BGP attributes for manipulate inbound traffic

[prashanma]

Hi Team,

Which two BGP attributes can be set with outbound policy to manipulate inbound traffic, if honored by the remote autonomous system ?

[balaji.bandi]

This is from my notes : hope this information helps you.

- 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.

-  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.

-  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.

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.

-  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.

[johnd2310]

Hi,

You can use AS-Path and MED

http://www.ciscopress.com/articles/article.asp?p=2738462&seqNum=2

http://www.ciscopress.com/articles/article.asp?p=2756480&seqNum=5

Thanks

John

[Seth Beauchamp]

Just to echo the others, you would use MED and/or AS prepending.

If for example you had two links to the same service provider, you could use MED to influence which link the service provider uses to send traffic into your network. MED will not carry over to your service provider's peers however, but in this particular case that should not matter since you are only connected to a single service provider.

If you had two links from two different providers, you may want to prepend your AS several times (i usually do 3x) toward one of the service providers to influence both your service providers network as well as the service providers external peers. If you have SP A and SP B, with the intent of SP A being primary, since MED doesn't transfer beyond the service provider's AS their external peers may prefer SP B for example, causing traffic to ingress on your SP B link. With AS prepending you are more likely to achieve the result of making SP A primary for all traffic.

Of course its all a "gentlemen's agreement", since the service providers dont necessarily have to honor your attributes.