Redistribution in ospf

Routing & Switching OSPF
1

Please help me on below two queries.

  1. In ospf , what is difference between
    under the router ospf
    redistribute connected and redistribute connected subnets ?

  2. In which scenario/use case E1 metric will be used. Is there any lesson/lab on this.

2

Hello Manish

Without the subnets keyword, only routes that are in the routing table with the default classful mask will be redistributed. When you use the subnets keyword, the subnets from the connected interfaces are redistributed with their appropriate prefixes. A detailed explanation can be found at this Cisco documentation:

The following lab may be of help for this question:

You may also find this post useful:

If you have any further questions, please let us know!

I hope this has been helpful!

Laz

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3

Thank you very Laz. One more doubt on OSPF, How routing loop is handled in ospf. Basically what scenario a loop can happen in ospf.

4

Hello Manish

OSPF, as a link-state protocol, enables each router to have a complete map of the network. This helps in preventing most routing loops. Secondly, as OSPF networks get larger, they are separated into distinct areas. These areas are all obligated to connect to area 0, thus creating a loop-free inter-area topology where traffic from one area to another must go through area 0.

This whole design of OSPF helps to alleviate the possibility of having routing loops. There is no “loop avoidance” technique employed by OSPF since loop avoidance is built into the shortest path first (SPF) algorithm.

Routing loops may be introduced into OSPF whenever you interconnect with other routing domains running different routing protocols, especially if you have two or more connections to these other domains. In such a case, you can use route tagging to avoid loops. This is further described in the following lesson:

I hope this has been helpful!

Laz

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5

Thank you Laz. Appreciate your response.

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6

Hi @ReneMolenaar @lagapidis

Currently in my work place i been asked to fix some configuration issues in our core devices. The set up is as below.. In our core switch ,we have quite a bit of vlans and we have SVI’s for those vlans and acting as the gateway for those vlans. Now we have ospf configuration between core switch and wan router where all the routes will be redistributed in to ospf from the core switch.
In our core switch we have the below configuration

router ospf 1 
redistributed connected

now i been asked to use route-map associated with prefix list to control what’s being redistributed in to ospf. 2 remove the summary statement hence i have the below test lab done .

CORE ---->WAN ROUTER

Core switch
==========

ip prefix-list FILTERTHIS seq 5 permit 192.168.0.0/24
ip prefix-list FILTERTHIS seq 10 permit 10.0.0.0/24
ip prefix-list FILTERTHIS seq 15 permit 20.0.0.0/24
ip prefix-list FILTERTHIS seq 20 permit 77.77.77.76/30
ip prefix-list FILTERTHIS seq 25 permit 192.168.60.0/24



route-map static_ospf permit 10
 match ip address prefix-list FILTERTHIS
route-map static_ospf deny 20



router ospf 1
 redistribute connected subnets route-map static_ospf
  1. would it be sufficient with the above route-map statements or any suggestions regarding the route-map is appreciated.
    2.. In our production switch we have summary address configured under ospf and i have been asked to remove that statement as well . will this cause any outage ?

Thanks

7

Hello Indika

At first glance, your route map and prefix list configuration should effectively filter the connected networks being redistributed into OSPF.

As for the summary address, if you simply remove it, OSPF should reconverge by sharing the specific routes rather than summarizing. You shouldn’t have anything more than a few seconds of downtime, however, NEVER implement this during normal operating hours, but do it during a maintenance window to minimize the impact.

Now having said that, I suggest you take a look at the following lesson concerning OSPF summarization to learn more about how it behaves. Once you understand it more fully, you’ll be in a position to make a more informed decision about how to proceed.

If at all possible, I suggest you emulate this whole process on an emulator like GNS3 or Cisco CML to see how it behaves before implementing it on your production network. That way, you can experience the actual behavior and make the changes more responsibly.

I hope this has been helpful!

Laz

8

Thanks @lagapidis appreciate your advise on that

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