BGP Regex to customers

Hello User. we are reviewing your question and we will answer you shortly. Thanks.

Hi,

What you need to match if you're talking about incoming prefixes then is "MyCustomer AS" & "Customer of MyCustomer AS". "MyAS" will only be seen once the prefixes leaves your AS to an upstream router.

For your scenery you can use the following Regex if the routes are only generated in AS 700|900:
1. Inbound MyCustomer AS = 500 > Customer of MyCustomer AS = 700 >> 500,700
^500_700+
2. Inbound MyCustomer AS = 500 > Customer of MyCustomer AS = 900 >> 500,900
^500_900+

You can test it using eNSP or routeviews:
http://routeviews.org/
Use the following command to test it:
show ip bgp regexp ^1221_38803+

Hope it helps!

Thanks @Herediano

You're right, AS300 will not be involved in this scenario, I complicated the scenario quoting it

As you said, the input filter is going to analyse only the Customer AS-Path (AS500) and Customers of Customer (AS700 or AS900)


I tough about this regex:

^(500)+((700)*|(900)*)$

Now I need to test if it will match some possibilities:

* Customer AS500 prefixes only (with prepends or not);
* Customer AS500 prefixes (with prepend or not) plus AS700 prefixes (with prepends or not)
* Customer AS500 prefixes (with prepend or not) plus AS900 prefixes (with prepends or not)

The next step is to test the underscore/underline wildcard placing to match all these situations

Hi,
The regex ^(500)+((700)*|(900)*)$ will match AS number sequence below:
500[500 500 ......][any other AS sequences][[700 700 700 ......][900 900 900 ......]]
I'm not sure what you mean 'with prepends or not'. Can you explain that?

Hi,

I mean to allow any combination of the AS inside parentheses (capture group) to use prepends ( + = one or more) ( * = zero or more)

Examples:

500
500,500,500,500

500,500,500,700
500,500,700,700

500,900,900,900
500,500,500,900

Yes, the regex could match your requirement. But you should modify the regex as below:
^(500_)+((700_)*|(900_)*)$

Hi,

I made a lab to test this scenario. I tried the following:

On R1:

#

sysname R1

#

bgp 300

 router-id 1.1.1.1

 peer 10.1.2.2 as-number 500

 peer 10.1.2.2 description R2

 peer 10.1.2.2 password simple 1122

 #

 ipv4-family unicast

  undo synchronization

  import-route direct

  peer 10.1.2.2 enable

  peer 10.1.2.2 route-policy AS500-IN import

#

route-policy AS500-IN permit node 10

 if-match as-path-filter AS500-IN

#

ip as-path-filter AS500-IN permit ^(500_)+((700_)*|(900_)*)$

--------------------------------------------------------------------------------

Another format I tried (multiples regex):

#

ip as-path-filter AS500-IN permit ^(500_)+$

ip as-path-filter AS500-IN permit ^(500_)+(700_)+$

ip as-path-filter AS500-IN permit ^(500_)+(900_)+$

#

All of them were accepted in both formats:

500

500 700

500 900

--------------------------------------------------------------------------------

Then, I made a new test, to block AS900, and intentionally changed AS900 to AS800 on as-path-filter: 

#

ip as-path-filter AS500-IN permit ^(500_)+((700_)*|(800_)*)$

#

After that, success! only AS500 and AS700 (even with prepends) were accepted by filter and AS900 was dropped:

 Total Number of Routes: 13

      Network              NextHop        MED        LocPrf    PrefVal Path/Ogn

 *>   1.1.1.1/32            0.0.0.0         0                     0      ?

 *>   1.11.11.11/32        0.0.0.0         0                     0      ?

 *>   2.2.2.2/32          10.1.2.2        0                     0      500 500 500 500?

 *>   2.22.22.22/32    10.1.2.2        0                     0      500 500 500 500?

 *>   3.3.3.3/32          10.1.2.2                              0      500 500 500 500 700 700 700 700 700?

 *>   3.33.33.33/32    10.1.2.2                              0      500 500 500 500 700 700 700 700 700?

 *>   10.1.2.0/30         0.0.0.0         0                     0      ?

                                  10.1.2.2        0                     0      500 500 500 500?

 *>   10.1.2.1/32          0.0.0.0         0                     0      ?

 *>   10.2.3.0/30         10.1.2.2        0                     0      500 500 500 500?

 *>   10.2.4.0/30         10.1.2.2        0                     0      500 500 500 500?

 *>   127.0.0.0             0.0.0.0         0                     0      ?

 *>   127.0.0.1/32        0.0.0.0         0                     0      ?

Thanks

Thanks for sharing