{"id":737,"date":"2018-07-08T13:17:41","date_gmt":"2018-07-08T06:17:41","guid":{"rendered":"https:\/\/lagonet.vn\/?p=737"},"modified":"2018-07-08T13:17:41","modified_gmt":"2018-07-08T06:17:41","slug":"manipulate-bgp-inbound-and-outbound-traffic","status":"publish","type":"post","link":"https:\/\/kb.lagonet.vn\/?p=737","title":{"rendered":"MANIPULATE BGP INBOUND AND OUTBOUND TRAFFIC"},"content":{"rendered":"

According to the BGP best route decision, best route will be chosed base on the the the sequence attributes ( Weight, Local preference, AS-Path, ). But I want to know do that attributes control the flow of inbound Traffic or outbound traffic or both directions?<\/p>\n

1. Local Preference is used to manipulate traffic going out of your Autonomous System. This attribute is only exchanged with IBGP peers and therefore has local significance to your Autonomous System.<\/p>\n

2. The Weight attribute is also used to manipulate traffic going out of your Autonomous System but this attribute is local to the router on which it is configured and is not exchanged with any IBGP or EBGP peers.<\/p>\n

3. A router uses the AS_PATH attribute in case of ties between the weight and local preference attributes and the route not being locally originated. When this attribute is used to select a particular route you are basically controlling traffic going out of your Autonomous System.<\/p>\n

When you are multi homed to two different providers it is sometimes required to prefer one provider’s link for some incoming traffic. In such cases a normal practise is to manipulate the AS_PATH attribute thereby making one path seem more attractive than the other (shorter AS_PATH). So in such circumstances you are using AS_PATH to manipulate traffic coming into your AS.<\/p>\n

4. Another attribute that is used to manipulate incoming traffic is called MED (Multi-Exit Discriminator). However, an EBGP peer receiving it will only share it with IBGP peers and will not forward it to another AS. This means that you can basically use this attribute for influencing traffic between two directly connected autonomous systems.<\/p>\n

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With point number 3 i was trying to convey the message that AS_PATH attribute can influence both traffic coming into and going out of your autonomous system. Lets consider two situations:<\/p>\n

Situation 1<\/u><\/b>:<\/p>\n

You have two edge routers each talking EBGP to a different service provider and talking IBGP to each other. You are receiving full internet routes from both providers. In this situation each edge router will have to decide on which route to install (for each internet destination), one available via an EBGP peer and the other via an IBGP peer. Assuming Local Preference and Weights are left at default the decision will come down to AS_PATH attribute. The routers will choose the route with the shortest AS_PATH and this will influence traffic leaving outside your autonomous system.<\/p>\n

Situation 2<\/u><\/b>:<\/p>\n

In this scenario we again have two edge routers speaking EBGP to two different providers and talking IBGP to eachother. The routers are configured to advertise the prefix assigned to your AS. Without any special configuration traffic destined to your AS will take the best route (shortest AS_PATH). So there is really no way of knowing how your inbound traffic flow will be. However, you might want to force all inbound traffic through one link and use the other as a backup. For this to work across two different providers you can manipulate the AS_PATH attribute in updates from one of your edge routers so that to the outside world it seems that this path is longer than the one through the other link.<\/p>\n

MED is useful when you have two or more links to the same provider and you want to control how traffic coming in should behave. You advertise MED to your EBGP peers so your internal IBGP configuration is more or less independent of this attribute unless you are a provider and are receiving MED but in that case I do not see why you will not have IBGP configured.<\/p>\n

Hope that makes things clearer.<\/p>\n

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BGP – Understanding Inbound Traffic Engineering<\/h1>\n
Created by\u00a0Shadowjk – CCNP(R&S), CCNA(Security; SP)<\/a>\u00a0on Dec 27, 2014 4:15 PM. Last modified by\u00a0Shadowjk – CCNP(R&S), CCNA(Security; SP)<\/a>\u00a0on Dec 27, 2014 4:22 PM.<\/div>\n<\/header>\n
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In this document I aim to explain the various options that you as a Network Operator can use to influence your inbound traffic flow via BGP. This topic is critical to understand for the CCNP ROUTE exam and introduces you to some good theory with regards to BGP policy enforcement.<\/p>\n

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Prerequisite knowledge<\/strong><\/p>\n

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