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BGP Inbound Route Preference

Posted on 2008-10-11
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Hello, I am completely new to BGP and was wondering how I can modify routes coming IN from the Internet. I have two carriers providing me each with a class C block of addresses. I'd like to have both blocks coming in through a single carrier which it currently is, but through the wrong carrier. How can I swap it to the second carrier? Thanks!
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Question by:MardukLitzer
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by:sstone55423
ID: 22694161
Weight Attribute
Weight is a Cisco-defined attribute that is local to a router. The weight attribute is not advertised to neighboring routers. If the router learns about more than one route to the same destination, the route with the highest weight will be preferred. In Figure 39-2, Router A is receiving an advertisement for network 172.16.1.0 from routers B and C. When Router A receives the advertisement from Router B, the associated weight is set to 50. When Router A receives the advertisement from Router C, the associated weight is set to 100. Both paths for network 172.16.1.0 will be in the BGP routing table, with their respective weights. The route with the highest weight will be installed in the IP routing table.
Figure 39-2 BGP Weight Attribute
 
See http://www.cisco.com/en/US/docs/internetworking/technology/handbook/bgp.html#wp1020576
 
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by:MardukLitzer
ID: 22694203
so both of my routers show weight 0 for the route in question... Should I set it higher on the one where I'd like it to come in?
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by:sstone55423
ID: 22694315
" the route with the highest weight will be preferred. " So, yes.
 
One thing I noted is that your question said "I'd like to have both blocks coming in through a single carrier which it currently is"  I am not sure I get that.  Isn;t one block coming in through one carrier, and the other block from the other carrier?
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JFrederick29 earned 500 total points
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Weight will only affect outbound routing.  If you want inbound traffic to prefer one ISP over the other, you can prepend your as-path a few times when advertising routes to the less preferred ISP.  This makes the primary ISP the more preferred path.  See the following for an example and explanation:

http://blog.ioshints.info/2008/02/bgp-essentials-as-path-prepending.html
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by:MardukLitzer
ID: 22704098
I will be giving this a shot tonight... Another question I have is, can I set the iBGP to send traffic originating from a class C through a particular router and the other class C through the second router?
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by:JFrederick29
ID: 22704215
You mean outbound traffic from yourself to the Internet, right?

Without knowing anything about your topology...

One way you can control this is via HSRP on your BGP routers.  You can make one router the active HSRP router for the subnet that you want routed out one ISP and the other router the HSRP active router for the other subnet.  If this method of control isn't a viable option, Policy Based Routing could be used on the hop prior to your BGP routers.
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by:MardukLitzer
ID: 22704266
Ok, I have a default route to one of the gateways and from there it goes out over our DS3 link, or if that is down it goes out over our other circuit...So currently the iBGP will always go out over the DS3 unless there is a failure... What could I change with it to sort of do a selection process based on source network?
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by:JFrederick29
ID: 22704309
What device has the default route to your BGP router?  Is it capable of policy routing?  Policy routing (PBR) in Cisco speak allows you to route based on source.  Are you only receiving a default route from your ISP's?  Do you have local preference for the default setup?
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by:MardukLitzer
ID: 22704351
My ASA has the default route, running 7.2(4). I am not sure if it is capable of policy routing. I am receiving only default routes, and currently I have local preferences set up (as of tonight if it works)...
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by:JFrederick29
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Yeah, the ASA doesn't do policy routing and local preference influences routes learned from your ISP (default in this case) so that won't help with your source routing dilemma.  You can use PBR on your edge BGP routers (assuming Cisco something) but you lose the dynamic element of routing via BGP.
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