topic Re: BGP MED Use Case in Network Security

BGP MED Use Case

Ganesh Devarshetty — Sun, 21 Jul 2024 15:54:44 GMT

Can i use MED in above case to influence inbound traffic.R1 & R2 are advertising same prefixes to three different ISPs.Outbound traffic is influence by local preference.

Re: BGP MED Use Case

MHM Cisco World — Sun, 21 Jul 2024 16:20:17 GMT

This all your topolgy?

If yes then there is one link between each AS so no need MED at all.

MHM

Re: BGP MED Use Case

M02@rt37 — Sun, 21 Jul 2024 19:43:24 GMT

Hello @Ganesh Devarshetty

Assuming ISP provide R1 and R2 are connected to ISP providers, using AS path prepending can effectively influence inbound traffic in your network setup.

By artificially lengthening the AS path attribute in BGP advertisements, you can make certain routes less attractive to your ISPs, thereby directing traffic to preferred entry points. For instance, if you want R1 to handle more traffic, you would prepend additional AS numbers to the advertisements from R2, making its path seem longer and less favorable. This method can help balance the load between R1 and R2 without upgrading your links.

The AS path is one of the BGP attributes, it's a well-known mandatory attribute which means that it's included with all prefixes that are advertised through BGP. When a BGP router advertises a prefix, it will include its own AS number to the left of the AS path attribute.

Re: BGP MED Use Case

Ganesh Devarshetty — Mon, 22 Jul 2024 13:10:53 GMT

Remember, MED is relevant when comparing routes from the same AS. If you want to consider MED across different ASes, use the always-compare-med command.

Re: BGP MED Use Case

Ganesh Devarshetty — Mon, 22 Jul 2024 13:11:32 GMT

Remember, MED is relevant when comparing routes from the same AS. If you want to consider MED across different ASes, use the always-compare-med command. Pls confirm

Re: BGP MED Use Case

MHM Cisco World — Mon, 22 Jul 2024 13:17:05 GMT

That true if AS-z and AS-B advertise same prefix

Then R4 and R5 can push MED to R1

And R1 can config to use MED to prefer path to AS-Z or AS-B

But the topology ypu share NOT mention that both AS z and b advertise or interconnect

That why my answer is no need.

And it true you need

Med always-compare

Since the R1 recieve MED from two different AS

MHM