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BGP attributes and traffic flow passes transit AS

As a general rule, Weight and Local-Preference are used to affect how traffic leaves the autonomous system. AS-Path and MED are used to affect how traffic enters the AS. Since Weight and Local-Preference are higher in the decision process than AS-Path and MED, you (generally) have control of how traffic leaves your AS.

Let us take an example,,,,traffic leaves AS 54 (originate) tarverse AS 400 towards AS 254 (destination),,,,,,,if question states that the configuration should be done on AS 400 as a requirement,,,,when the traffic leaves AS 400 (transit AS) towards AS 254 (destination), that means (to my understanding, may be I am wrong) I have to control the outgoing (outbound) tarffic leaving AS 400 going to AS 254.

That means I have to apply an attributes which affect outbound traffics,,,,as a subsequent,I have to use one of these two attributes (Weight, Local preferences,,,following the rule that I have mentioned above ), and as result, the direction to apply one of these two attributes (Weight, Loacl-Preference),will be inbound ,,,,Am I right in this conculsion ?

I am aware of the

"Traffic flow is always in the opposite direction of the flow of Routing information" from this the link below :

1-Filtering outgoing routing information inhibits traffic flow inbound

2- Filtering inbound routing information inhibits traffic flow outbound"

From the link below:

http://www.pacnog.org/pacnog1/day4/a1-6up.pdf

3 REPLIES

Re: BGP attributes and traffic flow passes transit AS

You're conclusion is correct.

The weight and local preference parameters will determine the preferred path through your AS, and these parameters apply to the local AS. To a lesser extent the MED can be used in the same way inside the local AS, but also can be propogated to influence the path in another AS, however they can choose to ignore this.

Your statement towards the end with regard to the filtering of routing information deals with a different subject. It applies to the filtering of routes. For example if you are connected to two external AS's, and both advertise the same network block and you choose to filter the advertisement of that network from one of the AS's (advertised inbound to you), then your outbound traffic will choose the other AS.

Similarly, if you filter your outbound advertisements to external AS's, then you affect how the traffic comes into your AS.

New Member

Re: BGP attributes and traffic flow passes transit AS

>>You're conclusion is correct.<<

Let us apply that to the following task from Internetwork Expert Lab :

http://www.internetworkexpert.com/downloads/iewb-rs.sample.lab.pdf

One of the Internetwork Expert Lab's task is:

((Parts of the acquisition agreement between AS400 and AS 100 stipulates that AS 400 will not provide transit for traffic coming from AS 54 and its customers that is destined for AS 254. Configure AS 400 to reflect this policy)).

Solution was :

router bgp 400

neighbor 204.12.1.254 route-map STOP_TRANSIT_TO_AS_254 out

!

ip as-path access-list 1 permit _254$

!

route-map STOP_TRANSIT_TO_AS_254 deny 10

match as-path 1

!

route-map STOP_TRANSIT_TO_AS_254 permit 20

Yes, may be there are another solutions which I do not bother at this stage, I am looking to interpret the general rule below to apply it to the task above

The general rule that internetwork expert lab has mentioned is:

((i- Weight and Local-Preference are used to affect how traffic leaves the autonomous system.

ii- AS-Path and MED are used to affect how traffic enters the AS.

Since Weight and Local-Preference are higher in the decision process than AS-Path and MED, you (generally) have control of how traffic leaves your AS.))

My interpretation to his task as follow (I want to find out where is my misunderstood)

1- Did he apply the same rule to the above task ?

2- AS 400 is transit AS.

3- Traffics leave AS400 towards AS 254.

4- That means we have outbound traffic because traffic is comming from AS 54 traveres AS 400 towards AS 254.

5- As internetwork expert rule said that (again may be there are other solutions) : "Weight and Local-Preference are used to affect how traffic leaves the autonomous system (AS in my caes or task is AS 400)".

6- According to the internetwork expert rule (my understanding to his rule), we can choose either "Weight" or "Local-Preference" and according to his rule we can not choose AS-Path and MED!!!!!!

7- According to the internetwork expert rule, (kindly see his table in the URL above), if we want to choose either "Weight" or "Local-Preference", we have to apply it as "in" not "out".

8- If you look to his solution , he has choosen one of the attribute "AS-PATH" which affects how the traffic enters AS (inbound), not how the traffic leaves AS (outbound),,,,but is his task he has got traffic leaves AS 400,,,,here I feel there is a contradiction.

9- According to his rule (kindly see the table in the URL above) we have to apply it as "in" not "out",,,,why did he apply it as "out" ? here also I feel there is other (beside to number 8) contradiction.

10- I am refering to the internetwork expert lab, because he is trying to solve the task above according to the rule that he stated in his booklet.

11- I am aware that the "weight" is locally to the AS.

Re: BGP attributes and traffic flow passes transit AS

The simplest way for AS400 not to provide transit for traffic from AS54 to AS254 would be for AS400 not to advertise AS254 routes to AS54. This would be done with an "as-path" access list applied on outbound BGP advertisements to AS54 as done in example provided. This way AS54 would not receive the AS254 routes from AS400 and would send no traffic in that direction.

However, in the example provided it does not state to which AS the filter is applied. I'd assume AS54. If it is AS254, I'd assume a typo. There would be no reason to apply it towards AS254!

Finally, consider the "as-path" filter as you would an access-list. If AS254 had 100 different originating networks an access-list of as many lines would be required to filter the routes towards AS54. Instead, defining the filter based on the AS is much simpler.

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