Controlling the BGP path

Regular Expression

AS path prepend matching regular Expression: ^([0-9]+)(_\1)*$
Match my own routes: ^$
matches prefixes from AS 51 that is directly connected to our AS : ^51_
matches prefixes that transit AS 51 :  _51_
matches prefixes that originated in AS 51, the $ ensures that it’s the beginning of the AS PATH :  _51$
match all AS: (.*)
^: Beginng of the List
$: End of the list
\: Removes the special meaning
_: white space
-: Matches single character

prefix list: Need to match network + mask
– Permit only default route: 0.0.0.0/0
– Permit anything : 0.0.0.0/0 le 32
– Pemit only defult route: 0.0.0.0/0 ge 32
– Permit class A subnet with mask exact 24: 0.0.0.0/1 ge 1 le 1

BGP Neighbour RESET:
-Soft reconfiguration will consumes memory since it will store the routes in filter.
– Inbound soft reconfiguration you need to configure first then need to apply the command.
– On 12.1 cisco added the ability which saves the memory called as Route Refresh. It will send request to other router to re-send all the routes again. This doesn’t need any pre-requist configuration.

Outbound Route filter(ORF):
-ORF transmits inbound filter to apply it outbound on other end. Neighbour must support ORF filter.
neighbour 1.1.1.1 capability orf prefix-list in/out/both

Communities:
-32 bit long(old format)
– You can use the Regular Expression in extended community.(100-199)

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