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vvasilev
Posts: 7
Registered: ‎10-11-2009
0

BGP as-override

Hello,

 

I am playing with BGP and as-override and I've come across the following:

 

PE(AS1)---ebgp---CE(AS65000)

 

The CE sends a route to its PE and the PE sends the same route back to the CE. The AS Path gets changed to "AS1 AS1" and the route is hidden on the CE. Why would the PE send the route back?

 

I've tested this on Cisco IOS and ExtremeOS (I was curious) and the route was not sent back.

 

Thanks!

 

Regards,

Vladislav A. VASILEV

Distinguished Expert
Distinguished Expert
pk
Posts: 801
Registered: ‎10-09-2008
0

Re: BGP as-override

Hi

 

This is a known behavior of Junos, described in KB

 

http://kb.juniper.net/InfoCenter/index?page=content&id=KB2222&actp=search&searchid=1234126984527&sml...

 

Shorlty, they don't do split horizon by default (not like IOS does).

 

This behavior is not related with as-override.

 

Best Regards,
Petr (PK)

Juniper Ambassador, Juniper Networks Certified Instructor,
JNCIE-SEC #98, JNCIE-ENT #393, JNCIE-SP #2253
[Juniper Authorized Education & Support in Russia]
Regular Visitor
Chintan
Posts: 4
Registered: ‎10-10-2009
0

Re: BGP as-override

Hi,

 

Unfortunately, I am not able to open link mentioned in reply so asking this again.

What should i do in this case with as-override feature so that PE don't send prefix back to CPE.

If i write export policy and match CPE AS , will as-override kick off first or policy ?

 

Regards,

Chintan

Distinguished Expert
Distinguished Expert
pk
Posts: 801
Registered: ‎10-09-2008
0

Re: BGP as-override

Here's cut-and-paste from that article. Again, it has nothing to do with as-override.

 

---

 

JUNOS does not perform a BGP Split Horizon.

Generally, Split Horizon in BGP is unnecessary, since any routes which might be received back by the originator will be less preferred due to AS Path length (for EBGP), AS Path loop detection (IBGP), or other BGP metrics. Advertising routes back to the neighbor from which they were learned will have a negligible effect on the router's performance, and is the correct thing to do.

If you do not want to advertise back routes learned via EBGP, an export policy can be used to provide a Split Horizon-like functionality to prevent advertising routes back to the EBGP peer from which they were learned. The following example policy does this by inspecting the AS Path attribute of the routes being advertised.

EXAMPLE
========

    [edit]
    policy-options {
        policy-statement policy_1 {
          term term_1 {
            from as-path tested-aspath;
            then reject;
          }
        }
        as-path tested-aspath 65500;
    }

Then export this policy in the specific EBGP peer group for the peers AS. 

Best Regards,
Petr (PK)

Juniper Ambassador, Juniper Networks Certified Instructor,
JNCIE-SEC #98, JNCIE-ENT #393, JNCIE-SP #2253
[Juniper Authorized Education & Support in Russia]
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