Distance Vector and Link State Routing Protocol
There are two major differences between Distance Vector routing protocols and Link State routing protocols. Distance Vector exchanges the routing updates periodically whether the topology is change or not, this will maximize the convergence time which increases the chance of routing loops while the Link State routing protocols send triggered change based updates when there is a topology change. After initial flood, pass small event based triggered link state updates to all other routers. This will minimize the convergence time that’s why there is no chance of routing loops. Secondly, the Distance Vector routing protocols rely on the information from their directly connected neighbours in order to calculate and accumulate route information. Distance Vector routing protocols require very little overhead as compared to Link State routing protocols as measured by memory and processor power while the Link State routing protocols do not rely solely on the information from the neighbours or adjacent router in order to calculate route information. Instead, Link State routing protocols have a system of databases that they use in order to calculate the best route to destinations in the network. An extra feature of Link State routing protocol is that they can detect media types along with other factors. This could increase the overhead as compare to Distance Vector routing protocols in order to measure by processor power and memory. Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) are the examples of Distance Vector routing protocols while the Open Shortest Path First (OSPF) is a classic example of Link State routing protocols.
The other differences of both types of routing protocols are as follows:
Distance Vector
- Distance Vector routing protocols are based on Bellma and Ford algorithms.
- Distance Vector routing protocols are less scalable such as RIP supports 16 hops and IGRP has a maximum of 100 hops.
- Distance Vector are classful routing protocols which means that there is no support of Variable Length Subnet Mask (VLSM) and Classless Inter Domain Routing (CIDR).
- Distance Vector routing protocols uses hop count and composite metric.
- Distance Vector routing protocols support dis-contiguous subnets.
- Common distance vector routing protocols include: Appletalk RTMP, IPX RIP, IP RIP, IGRP
Link State
- Link State routing protocols are based on Dijkstra algorithms.
- Link State routing protocols are very much scalable supports infinite hops.
- Link State routing protocols are classless which means that they support VLSM and CIDR.
- Cost is the metric of the Link State routing protocols.
- Link State routing protocols support contiguous subnets.
Limitations of Distance Vector
For Distance Vector routing protocols such as RIP, IGRP as well as hybrid routing protocols with the characteristics of Distance Vector such as EIGRP while maintaining routing information, the routing loops have been occurred. It is because the Distance Vector routing protocols send periodic routing updates and each node maintain the distance from itself to each possible destination network, for this the convergence time of Distance Vector routing protocols is slow. Slow convergence produces inconsistent routing. When the topology of network changes and a network has gone down, the packets for the network bounce between routers and the hop count for specific network counts to infinity, the solution is split horizon. Split horizon follows the rule that it is never useful to send information about a route back in the direction from which the original packet came. Split horizon can be disabled for all Distance Vector routing protocols.
For Distance Vector routing protocols such as RIP, IGRP as well as hybrid routing protocols with the characteristics of Distance Vector such as EIGRP while maintaining routing information, the routing loops have been occurred. It is because the Distance Vector routing protocols send periodic routing updates and each node maintain the distance from itself to each possible destination network, for this the convergence time of Distance Vector routing protocols is slow. Slow convergence produces inconsistent routing. When the topology of network changes and a network has gone down, the packets for the network bounce between routers and the hop count for specific network counts to infinity, the solution is split horizon. Split horizon follows the rule that it is never useful to send information about a route back in the direction from which the original packet came. Split horizon can be disabled for all Distance Vector routing protocols.
Static Routing Tutorial
In studying for your CCNA exam and preparing to earn this valuable certification, you may be tempted to spend little time studying static routing and head right for the more exciting dynamic routing protocols like RIP, EIGRP, and OSPF.
This is an understandable mistake, but still a mistake. Static routing is not complicated, but it’s an important topic on the CCNA exam and a valuable skill for real-world networking.
To create static routes on a Cisco router, you use the ip route command followed by the destination network, network mask, and either the next-hop IP address or the local exit interface. It’s vital to keep that last part in mind – you’re either configuring the IP address of the downstream router, or the interface on the local router that will serve as the exit interface.
Let’s say your local router has a serial0 interface with an IP address of 200.1.1.1/30, and the downstream router that will be the next hop will receive packets on its serial1 interface with an IP address of 200.1.1.2/30. The static route will be for packets destined for the 172.10.1.0 network. Either of the following ip route statements would be correct.
R1(config)#ip route 172.10.1.0 255.255.255.0 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 172.10.1.0 255.255.255.0 serial0 ( local exit interface)
You can also write a static route that matches only one destination. This is a host route, and has 255.255.255.255 for a mask. If the above static routes should only be used to send packets to 172.10.1.1., the following commands would do the job.
R1(config)#ip route 172.10.1.1 255.255.255.255 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 172.10.1.1 255.255.255.255 serial0 ( local exit interface)
Finally, a default static route serves as a gateway of last resort. If there are no matches for a destination in the routing table, the default route will be used. Default routes use all zeroes for both the destination and mask, and again a next-hop IP address or local exit interface can be used.
R1(config)#ip route 0.0.0.0 0.0.0.0 200.1.1.2 (next-hop IP address)
R1(config)#ip route 0.0.0.0 0.0.0.0 200.1.1.2 (next-hop IP address)
OR
R1(config)#ip route 0.0.0.0 0.0.0.0 serial0 (local exit interface)
IP route statements seem simple enough, but the details regarding the next-hop IP address, the local exit interface, default static routes, and the syntax of the command are vital for success on CCNA exam day and in the real world.
- Static Routing
- Static routing is the term used to refer to a manual method that is used to set up routing between networks. The network administrator configures static routes in a router by entering routes directly into the routing table of a router. Static routing has the advantage of being predictable and simple to set up. It is easy to manage in small networks but does not scale well. Compare this with dynamic routing.
ADVANTAGES
- Static routes are simple and quick to configure.
- Static routing is supported on all routing devices and all routers.
- Static routes are easy to predict and understand in small networks.
DISADVANTAGES
- Static routes require extensive planning and have high management overhead. The more routers exist in a network, the more routes that need to be configured. If you have ‘N’ number of routers and a route between each router is needed, then you must configure N x N routes, so, for a network of nine routers, you need a minimum of 81 routes (9 x 9 = 81).
- Static routes do not dynamically adapt to network topology changes or equipment failures.
- Static routing does not scale well in large networks.
STATIC ROUTE CONFIGURATION (Cisco)
- Default Route
- Static Null Route
- Preferred Routes
- Backup Routes
- Static Load Balancing
Default Route
A default route is often called the ‘route of last resort’. It is the last route tried by a router when all other routes fail because it has the fewest number of network bits matching and is therefore less specific. A default route is configured on a Cisco router with the following command:
CISCO ‘default ip route’ command:
ip route 0.0.0.0 0.0.0.0 <next hop or exit interface>
Static Null Route
A null route directs traffic to a non-existent interface called the null interface. The “Null 0″ interface is often referred to as a ‘bit bucket’. Network packets directed to the “Null 0″ interface are discarded as soon as they are received. A null route is useful for removing packets that cannot make it out of the network or to their destination, and/or to decrease congestion created when packets with no currently reachable destination float around the network, or the destination is under a denial of service attack. During a denial of service attack, a null route can temporarily be placed on the next to last hop closest to destination which will cause that device to drop all traffic generated by the attack.
CISCO ‘null route’ command:
! Null route (less specific) ip route 202.148.224.0 255.255.255.0 null0 ! Preferred routes (more specific) ip route 202.148.224.0 255.255.255.128 e1 ip route 202.148.224.128 255.255.255.128 e2
Preferred Route
The route which has the greatest number of network bits in the network address matching the destination address stored in the packet is the preferred route to a destination. Routers are programmed to select a preferred route based on how closely the bits in the route and packet match. This is also referred to as ‘longest prefix match’.
CISCO ‘preferred route’ commands:
! Standard route ip route 202.148.224.0 255.255.255.0 e0 ! Preferred routes ip route 202.148.224.0 255.255.255.128 e1 ip route 202.148.224.128 255.255.255.128 e2
Backup Routes
In cases where redundancy is needed and where more than one network connection between two network locations exists, a second route can be configured to be used when the primary network connection has failed. A backup route causes traffic to flow over a different physical path. When the first route fails, the second route over one or more less preferred path(s) will be used. By using a secondary routes the network administrator increases the redundancy and reliability of the network. Backup routes help compensate for network failures.
CISCO backup route configurations:
Scenario 1: Using more Specific Route(s)
SPECIFIC ROUTES (used unless down) ip route 202.148.224.0 255.255.255.128 e0 ip route 202.148.224.128 255.255.255.128 e1 BACKUP ROUTES (used when one of the specifics are down) ip route 202.148.224.0 255.255.255.0 e0 ip route 202.148.224.0 255.255.255.0 e1
Scenario 2: Using Cisco’s Weight mechanism
ip route 202.148.224.0 255.255.255.0 e0 ip route 202.148.224.0 255.255.255.0 e1 250
Static Load Balancing
Using static load balancing, you can configure traffic load balancing on a per-packet basis without using a dynamic routing protocol. Most routers will perform load balancing automatically if several equal cost paths to a destination exist on multiple interfaces. To configure this using static routing, you need only create multiple static routes for more than one interface. This creates more than one equal cost path which will balance the load.
Cisco routers perform load balancing on a per-destination, per-source-destination and per-packet basis. Load can be balanced across up to six equal cost paths.
CISCO router commands:
ip route 202.148.224.0 255.255.255.0 e0 ip route 202.148.224.0 255.255.255.0 e1 -or- ip route 202.148.224.0 255.255.255.0 199.212.18.1 ip route 202.148.224.0 255.255.255.0 199.212.18.17 ip route 202.148.224.0 255.255.255.0 199.212.18.33
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