What does Hot Potato Routing mean ? Why hot potato routing is important ? What is the default policy of the Internet Service Providers ? In which case Hot Potato Routing is not wanted ? I will explain these questions and more in this post.
What does Hot Potato Routing Mean ?
If someone tosses you hot potato, you can’t hold in your hand too much , because it is too hot. It can hurt you. You want to throw it away as soon as possible.
It is exactly the same for networking. When the network traffic needs to leave from the Autonomous System, sending the traffic to the closest exit is called as Hot Potato Routing. For example, if traffic will go to the Internet, sending it to the closes exit point from the network is a Hot Potato Routing.
What does ” closest exit point ” mean ?
Closes Exit point is calculated from the IGP cost distance. If there are two paths to reach to the Internet from my network, whichever has an end to end lower IGP cost, that is considered as closest path to the exit point.
Why Hot Potato Routing is Important ?
If the network keeps network traffic longer in their network, so the more links the network traffic passes, cost of the transport of network traffic increases for the network owner.
Imagine Internet Service Providers. When ISPs receives customer traffic, ISPs by default perform Hot Potato Routing. It sends the customer traffic to the closest BGP next hop which mean closest exit point where ISP is connected with the other networks.
What is the default Traffic Engineering Policy for the Internet Service Providers ?
It is obvious at this point I think that ISPs by default always prefer Hot Potato Routing.
I said by default, because customer has a capability and right to change the ISPs BGP policy.
Instead of sending customers traffic by performing Hot Potato Routing, customer with the BGP Path Attributes, can change automatically the ISP Policy and force ISP to perform Cold Potato Routing.
BGP Policies between customer and the providers for Hot Potato Routing or Cold Potato Routing are done via BGP Communities in general.(AS-Path and MED attribute can be used as well)
Why Hot Potato Policy may not be wanted ?
Let’s look at above topology to understand Hot Potato operation better and discuss why Hot Potato Routing may no be wanted and customer can change the Providers traffic engineering behavior from Hot Potato Routing to Cold Potato Routing.
We have two Providers in the above figure. AS 200 which is the customer, AS 100 is the Provider. Basically AS 100 is receiving an Internet Transit Service from the AS100.
I provided an IGP costs for the links.
We have two connections between the AS 200 and AS 100. San Francisco and New York.
Customer AS which is AS200 wants to send its customer traffic to the content network behind AS 100.
AS 200 performs Hot Potato Routing for its customers , that’s why send the traffic to the closes exit (Lowest IGP cost) which is SF.
When AS100 receives the traffic from AS 200, It basically sends the traffic to the content network. Return traffic from AS 100 to AS 200 goes to NY , because AS 100 does Hot Potato Routing as well.
Here is the trick.
AS200 by basically playing with the BGP Attributes, can receive the traffic from the any interconnection points it wants.
So, instead of receiving the traffic from AS 100 through NY link, AS 200 can receive the traffic over SF circuit which is closer to AS 200 customers.
AS 200 can do this with BGP MED , AS-Path Prepending and BGP Communities.
By sending with the lower MED over SF circuit compare to NY , by prepending AS numbers over the NY or by sending the prefixes from it’s network with BGP community which increases the Local Preference value over the SF circuit, AS 200 have three possible Traffic Engineering Policy for the EBGP connections.
Content farm in the AS 100 network is closer to the NY location from the IGP cost point of view.
But, as AS 200 , as a customer , wants to receive the traffic from its SF connection point , Cold Potato Routing is performed in the AS 100 network.
Cold Potato is called as Best Exit. Best Exit from the neighboring network point of view.
In our example, AS 100 is doing Cold Potato Routing because, AS 200 wanted this and signal that request with one of the above BGP Attribute.
If you have any question regarding this post or want to share your insights , please share in the comment box below.