Type s of distance vector routing protocols
RIP
Routing Information Protocol (RIP) was originally specified in RFC 1058. It has the following key characteristics:
Hop count is used as the metric for path selection.
If the hop count for a network is greater than 15, RIP cannot supply a route to that network.
Routing updates are broadcast or multicast every 30 seconds, by default.
IGRP
Interior Gateway Routing Protocol (IGRP) is a proprietary protocol developed by Cisco. IGRP has the following key design characteristics:
Bandwidth, delay, load and reliability are used to create a composite metric.
Routing updates are broadcast every 90 seconds, by default.
IGRP is the predecessor of EIGRP and is now obsolete.
EIGRP
Enhanced IGRP (EIGRP) is a Cisco proprietary distance vector routing protocol. EIGRP has these key characteristics:
It can perform unequal cost load balancing.
It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path.
There are no periodic updates as with RIP and IGRP. Routing updates are sent only when there is a change in the topology.
The meaning of Distance vector
The Meaning of Distance Vector
The meaning of Distance Vector
As the name implies, distance vector means that routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric such as hop count and direction is simply the next-hop router or exit interface.
Distance vector routing protocols share certain characteristics;
Periodic updates,
Neighbors-routers that share a link and use the same routing protocol,
Broadcast updates-all updated routing information is sent to the broadcast address of 255.255.255.255,
Most distance vector routing protocols send the entire routing table in the updates
The purpose of the Routing protocol Algorithm
The algorithm used for the routing protocols defines the following processes:
Mechanism for sending and receiving routing information.
Mechanism for calculating the best paths and installing routes in the routing table.
Mechanism for detecting and reacting to topology changes.
Even though that when the routing protocol is configured on router for example RIP, this action "turns of the functionality of the routing protocol on the router" the next step would be to tell the routing protocol which networks will be routed. This step dose not add information into the routing table. The algorithm
Routing Protocol Characteristics
Screen clipping taken: 23.06.2011 16:55
Time to Convergence
Scalability
Classless or Classful
Resource Usage
Implementation and Maintenance
Network Discovery
A directly connected device is not a another device that is connected with the router,
Ist is a network that is directly connected to the router
After a cold start routers will only send updates about their directly connected neighbors
And the next step will be to have information about their immediate neighbors in the routing
Table. The last step is for the routing protocol to send updates about networks beyond their
Immediate neighbors.
Routing Table Maintainance
The main cause of changes in the routing table are because of:
Failure of a link
Introduction of a new link
Failure of a router
Change of link parameters
RIP uses periodic updates to keep the routing table up to date, by default it sends a update
Every 30 seconds, but also uses other timers:
Invalid Timer: default 180 seconds If no update is received for a route after the time it marked invalid
And will be removed form the routing table after the flush timer expires
Flush Timer: default 240 seconds used to remove invalid or unreachable networks from the routing table
Holddown Timer:default 180 seconds, time waited once a route is marked unreachable
For the network to converge
Show ip route- allows u too so see when the route was last updated
Show ip protocols - allows u to see all timers
EIGRP uses bounded updates
Non-periodic because they are not sent out on a regular basis.
Partial updates sent only when there is a change in topology that influences routing information.
Bounded, meaning the propagation of partial updates are automatically bounded so that only those routers that need the information are updated.
RIP also uses triggered updates when one of the follows occur:
Triggered updates are sent when one of the following occurs:
An interface changes state (up or down)
A route has entered (or exited) the "unreachable" state
A route is installed in the routing table
Triggered updates dont wait until the timer expires, after the router notices the change
The updates are sent to other router, and then the other router also send triggered updates.
Triggered updates have 2 main problems 1) packets can be lost or corrupted 2) they take too
Long to propagate across the network and could get mixed up with other routing information
Jitter
Over time updates may become synchronized, but this can cause problems in
distance vector routing Protocols because packets can collide. Cisco IOS uses RIP_JITTER
To get rid of this problem. JITTER subtracts a variable amount of time to the update interval for each router in the network. This random jitter, or variable amount of time, ranges from 0% to 15% of the specified update interval.
Routing Loops
A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network.
The loop may be a result of:
Incorrectly configured static routes
Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol and is discussed in CCNP-level courses)
Inconsistent routing tables not being updated due to slow convergence in a changing network
Incorrectly configured or installed discard routes
A routing loop can create the following conditions:
Link bandwidth will be used for traffic looping back and forth between the routers in a loop.
A router's CPU will be strained due to looping packets.
A router's CPU will be burdened with useless packet forwarding that will negatively impact the convergence of the network.
Routing updates may get lost or not be processed in a timely manner. These conditions would introduce additional routing loops, making the situation even worse.
Packets may get lost in "black holes."
There are a number of mechanisms available to eliminate routing loops,
Defining a maximum metric to prevent count to infinity in RIP the metic used is hop count and the maximum is 16
Holddown timers
Holddown timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad. Holddown timers instruct routers to hold any changes that might affect routes for a specified period of time. If a route is identified as down or possibly down, any other information for that route containing the same status, or worse, is ignored for a predetermined amount of time (the holddown period).
Holddown timers work in the following way:
1. A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible.
2. The router marks the network as possibly down and starts the holddown timer.
3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed.
4. If an update from any other neighbor is received during the holddown period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated.
5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. If the destination network truly is unavailable and the packets are forwarded, black hole routing is created and lasts until the holddown timer expires.
Split horizon
The split horizon rule says that a router should not advertise a network through the interface from which the update came.Complete routing updates are exchanged, with the exception of routes that violate the split horizon rule
Route poisoning
Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. Unreachable is interpreted as a metric that is set to the maximum. For RIP, a poisoned route has a metric of 16.
Poison Reverse
The rule for split horizon with poison reverse states when sending updates out a specific interface, designate any networks that were learned on that interface as unreachable.
IP and TTL
Time to Live (TTL) is an 8-bit field in the IP header that limits the number of hops a packet can traverse through the network before it is discarded.
Triggered updates
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