these computers, a connection for electric power, and a number of LED lights to display device status. While routers, hubs and switches all share similiar physical appearance, routers differ substantially in their inner workings.Traditional routers are designed to join multiple area networks (LANs and¬†WANs). On the Internet or on a large corporate network, for example, routers serve as intermediate destinations for network traffic. These routers receive¬†TCP/IP packets, look inside each packet to identify the source and target¬†IP addresses, then forward these packets as needed to ensure the data reaches its final destination.Routers for home networks (often calledbroadband routers) also can join multiple networks. These routers are designed specifically to join the home (LAN) to the Internet (WAN) for the purpose of Internet connection sharing. In contrast, neither hubs nor switches are capable of joining multiple networks or sharing an Internet connection. A home network with only hubs and switches must designate one computer as the gateway to the Internet, and that device must possess two¬†network adapters for sharing, one for the home LAN and one for the Internet WAN. With a router, all home computers connect to the router equally, and it performs the equivalent gateway functions.

Router Switch Hub

Additionally, broadband routers contain several features beyond those of traditional routers. Broadband routers provide DHCP server and proxy support, for example. Most of these routers also offer integrated firewalls. Finally, wired Ethernet broadband routers typically incorporate a built-in Ethernet switch. These routers allow several hubs or switches to be connected to them, as a means to expand the local network to accomodate more Ethernet devices.

In home networking, hubs and switches technically exist only for wired networks. Wi-Fiwireless routers incorporate a built-in access point that is roughly equivalent to a wired switch.

The Basics of BGP Route Reflection

Configuring Border Gateway Protocol (BGP) can be quite onerous, particularly with large numbers of peering sessions that must be configured manually. In fact, in a large network, the full-mesh requirement for IBGP can be a provisioning nightmare.

BGP’s answer to the IBGP pairing configuration nightmare that is the full mesh is called route reflection. Route reflection allows sharing of routing information among a group of routers without having to send the exact same information to each of them individually. It’s sort of like giving information to one person and having them distribute it to all their peers.

IBGP comes with a significant restriction: IBGP peers should not re-advertise IBGP-learned routes to other IBGP speakers, which is why they all need to be fully meshed. If you can’t re-advertise IBGP routes, you must be directly connected to the originator of the route, hence the full mesh requirement. Remember, IBGP has no dedicated loop prevention mechanism, and this is why you need route reflectors for large networks.

The concept of route reflection allows you to designate one or more of your routers as route reflectors. BGP relaxes the re-advertising restriction on these route reflectors, allowing them to accept and propagate IBGP routes to their clients.




Because of the IBGP full-mesh requirement, this topology would require 15 IBGP peering sessions per router, or 120 distinct IBGP sessions within the network. However, if you designate router 4 as a route reflector, you can start to minimize this requirement. For example, look at what happens in with the routers directly connected to router 4.




In this part of the topology, router 4 has three directly-connected routers. If just this part of the topology is running IBGP, you have to configure a full mesh between the 4 routers. However, if you designate router 4 as a route reflector, BGP only requires that every route reflector client have an IBGP connection to the route reflector (not to each other).




With the new configuration, the IBGP routes from routers 1, 2, and 3 are sent to the route reflector. Router 4, acting as the route reflector, re-advertises these routes to all of its clients.

In this way, router 1 and router 2 are connected via IBGP, through their shared route reflector, router 4. This group of routers is called a cluster, and each cluster is uniquely identified by its cluster ID (a 32-bit number similar to an IP address).

Looking back at the original 16-router network, if you make similar route reflectors with routers 8, 12, and 16, you can create four route reflectors and reduce the number of IBGP sessions.



The 16-router fully meshed route reflector network

However, all 16 routers are still in the same AS, which means that IBGP has to fully connect all 16 routers. How do you do this?

Ultimately, you must have connectivity somewhere. That connectivity occurs at the route reflector level. The route reflectors must be fully meshed, meaning that you must have IBGP peering sessions between each of the four route reflectors.

Essentially, you have drastically reduced the number of IBGP sessions in your network. Where you previously needed 120 sessions to fully mesh your network, you now need only three sessions from each route reflector to its clients and an additional six sessions to fully mesh the route reflectors (for a total of 18 IBGP sessions).

what BGP Attributes

BGP Attributes

  1. AS path
  2. Origin
  3. Local Preference
  4. Multi-Exit Discriminator (MED)
  5. Weight


AS path: An ordered list of all the autonomous systems through which this update has passed. Well-known, mandatory.

Origin: How BGP learned of this network. i = by network command, e = from EGP, ? = redistributed from other source. Well-known, mandatory.

Local Preference: A value telling IBGP peers which path to select for traffic leaving the AS. Default value is 100. Well-known, discretionary.

Multi-Exit Discriminator (MED): Suggests to a neighboring autonomous system which of multiple paths to select for traffic bound into your autonomous system. Lowest MED is preferred. Optional, non-transitive.

Weight: Cisco proprietary, to tell a router which of multiple local paths to select for traffic leaving the AS. Highest weight is preferred. Only has local significance.

IGPs, such as EIGRP or OSPF, choose routes based on lowest metric. They attempt to find the shortest, fastest way to get traffic to its destination. BGP, however, has a different way of route selection. It assigns various attributes to each path; these attributes can be administratively manipulated to control the path that is selected. It then examines the value of these attributes in an ordered fashion until it can narrow all the possible routes down to one path.

BGP chooses a route to a network based on the attributes of its path. Four categories of attributes exist as follows:
Well-known mandatory: Must be recognized by all BGP routers, present in all BGP updates, and passed on to other BGP routers. For example, AS path, origin, and next hop

Well-known discretionary: Must be recognized by all BGP routers and passed on to other BGP routers but need not be present in an update, for example, local preference.

Optional transitive: Might or might not be recognized by a BGP router but is passed on to other BGP routers. If not recognized, it is marked as partial, for example, aggregator, community.

Optional non-transitive: Might or might not be recognized by a BGP router and is not passed on to other routers, for example, Multi-Exit Discriminator (MED), originator ID.


BGP Attributes Symbol

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