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When to use Routing Protocols

Posted on 2014-04-06
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Last Modified: 2014-04-23
I believe we still can build a Network without using any Routing protocol (RIP,OSPF,EIGRP,BGP,etc..).
OR anything that involves a router in between subnets needs a Routing table ?


I would like to know in which case Routing Protocol usage is mandatory.

Thank you
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Question by:jskfan
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by:Shaik M. Sajid
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in case u have small network, minimum routes, small router...  yes you can build any kind of network using static routes... but dynamic routes will reduce your overhead burdens...
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by:Don Johnston
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Routing protocols are beneficial when there are multiple paths between networks and you want the traffic to failover if a failure occurs on the primary (or preferred) path.

Also when the number of networks and routers gets to the point that creating static routes becomes labor intensive.
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by:skullnobrains
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in my opinion, there are no cases where using a routing protocol is mandatory and very few in which it actually makes things simpler or more robust. most people won't agree because they learnt how to build a network from cisco's whitepapers or from people who did.

they do get useful when you want a huge lan to be separated in several small lans, (likely in different locations) and link them together randomly without using a central core. you also would not need centralised firewall management or be ready to pay for a distributed firewall.
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by:giltjr
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I agree that they are not mandatory.  

However, as donjohnston and skullnobrains have stated there are times when they are very useful because they allow you to do things that you can't otherwise do.

Such as having multiple network paths to remote networks and choosing the "best" route to the remote network and having fail over capability.
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by:skullnobrains
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having multiple network paths to remote networks and choosing the "best" route to the remote network and having fail over capability

actually this can easily be achieved without routing protocols. linux even has support for basic failover (which i would definitely not recommend for production). but a simple shell script can usually address real-world scenarios much better than any routing protocol when it comes to simple failover.

an example scenario could be : use that less expensive ($$) route as long as it is up, failover to whichever redundant link has the best RTT to server X when that route is down.

this scenario has specificities of it's own : the bandwidth is not taken into account, a link has privilege at all times because it is less expensive (in terms of money) and the RTT is the criteria for selecting the failover link. quite simple to script but really hard to handle with a routing protocol.

my point here being : they are actually usefull in such scenarios (espetially if you have an unknown number of remote networks to connect to), but "you can't otherwise do" is something I don't really agree with in this context.

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maybe an example case where you would "need" routing protocols is if your infrastructure is full-cisco and you want to achieve load sharing over multi-hop routes

basically each approach to a network will let you do easily some things and be a real pain to handle others

if you want to combine dynamic routing ( failover / best route selection ) with load sharing, routing protocols are not very well featured (and provide little inter-operability and sometimes no ability to work without as frien peer), and scripting requires lots of time and knowlege so in both cases people usually end up with something just as good as it can be.
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by:jskfan
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If I create VLANs on all our L3 Switches in our LAN, then do I need kind of routing protocols (including static routes)to route traffic between VLANs ?
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by:Don Johnston
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If you only have one L3 switch, then no, you do not need static or dynamic routes as all networks are directly connected.

If you have more than one L3 switch, then you will need routes to the non-directly connected networks.

Whether you use static or dynamic routes is dependent on the topology and your requirements.
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by:skullnobrains
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if you create VLANs, you'll need each VLAN to correspond to a different subnet. if you create vlans and all the machines are on the same network, they will not be able to communicate.

you should not need routing protocols or to setup static routes on the machines, but you may need to create static routes on the routers depending on your topology. basically if 2 VLAN's gateways are on the same router, they will be able to communicate through that router without any configuration (other than allowing the traffic to flow). this will defeat the purpoise of the VLAN but efficiently isolate broadcasts.

post about your topology and requirements and we should be able to help. in many cases, you'll mix L2 and L3. unless you have a huge number of L3 switches connected randomly or in circle, you most likely don't need to setup routing protocols. i'll strongly advise against it if you're not already familiar with LAN, VLAN, and IP routing concepts
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by:jskfan
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<<<if you create VLANs, you'll need each VLAN to correspond to a different subnet>>>

I thought you can create SVIs, and run IP ROUTING command on each Switch, and make those SVIs the default gateway of each computer. This way computers can talk to each other without any routing protocol…
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by:Don Johnston
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If you have have one switch, that is correct since all the networks (VLANs) are directly connected to the switch.

But if you have more than one switch, you will need routes in the switches to networks that are not directly connected.
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by:giltjr
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If you have multiple switches,  you don't need a SVI on each of them for the same VLAN.

That is, if you have VLAN10 and you have 5 switches that all have VLAN10, you only need a SVI on one of them.

For redundancy you could setup SVI's on two of them and use HSRP for fail over.
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by:skullnobrains
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if you put a virtual interface in each VLAN, each of them will be in a different subnet

if all virtual interfaces are not on the same switch (or virtual switch if you do HSRP as suggested above), you'll need the switch hosting each VI to know where to find the other networks
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by:jskfan
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Ok… if I have a bunch of switches that have different VLANs then
 I decide to use static route , on which port should I configure that?

I am sure all switches will have Trunk ports to a backbone switch.
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by:giltjr
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If all VLAN's exist on the "backbone switch" then the SVI should exist on that switch and let it do all the routing.
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by:skullnobrains
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you really (!) should post your topology if you expect better help !

then you don't configure a route on a port : the setting is router/host-wide

as a general rule, you will need to create a route on the router that has each gateway telling it to use each of the other gateways to reach the corresponding network. when the 2 gateways are on the same router, you don't need a route

--

example (with a dedicated lan for routing which is recommended in such setups)
routing lan iss 10.250.0.0/24 in the example

RT1
addr 10.1.0.1/16
connected to a bunch of hosts in the 10.1.0.0/16 network using gw 10.1.0.1
addr 10.250.0.1/24
 |
RT2
addr 10.250.0.2/24
addr 10.2.0.1/16
connected to a bunch of hosts in the 10.2.0.0/16 network using gw 10.2.0.1

route on RT1
dest 10.2.0.1
mask 255.255.0.0
gw  10.250.0.2

route on RT2
dest 10.1.0.1
mask 255.255.0.0
gw  10.250.0.1
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by:jskfan
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giltjir:
<<<If all VLAN's exist on the "backbone switch" then the SVI should exist on that switch and let it do all the routing.>>>

I meant all VLANs are trunked back to the Backbone switch.

for instance:

SW1: has Vlans 10 and 100
SW2: has Vlans 100 and 20
SW3 has Vlans 20 and and 200
SW4 has Vlans 200 and 30
SW5 has Vlans 55 and 66

The Switches above will all have a trunk port that connects to other trunk ports on switch HUBSWITCH.

how do I make all computers be able to communicate each other? do I need a router? if so what should I configure on the router to achieve the goal? if I need a HUBSWITCH to be L3 then what do I need to configure  on the HUBSWITCH?

Thanks
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by:Don Johnston
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Are any of the switches multilayer switches?
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skullnobrains earned 213 total points
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if all vlans are trunked back to the backbone, then the gateways are either on the backbone (if it is a so-called layer3 switch ie actually a router) or on another equipment upstream (firewall or router)

in that case, you don't need to do anything other than allowing traffic to flow from a vlan to another on the equipment that has all the gateways. if it is a router (or layer3 switch), it will default to allow, if it is a firewall, you'll need to create a rule. routes will be automatically setup just by creating the IPs you use as gateways for the various networks.

you may want to wonder about your goals : if you want to route all traffic between VLANs, you probably don't need vlans at all.
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by:giltjr
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--> I meant all VLANs are trunked back to the Backbone switch.

That should mean that all VLAN's exist on the "Backbone switch."  As we have all stated, if the "Backbone switch" is a L3 device, then it should have a IP address within each VLAN and it would be the router for each VLAN.
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by:jskfan
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Thank you Guys!
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