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BGP Community and AS Path Prepend

rhap4boyz
Level 1
Level 1

Trying to analyze a dual ISPs BGP configurations. This is for both Lumen and AT&T ISPs

1.I have the following BGP community.

set community 202:110 202:202

How do I find out what they maps to?  Is there a way to look those up?

2. We have AS number prepended couple times for AS Path for both ISPs

How do I find out whether they are taking effect from ISP's point of view?

Tried looking glass but couldn't find it.  I must be looking at the wrong place.

What is the command to look that up at the CLI level on the router?

 

 

2 Replies 2

M02@rt37
VIP
VIP

Hello @rhap4boyz,

To find out what the BGP communities 202:110 and 202:202 map to, you can refer to the BGP community documentation provided by your ISPs. Typically, ISPs define and document their BGP communities and their intended meanings. Contact your ISPs' technical support or consult their documentation to understand the specific mappings for the communities you are using.

To determine whether the AS path prepending is taking effect from the ISP's perspective, you can use the following steps:

--Verify that the BGP session is established and the routes are being received from the ISPs. Use the command "sh ip bgp summary" to view the BGP session status and ensure that the BGP neighbors are established.

--Use the command "sh ip bgp" to examine the BGP routing table entries received from the ISPs. Look for the prefixes advertised by the ISPs and check their AS paths. Confirm whether the AS path contains the desired number of AS prepends.

--Use the command "show ip bgp neighbor <neighbor-IP> advertised-routes" to view the routes being sent to the ISP. Verify that the AS path being advertised to the ISPs includes the desired number of AS prepends.

--If you couldn't find the required information using the ISP's looking glass service, you can try using an external looking glass service. There are various public looking glass services available that allow you to query the BGP routes and AS path information from multiple perspectives. Examples of popular looking glass services include Hurricane Electric (http://lg.he.net) and bgp.tools (https://bgp.tools/).

Best regards
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can you more elaborate, 
set community is config under the route-map, the name of route-map must match route-map you use in BGP config. or I am wrong ?
also for as prepend can you more elaborate 

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