Redistribution from BGP to OSPF

Answered Question
Jan 28th, 2008
User Badges:

I've private AS connected to other private AS's via 2 routers. I received route at first router with as-path 3 and on the second with as-path 4(i've made it for influence traffic). But during redistribution to OSPF all as-path information will be lost. How to preserve that info and use it later in OSPF domain for managing routes?

Correct Answer by Jon Marshall about 9 years 2 months ago

Ah well, you never said you wanted to do anything with them :)


If you want to set metrics you are better doing it when you redistribute - on the 7200 in our example. So you still use the as-paths to identify the relevant routes but then just set the metric you need.


Jon

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4.5 (2 ratings)
Loading.
royalblues Mon, 01/28/2008 - 07:36
User Badges:
  • Green, 3000 points or more

You can always modify the metric while redistributing so that one router is always preferred inside your ospf domain


Narayan

Jon Marshall Mon, 01/28/2008 - 07:58
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Hi


I've just checked this in our lab. The as-path information is preserved as a tag automatically for you. To see this do a


sh ip route "IP subnet" "subnet mask"


on an OSPF route redistributed from BGP and you will see the tag.


You can then use a route-map to match these tags and apply whatever policies you want to them.


Jon

rmv72 Mon, 01/28/2008 - 22:42
User Badges:

Hi

Position tag shows me only last AS number ( Tag 65334)-


pdc-cat6506-obs# sh ip route 10.20.9.0

Routing entry for 10.20.9.0/24

Known via "ospf 1", distance 110, metric 1

Tag 65334, type extern 2, forward metric 1

Last update from 9.9.9.2 on GigabitEthernet5/1, 18:19:09 ago

Routing Descriptor Blocks:

* 9.9.9.2, from 172.22.21.222, 18:19:09 ago, via GigabitEthernet5/1

Route metric is 1, traffic share count is 1

Route tag 65334


pdc-cat6506-obs# sh ip route 10.20.16.0

Routing entry for 10.20.16.0/24

Known via "ospf 1", distance 110, metric 1

Tag 65334, type extern 2, forward metric 1

Last update from 9.9.9.2 on GigabitEthernet5/1, 18:19:22 ago

Routing Descriptor Blocks:

* 9.9.9.2, from 172.22.21.222, 18:19:22 ago, via GigabitEthernet5/1

Route metric is 1, traffic share count is 1

Route tag 65334



Here info from BGP table -

*> 10.20.9.0/24 2.2.2.100 0 65334 i

*> 10.20.16.0/24 2.2.2.201 0 65334 2854 65011 i

dongdongliu Mon, 01/28/2008 - 23:39
User Badges:

Ospf does not understand the attributes of bgp ,you1`d better use metric xx command when redistribute was configured

rmv72 Mon, 01/28/2008 - 23:55
User Badges:

Should i use route policy during redistribution from bgp to ospf where i can match as-path and set metric?

Jon Marshall Mon, 01/28/2008 - 23:59
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Hi


This is exactly what i'm trying at the moment in the lab - can't seem to get it working just yet but there is a lot going on in my lab :)


I can't see why this wouldn't work though.


** Edit - okay just got it working, my mistake on the as-path access-list. So yes, you can do this on the router that redistributes between BGP and OSPF. **



Jon

rmv72 Tue, 01/29/2008 - 00:05
User Badges:

Jon, can you post part of config for example?

Jon Marshall Tue, 01/29/2008 - 00:13
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Okay, i did not emulate your setup entirely so you will need to use regexs in your as-path access-lists.


R5 -> R1 -> 7200 -> R4


R5 is in BGP AS 20

R1 is in BGP AS 3

7200 is in BGP 10


R5 peers with R1, R1 peers with 7200 and 7200 uses OSPF to peer with R4.


The routes received by 7200 from R1


192.168.31.0 - as path 3

192.168.41.0 - as path 3 20 <-- this is the one we are setting tag of 200 on.


router ospf 10

log-adjacency-changes

redistribute bgp 10 subnets route-map MPL

network 172.16.22.0 0.0.0.255 area 0

!

router bgp 10

no synchronization

bgp log-neighbor-changes

network 10.9.1.0 mask 255.255.255.240

network 172.16.22.0 mask 255.255.255.0

neighbor 192.168.5.1 remote-as 3

no auto-summary

!


ip as-path access-list 2 permit 3 20

!


route-map MPL permit 10

match as-path 2

set tag 200

!

route-map MPL permit 20



Output from R4


R4#sh ip ro 192.168.31.0

Routing entry for 192.168.31.0/24

Known via "ospf 1", distance 110, metric 1

Tag 3, type extern 2, forward metric 1

Last update from 172.16.22.1 on FastEthernet0/1, 00:12:39 ago

Routing Descriptor Blocks:

* 172.16.22.1, from 192.168.6.2, 00:12:39 ago, via FastEthernet0/1

Route metric is 1, traffic share count is 1

Route tag 3



R4#sh ip ro 192.168.41.0

Routing entry for 192.168.41.0/24

Known via "ospf 1", distance 110, metric 1

Tag 200, type extern 2, forward metric 1

Last update from 172.16.22.1 on FastEthernet0/1, 00:13:04 ago

Routing Descriptor Blocks:

* 172.16.22.1, from 192.168.6.2, 00:13:04 ago, via FastEthernet0/1

Route metric is 1, traffic share count is 1

Route tag 200


HTH


Jon


rmv72 Tue, 01/29/2008 - 00:24
User Badges:

And now i can have routes at R4 with different route tags. But how to use it is later? I mean how to convert Route tag to metric which can influence which route to use )?

Correct Answer
Jon Marshall Tue, 01/29/2008 - 00:37
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Ah well, you never said you wanted to do anything with them :)


If you want to set metrics you are better doing it when you redistribute - on the 7200 in our example. So you still use the as-paths to identify the relevant routes but then just set the metric you need.


Jon

royalblues Tue, 01/29/2008 - 00:40
User Badges:
  • Green, 3000 points or more

you would be better off configuring the metric while redistribution


Make sure the router that is setting the as-path prepend redistributes with a higher metric


Narayan

Jon Marshall Tue, 01/29/2008 - 00:44
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Narayan


After messing around in a lab with route-maps, as-path's etc. i came to the same conclusion as you. The fact that you got straight to the point while i went round the houses should tell me something :)


Jon

royalblues Tue, 01/29/2008 - 00:54
User Badges:
  • Green, 3000 points or more

There is always something we get to learn when you play around in the lab. worth the time spent


I keep doing it all the time :-)


good day..

can i get to know your mail address



Narayan

rmv72 Tue, 01/29/2008 - 00:52
User Badges:

Narayan, i didn't understand your warning.

If i use as-path prepending i don't set metric because (if i correct) during flowing through transit AS MED will be lost?

Or did you mean something else?

royalblues Tue, 01/29/2008 - 01:09
User Badges:
  • Green, 3000 points or more

When i meant metric it is the ospf metric and not the BGP MED so that the internal routers chose the correct border router for exit


Narayan



Actions

This Discussion