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Advice on BGP path selection please


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#1 LTHLRB

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Posted 06 March 2013 - 04:22 PM

Hi All, 

 

I have re-designed our corp Internet design for our company. Everything is working, however we have some routing issues which I am trying to sort.

 

Scenario:

 

2 Data Centres - Both have 2 Internet Links from our ISP(1 is for National Traffic, and 1 is International).

 

We have 2 routers in each DC, running HSRP.

We are running OSPF on the LAN side and have failover running via GRE tunnels - The international router has a GRE tunnel to the OTHER DC's Int/Nat Router, and vice versa - If one of the DC's WAN link breaks, it fails over to the other DC via GRE tunnels. 

 

With me so far? 

 

We are running BGP between our routers, and the ISP. Please note, that our 4 routers ARENT running IBGP, and all are running the same AS.

 

The issue - INBOUND - Path selection for certain subnets. We own a /24 subnet, and each of the 4 routers adv's a /25(In the event of a circuit failure, we adv a /24 as well, which goes across the GRE tunnels

 

With me so far? lol.

 

Actual issue All traffic coming into 1 of the DC's is coming in via the International router - This INCLUDES National traffic as well!!!

 

What SHOULD happen - ALL National traffic SHOULD go out AND come back in VIA the Nat. All International traffic Should go out the Int. Int the event that the Nat router DOESNT have a route, it passes the traffic to the Int(Deemed Int, and passes it to its gateway), also, if the Nat router dies, the nat traffic goes OUT via the Int(It doesnt allow Nat traffic to come in)

 

Please note, that OUTBOUND, its working fine, its ONLY incoming traffic.

 

What I did - Used MED's to influence the National routes to come via the National router - When doing tracert tests, ALL traffic is coming in via the National now, lol wtf!

 

The Route-map set meds for a /25 only, and I also did an implicit allow and set the meds for any other route to 100(default).

 

My question is - How do I influence the National router to ONLY receive the traffic destined for our /25 subnet? In essense - All National traffic - come via the national router, all int traffic - come via Int. 

 

I hope thats not too confusing!

 

I would have thought that sticking meds will do the trick but obviously not.


Edited by LTHLRB, 06 March 2013 - 04:26 PM.

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#2 Scales

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Posted 21 March 2013 - 10:59 AM

I haven't read everything above or have the time to try to fully understand what you are doing.

However, MED is a fairly weak BGP attribute and not all AS's honour it.

 

A better way to influence traffic inbound is to use AS path pre-pending.

 

Good luck.

 


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