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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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Filtering on an AS-Path Attribute

You can use the BGP attribute, AS_PATH, to manipulate routing policies.

The AS_PATH attribute contains a sequence of AS numbers representing the route’s path. As the route traverses an AS, the ASN is prepended to the route. You can manipulate routes based on their AS_PATH to affect interdomain routing. By identifying certain ASN in the AS_PATH, you can permit or deny routes based on the number in its AS_PATH.

AS-PATH ACLs use regular expressions to search AS_PATH values. AS-PATH ACLs have an “implicit deny.” This means that routes that do not meet any Match filter are dropped.

To configure an AS-PATH ACL to filter a specific AS_PATH value, use these commands in the following sequence.

  1. Assign a name to a AS-PATH ACL and enter AS-PATH ACL mode. CONFIGURATION mode ip as-path access-list as-path-name
  2. Enter the parameter to match BGP AS-PATH for filtering. CONFIG-AS-PATH mode {deny | permit} filter parameter This is the filter that is used to match the AS-path. The entries can be any format, letters, numbers, or regular expressions. You can enter this command multiple times if multiple filters are desired. For accepted expressions, refer to Regular Expressions as Filters .
  3. Return to CONFIGURATION mode. AS-PATH ACL mode exit
  4. Enter ROUTER BGP mode. CONFIGURATION mode router bgp as-number
  5. Use a configured AS-PATH ACL for route filtering and manipulation. CONFIG-ROUTER-BGP mode neighbor { ip-address | peer-group-name} filter-list as-path-name {in | out} If you assign an non-existent or empty AS-PATH ACL, the software allows all routes.

Example of the show ip bgp paths Command

To view all BGP path attributes in the BGP database, use the show ip bgp paths command in EXEC Privilege mode. 

Dell#show ip bgp paths
                                 Total 30655 Paths
                                 Address   Hash Refcount Metric Path
                                 0x4014154 0    3        18508 701 3549 19421 i
                                 0x4013914 0    3        18508 701 7018 14990 i
                                 0x5166d6c 0    3        18508 209 4637 1221 9249 9249 i
                                 0x5e62df4 0    2        18508 701 17302 i
                                 0x3a1814c 0    26       18508 209 22291 i
                                 0x567ea9c 0    75       18508 209 3356 2529 i
                                 0x6cc1294 0    2        18508 209 1239 19265 i
                                 0x6cc18d4 0    1        18508 701 2914 4713 17935 i
                                 0x5982e44 0    162      18508 209 i
                                 0x67d4a14 0    2        18508 701 19878 ?
                                 0x559972c 0    31       18508 209 18756 i
                                 0x59cd3b4 0    2        18508 209 7018 15227 i
                                 0x7128114 0    10       18508 209 3356 13845 i
                                 0x536a914 0    3        18508 209 701 6347 7781 i
                                 0x2ffe884 0    1        18508 701 3561 9116 21350 i
                                 0x2ff7284 0    99       18508 701 1239 577 855 ?
                                 0x2ff7ec4 0    4        18508 209 3561 4755 17426 i
                                 0x2ff8544 0    3        18508 701 5743 2648 i
                                 0x736c144 0    1        18508 701 209 568 721 1494 i
                                 0x3b8d224 0    10       18508 209 701 2019 i
                                 0x5eb1e44 0    1        18508 701 8584 16158 i
                                 0x5cd891c 0    9        18508 209 6453 4759 i
                                 --More--

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