Harriet Nyakaana
| Distance Vector |
| Problems With Distance Vector |
| Solutions |
| References |
Distance-Vector is an algorithm which uses a direction to any link in the interconnection network to determine the best route.
There are two very popular Distance Vector routing protocols that is the Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP). RIP is a relatively old but still commonly used interior gateway protocol created for use in small, homogeneous networks. It is a classical distance-vector routing protocol. RIP is documented in RFC 1058.
With RIP, routers periodically exchange entire tables. Because this is inefficient, RIP is gradually being replaced by a newer protocol called Open Shortest Path First (OSPF). http://www.webopedia.com/TERM/R/Routing_Information_Protocol.html
RIP is a widely-used protocol for managing
router information within a self-contained network such as a corporate local
area network (
There are various metrics that Routing Protocols use to rate the value of different
routes. The RIP uses the hop
count
which
is the number of routers that can be traversed in a route. A directly connected
network has a metric of zero; an unreachable network has a metric of 16. This
small range of metrics makes RIP an unsuitable routing protocol for large
networks.
So the router makes decisions about the way a packet will travel, based on the
amount of hops it takes to reach the destination and if it had 2 different ways
to get there, it will simply send it via the shortest path, regardless of the
connection speed. This is known as pinhole congestion.
Below is a diagram which shows a router using Distance Vector routing protocols:
Inter-Gateway Routing Protocol – IGRP is a Cisco proprietary, distance-vector, routing protocol used by routers to exchange routing information. The goal of IGRP was to create a robust protocol for routing within an autonomous system (AS). http://eie507.eie.polyu.edu.hk/ss-submission/C3-IGRP/IGRP.htm
Problems with
Distance
-Vector
The main problem with distance -vector is that it updates the network step by step so it requires more bandwidth to process the information.
More problems include: Routing loops and Counting to infinity.
Routing loops arise in a Distance Vector Algorithm when routes 'backwash' from the receiver to the sender. Counting to Infinity is when wrong information is communicated between routers.
Solutions to such problems are setting infinity values, split horizon, hold-down timers, same neighbour problem, different neighbour better metric, different neighbour poorer metric.
Below is abit of the descriptions of the solutions to Distance Vector problems.
Counting to Infinity
The problem of counting to infinity is normally caused by gossip and wrong information being communicated between the routers. Because there is nothing to protect against this type of loop, the hop count will keep on increasing each time the packet goes through a router. Therefore to solve this problem there is need to define a maximum hop count. Distance Vector (RIP) permits a hop count of up to 15, so anything that needs 16 hops is unreachable. So if a loop occurred, it would go around the network until the packet reached a hop count of 15 and the next router would simply discard the packet.
Split Horizon
With Split Horizon, it's never useful to send information about a router back to the destination from which the original packet came for it would be useful sending back the same information.
Hold Down Timers
With Hold Down Timers, routers keep an entry for the network-down state, allowing time for other routers to recompute for this topology change, this way, allowing time for either the downed router to come back or the network to stabilise somewhat before changing to the next best route.
Neighbour Problems
When a router receives an update from a neighbour indicating that a previously accessible network is not working and is inaccessible, the hold-down timer will start. If a new update arrives from a neighbour with a better metric than the original network entry, the hold-down is removed and data is passed. But if an update is received from a neighbour router before the hold-down timer expires and it has a lower metric than the previous route, the update is ignored and the hold-down timer keeps ticking. This allows more time for the network to converge.
There are three instances when triggered updates will reset the hold-down timer:
1) The hold-down timer expires
2) The router received a processing task proportional to the number of links in the internet work.
3) Another update is received indicating the network status has changed.
http://homepages.uel.ac.uk/u0222251/website/Distance_vector.htm
http://homepages.uel.ac.uk/u0217358/main%20Routing%20protocols.htm
http://www.cs.purdue.edu/homes/yilu/papers/percom.pdf
http://homepages.uel.ac.uk/u0222251/website/Distance_vector.htm
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/rip.htm#xtocid1
http://www.netbook.cs.purdue.edu/othrpags/qanda69.htm