Redistribute Meaning

[prashant]

Hi,

    Can anyone explain me briefly what is meaning of using redistribute cmd in any routing protocol.

For Example: Let’s assume I am adervetising the network in to EIGRP via redistribution then I put the below cmd

Router(config)#router eigrp 100

Router(config-router)#redistribute ?

  bgp        Border Gateway Protocol (BGP)

  connected  Connected

  eigrp      Enhanced Interior Gateway Routing Protocol (EIGRP)

  isis       ISO IS-IS

  iso-igrp   IGRP for OSI networks

  metric     Metric for redistributed routes

  mobile     Mobile routes

  odr        On Demand stub Routes

  ospf       Open Shortest Path First (OSPF)

  rip        Routing Information Protocol (RIP)

  route-map  Route map reference

  static     Static routes

  <cr>

If I apply any cmd like that "redistribute connected" or "redistribute static" or redistribute bgp 1

Now what is the meaning of applying this cmd or what will happen if I apply like that.

Why I am asking because i m bit confuse to apply this cmd.

Once I know the actual meaning then I can apply the exact one.

Kindly help me to understand this.

Regards

Prashant

[Peter Paluch]

Hi Prashant,

The term redistribution refers to a process where a routing protocol retakes and advertises networks learned from a different source than itself. Any routing protocol and its particular instance (if multiple instances are run on a single router) by default disseminates only information that it has learned itself, plus the directly connected networks added by the network command. However, if there are multiple routing protocols running on the same router and each of them inserts a set of its own networks into the routing table, these network won't "mix" and get advertised by all these routing protocols. Even though the routing table can contain networks from all these routing protocols, each of them still keeps track of its own routes and continues to advertise only its own routes, disregarding networks learned from other routing protocols.

If you need to "leak" one routes learned from one source to a different routing protocol, you use the redistribution. What you do is to instruct the target routing protocol to look for all routes present in the routing table that were learned by the selected source, and redistribute - or inject them - into its own database and advertise them further. Without configuring redistribution, all routing protocols keep their own knowledge separate. With redistribution, you are allowing routing information from one routing protocol to leak into another.

Assuming you are running router eigrp 100 then these are the effects of configuring redistribution using the different commands you have suggested:

• redistribute connected will cause all directly connected networks that are visible in the routing table with the 'C' flag to be advertised in EIGRP 100. You do not need to have them individually covered by network commands
• redistribute static will cause all statically configured routes that are visible in the routing table with the 'S' flag to be advertised in EIGRP 100
• redistribute bgp 1 will cause all external (eBGP) routes learned from BGP that are visible in the routing table with the 'B' flag to be advertised in EIGRP 100. Redistributing internal BGP (iBGP) routes is possible with this command as well but it requires additional configuration in the BGP process.

Injecting locally connected networks using redistribute connected is different from the network command in two important aspects:

1. With redistribute connected, the routing protocol does not attempt to communicate with neighbors on local interfaces. These interfaces are not truly added to the routing protocol, just the IP networks on these interfaces are injected into the routing protocol. If you need to communicate over a particular interface with other routers on that interface, you must add that interface into the routing protocol using the network command.
2. With redistribute connected, the local networks are injected into the routing protocol as so-called external routes (if the protocol is capable of distinguishing between internal and external networks - OSPF and EIGRP are). External networks can be treated differently in the routing protocol, most often having a lower trustworthiness than internal routes. If a routing protocol chooses between an internal or an external path to the same destination network, it will prefer the internal path even if the external path has a lower metric.

Redistribution can be a quite convoluted concept which is why it is being taught at the CCNP level, and on CCNA, only a very few basic comments about it are given. Do not worry if this sounds too complicated at the first sight.

Feel welcome to ask further!

Best regards,

Peter

[prashant]

Hi Peter,

            Thanks for your response. It’s really good information for me. I little bit try to understated your answer. Still I am not able to fully digest your comment on my mind. I am trying to understand this.

Meanwhile can you explain little bit more on this so I can understand myself.

In EIGRP if I applied the redistribute commend then what will happen

If I apply the cmd " redistribute ospf" means ospf network will advertised.

If I apply the commend " redistribute connected" means only connected network will advertise in to EIGRP.

Conneted network means directly connected network, not the network cmd applied , like below

networks network 192.168.10.0 0.0.0.255 area 0 for example.

Like the same for static network, BGP network and all other network.

And lastly can you suggest me any good book for Redistribution to understand the full concept.

Regards

Prashant

[Peter Paluch]

Hi Prashant,

Sure, take your time. Redistribution can be a difficult concept.

In EIGRP if I applied the redistribute commend then what will happen

Depending on what keyword would be placed after the redistribute command, those foreign routes will be imported and advertised in EIGRP along with other routes already present and advertised in EIGRP.

If I apply the cmd "redistribute ospf" means ospf network will advertised.

Yes, this is correct. Inside EIGRP configuration, the redistribute ospf tells EIGRP to take OSPF-learned routes from the routing table and advertise them in EIGRP, along with other routes already present and advertised in EIGRP.

If I apply the commend "redistribute connected" means only connected network will advertise in to EIGRP.

Not "only connected networks" - that would be incorrect. The redistribute connected means that along with other routes already present in EIGRP, all directly connected networks will be also advertised.

Conneted network means directly connected network,

A connected, or a directly connected network, is a network on your own interface that is in "Interface up, line protocol up" state. In other words, it is the IP network that is configured on your own active interface.

The network command merely refers to directly connected networks but it never creates them. A directly connected network can be created only by configuring an interface with an IP address and netmask. A network command can be used afterwards to add this interface into the routing protocol, but this command does not create a directly connected network itself.

And lastly can you suggest me any good book for Redistribution to understand the full concept.

I believe there are plenty of them. Look for CCNP:ROUTE textbooks - either the Foundation Learning Guide or the Certification Guide from Cisco Press. And simply by going to Google and looking for "routing protocol redistribution", you'll get dozens of blogs, articles and examples of how to master and use the redistribution.

Best regards,

Peter

[Joseph-E]

Hey Peter,

You are....... an absolute legend! Great post and great way of describing redistribution. Couldn't have put it better, Thanks man.