From: Jay Hennigan (jay@west.net)
Date: Mon May 12 2003 - 14:44:45 GMT-3
On Sun, 11 May 2003, Robert Yee wrote:
> Given the following scenario, how can I prevent my AS from becoming a transit AS:
>
> ----- -----
> |AS100| |AS300|
> ----- -----
> \ /
> \ /
> ---------------------------
> | \ / |
> | R1----iBGP---R2 |
> | \ / |
> | \ / |
> | OSPF OSPF |
> | \ / |
> | \ / |
> | R3 |
> |AS200 (OSPF) |
> ---------------------------
>
> AS 300 advertises 10.1.1.0 /24 to AS 200 through R2.
>
> The 2 edge routers in AS 200 are running BGP and OSPF. R3 is running OSPF and has numerous routes that it is advertising to R1 and R2. Syncronization in BGP is disabled, so there should be no issue with bgp/ospf sync.
>
> The 2 edge routers are running mutual redistribution between the IGP and EGP.
>
> R2 redistributes 10.1.1.0 /24 from BGP into OSPF. Eventually, this route will reach R1 from OSPF (and from iBGP) and will be redistributed back into BGP from OSPF. Since OSPF has a better AD than iBGP, the OSPF route will be placed into routing table, and BGP will pick the redisributed route as the best path. This route will then get advertised to AS 100.
>
> Because OSPF is redistributing many routes to BGP, I think the best solution would be to put the following filter-list (skip ditribute-list, prefix-list, route-map) on each edge router:
>
> ip as-path access-list 1 permit ^$
>
> This filter list should only allow routes out that have no AS-PATH attribute associated with it.
>
> This should prevent AS 200 from being a transit area (if we were only running BGP), BUT since we are mutually redistributing, 10.1.1.0 /24 now is seen by BGP on R1 as route w/out an AS-PATH attribute.
>
> Then 10.1.1.0 /24 gets advertised out to AS 100.
>
> Is there any way around this?
Look at how the BGP routes are injected into OSPF, and how you can control
this. Then look at how you can control what gets redistributed into BGP.
Hint:
(config-router)# redistribute ospf 1 match ?
-- Jay Hennigan - CCIE #7880 - Network Administration - jay@west.net NetLojix Communications, Inc. - http://www.netlojix.com/ WestNet: Connecting you to the planet. 805 884-6323
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