CCNP Large Scale BGP: AS-Path ACL, Regular expressions (regex): GNS3 CCNP Lab 1.6: Answers Part 7

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AS-path Access list: ACL
ip as-path access-list

To configure an access-list filter for Border Gateway Protocol (BGP) autonomous system (AS) numbers, use the ip as-path access-list command. To remove the filter, use the no form of this command.

ip as-path access-list name { deny | permit } regexp

no ip as-path access-list name { deny | permit } regexp

Regex:
A regular expression is entered as part of a command and is a pattern made up of symbols, letters, and numbers that represent an input string for matching (or sometimes not matching). Matching the string to the specified pattern is called pattern matching.

Pattern matching either succeeds or fails. If a regular expression can match two different parts of an input string, it will match the earliest part first.

Cisco configurations uses regular expression pattern matching in several implementations

Route Filtering and Manipulation
Route filtering is a method for selectively identifying routes that are advertised or received from neighbor routers. Route filtering may be used to manipulate traffic flows, reduce memory utilization, or to improve security. For example, it is common for ISPs to deploy route filters on BGP peerings to customers. Ensuring that only the customer routes are allowed over the peering link prevents the customer from accidentally becoming a transit AS on the Internet. Filtering of routes within BGP is accomplished with filter-lists, prefix-lists, or route-maps on IOS and NX-OS devices.

Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. The protocol is classified as a path vector protocol. The Border Gateway Protocol makes routing decisions based on paths, network policies, or rule-sets configured by a network administrator and is involved in making core routing decisions.

BGP may be used for routing within an autonomous system. In this application it is referred to as Interior Border Gateway Protocol, Internal BGP, or iBGP. In contrast, the Internet application of the protocol may be referred to as Exterior Border Gateway Protocol, External BGP, or eBGP.

BGP neighbors, called peers, are established by manual configuration between routers to create a TCP session on port 179. A BGP speaker sends 19-byte keep-alive messages every 60 seconds to maintain the connection. Among routing protocols, BGP is unique in using TCP as its transport protocol.


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