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Before a data packet is forwarded, a chart is reviewed to determine the best possible path for the data to reach its destination. This chart is the computer's routing table. Maintaining an accurate routing table is essential for effective data delivery. Every computer on a TCP/IP network has a routing table stored locally.
Static vs. dynamic
Static routing requires manual inputting of the route table, takes times and human errors can occur. where as dynamic is automatically done so if a change occurs in the topology the dynamic routing protocol can re-calculate its route where as a static could not.
The "route add" command adds a static route to the routing table. The route add command with the -p switch makes the static route persistent.on a host machine
With distance-vector router communications, each router on the network communicates all the routes it knows about to the routers to which it is
directly attached. In this way, routers communicate only with their router neighbors and are unaware of other routers that may be on the network. The communication between distance-vector routers is known as hops.
A distance-vector routing protocol sends a full copy of its routing table to its directly attached neighbors. This is a periodic advertisement, meaning that even if there have been no topological changes, a distance-vector routing protocol will, at regular intervals, re-advertise its full routing table to its neighbors.
Slow convergences times and heavy use of bandwidth and potential of routing loops due to slow convergences times.
A router that uses a link-state protocol differs from a router that uses a distance- vector protocol because it builds a map of the entire network and then holds that map in memory. On a network that uses a link-state protocol,
routers send link-state advertisements (LSAs) that contain information about the networks to which they connect. The LSAs are sent to every router on the network, thus enabling the routers to build their network maps.
When the network maps on each router are complete, the routers update each other at a given time, just like with a distance-vector protocol; however, the updates occur much less frequently with link-state protocols than with distance- vector protocols.
A router that uses link-state protocols must maintain a database of all the routers in the entire network.
A hybrid routing protocol is one that uses both the distance and link state protocol to make its routing decision. EIGRP is an example of a hybrid.
IGP vs. EGP
Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs)
Routing protocols can be categorized based on the scope of their operation. Interior Gateway Protocols (IGP) operate within an autonomous system (AS), where an AS is a network under a single administrative control. Conversely, Exterior Gateway Protocols (EGP) operate between autonomous systems.
Believability of a Route
The index of believability is called administrative distance (AD).
It governs the route a router will chose when their is more than one route with different routing protocols.
Directly connected network 0
Statically configured network 1
External EIGRP 170
Unknown of unbelievable 255 (considered to be unreachable)
Enhanced Interior Gateway Routing Protocol
A Cisco-proprietary protocol.
EIGRP is an IGP with fast convergence and is very scalable
By default, EIGRP uses bandwidth and delay in its metric calculation but can use reliability, load, and maximum transmission unit (MTU) size.
Some literature calls EIGRP an advanced distance-vector routing protocol, while some literature calls EIGRP a hybrid routing protocol.
EIGRP uses information from its neighbors to help it select an optimal route (like distance-vector routing protocols). However, EIGRP also maintains a database of topological information (like a link-state routing protocol). The algorithm EIGRP uses is Diffusing-Update Algorithm ( DUAL).
Open Shortest Path First A link-state routing protocol based on the SPF (Shortest Path First) algorithm to find the least-cost path to any destination in the network. In operation, each router using OSPF sends a list of its neighbors to other routers on the network. From this
information, routers can determine the network design and the shortest path for data to travel.
Routing protocol that uses a metric of cost , which is based on the link speed between two routers. OSPF is a popular IGP, because of its scalability, fast convergence, and vendor-interoperability.
Intermediate System to Intermediate System
This link-state routing protocol is similar in its operation to OSPF. It uses a configurable, yet dimensionless, metric associated with an interface and runs Dijkstra's Shortest Path First algorithm. Although IS-IS as an IGP offers the scalability, fast convergence, and vendor-
interoperability benefits of OSPF, it has not been as widely deployed as OSPF.
IS-IS routers distribute topology information to other routers, enabling them to make the best path decisions.
Routing Information Protocol version 1
RIP is a distance-vector routing protocol. RIP is limited to a maximum of 15 hops. One of the downsides of the protocol is that the original specification required router updates to be transmitted every 30 seconds. On smaller networks this is acceptable; however, this can result in a huge traffic load on larger networks. The original RIP specification also did not support router authentication, leaving it
vulnerable to attacks.
Routing Information Protocol version 2
The second version of RIP dealt with the shortcomings of the original design. Authentication was included to enable secure transmissions, also, it changed from a networkwide broadcast discovery method
to a multicast method to reduce overall network traffic. However, to maintain compatibility with RIP, RIPv2 still supports a limit of 15 hops.
Border Gateway Protocol
A routing protocol often associated with the Internet. BGP can be used between gateway hosts on the Internet. BGP examines the routing
table, which contains a list of known routers, the addresses they can reach, and a cost metric associated with the path to each router so that
the best available route is chosen. BGP communicates between the routers using TCP.
Some literature classifies BGP as a distance-vector routing protocol, it can more accurately be described as a path-vector routing protocol, meaning that it can use as its metric the number of AS hops that must be transited to reach a
destination network, as opposed to a number of required router hops. BGPs path selection is not solely based on AS hops, however. BGP has a variety of other parameters that it can consider. Interestingly, none of those parameters
are based on link speed. Also, although BGP is incredibly scalable, it does not quickly converge in the event of a topological change.
Routing metrics: Hop counts
Hop counts are the number of hops necessary to reach a node. A hop count of infinity means the route is unreachable.
Routing metrics:MTU, bandwidth
The Maximum Transmission Unit (MTU) defines the largest data unit that can be passed without fragmentation.
Costs are the numbers associated with traveling from point A to point B (often hops). The lower the total costs (the less links in the route), the
more that route should be favored.
Latency is the amount of time it takes for a packet to travel from one location to another.
Convergence (steady state)
The time it takes the routers to updat their table once a change has occurred in the network.
Stopping Router Loops
Split horizon: Works by preventing the router from advertising a route back to the other router from which it was learned. This prevents two nodes from bouncing packets back and forth between them, creating a loop.
Poison reverse (also called split horizon with poison reverse):
Dictates that the route is advertised back on the interface from which it was learned, but it has a hop count of infinity, which tells the node that the route is unreachable.
Defined by the IEEE 802.1D standard, it allows a network to have redundant Layer 2 connections, while logical preventing a loop, which could lead to symptoms such as broadcast storms and MAC address table corruption.
STP is used with network bridges and switches.
STP refers to Layer 2
With the help of Spanning Tree Algorithm (STA), STP avoids or eliminates loops on a Layer 2 bridge.
STP actively monitors the network, searching for redundant links. When it finds some, it shuts them down to prevent switching loops. STP uses STA to create a topology database to find and then remove the redundant links. With STP operating
from the switch, data is forwarded on approved paths, which limits the potential
STP actions on a port
If a particular port has a problem, STP can perform a number of actions, including blocking the port, disabling the port, or forwarding data destined for that port to another port. It does this to ensure that no redundant links or paths are found in the spanning tree and that only a single active path exists between any two network nodes.
STP uses bridge protocol data units (BPDUs) to identify the status of ports and bridges across the network. BPDUs are simple data messages exchanged between switches. BPDUs contain information on ports and provide the status of those ports to other switches. If a BPDU message finds a loop in the network, it is managed by shutting down a particular port or bridge interface.
STP port modes
Disabled: The port is offline and does not accept BPDU messages.
A blocked port accepts BPDU messages but does not forward them.
Listening: A listening port receives BPDU messages and monitors for changes to the network topology.
Learning: In a learning state, the port is not part of the active spanning tree topology but can take over if another port fails. Learning ports receive BPDUs and identify changes to the topology when made. Learning MAC addresses
Forwarding: The port is part of the active spanning tree topology and forwards BPDU messages to other switches. Learning MAC addresses
Rapid Spanning Tree Protocol 802.1w
The 802.1D disabled, blocking, and listening states are merged into a unique 802.1w discarding state.
It is a quicker way of updating the switches with a new topology once there has been a change.
VLANs enable you to create multiple broadcast domains on a single switch. In essence, this is the same as creating separate networks for each VLAN.
VLANs are used for network segmentation, a strategy that significantly increases the network's performance capability, removes potential performance bottlenecks, and can even increase network security. A VLAN is a group of connected computers that act as if they are on their own network segments, even though they might not be.
VLAN Trunking Protocol (VTP),
One challenge with carving a switch up into multiple VLANs is that several switch ports (that is, one port per VLAN) could be consumed to connect a switch back to a router. A more efficient approach is to allow traffic for multiple VLANs to travel over a single connection.
You need some way to monitor network traffic and monitor how well a switch works. This is the function of port mirroring. To use port mirroring, administrators configure a copy of all inbound and outbound traffic to go to a certain port. A protocol analyzer examines the data sent to the port and therefore does not interrupt the flow of regular traffic.
Broadcast domain vs. collision domain
A broadcast domain is created by routers or VLANs and stops broadcast messages from leaving their network and creating congestion.
HUBs create one collision zone where as switches create one for each port . This stop congestion caused by nods trying to communicate at the same time on the same collision domain.
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