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BGP Regular Expressions Cheat Sheet

by ShahedAugust 26, 2011October 22, 20150

BGP Regular Expressions Cheat Sheet Regular expressions are strings of special characters that can be used to search and find character patterns. Within the scope of BGP in Cisco IOS regular expressions can be used in show commands and AS-Path access-lists to match BGP prefixes based on the information contained in their AS-Path. [boxads] In order to understand how to build regular expressions we first need to know what the character definitions are for the regex function of IOS. The below table illustrates the regex characters and their usage. This information is contained in the Cisco IOS documentation under the …

by ShahedAugust 1, 2011October 22, 20150

BGP Best Path Selection Algorithm Cisco router BGP is the protocol used to announce prefixes throughout the internet. It’s a very robust protocol, and very useful to carry lot of prefixes, such as the Internet prefixes or internal client prefixes of an ISP. BGP Best Path Selection Algorithm Cisco router when a prefix is received in BGP, the path passes through two steps before being chosen as candidate to populate the RIB.[boxads] The first step consists on checking if the path is valid. If it is, the prefix will get into the BGP table, and later the second step of …

by ShahedJuly 27, 2011October 22, 20150

BGP MED Attribute Configuration example BGP MED Attribute Configuration example BGP MED is an optional non-transitive attribute meaning its not propagated throughout the whole internet but just to adjacent AS. The word “optional” means that this is not necessarily by default sent with the BGP updates. The purpose of MED is to influence how other autonomous systems enter your AS to reach a certain prefix. If the other attributes are set to default, MED will be the attribute used for path selection however, if Weight or Local preference is configured on the adjacent AS router, then MED will not be …

by ShahedJuly 24, 2011October 22, 20150

Removing Private AS Numbers in BGP Autonomous System (AS) is identified by a number in the range 1 to 65535. AS numbers in the range 1 to 64511 are assigned by InterNIC. These are reserved and globally unique AS numbers. AS numbers in the range 64512 to 65535 are known as private AS numbers, and like private IP addresses, these can be used to address ASs when a globally unique AS number is not necessary. It is important to ensure that the private AS numbers are not introduced into the global BGP table through the AS-Path attribute. To removing private …

by ShahedJuly 23, 2011October 24, 20150

BGP as-override and allowas-in: Today i will discuss basic difference between as-override and allowas-in command & which are configure which router. [boxads] These two functions are pretty similar, just with subtle differences. They can be used in an environment where a customer is using one AS number for many sites that are connected to an ISP. This is shown in the example below. You can see that AS 65001 connects to the ISP at two locations. So when R2 receives the prefix 99.99.99.99/32, he will see that the AS path is via 1, 65001. Because of the loop prevention mechanism, …

by ShahedJuly 20, 2011October 22, 20150

BGP AS path Prepending outbound routes BGP AS path Prepending outbound routes is rich in features that you can have more control than on what IGP’s offer however, you can only have control on how the traffic leaves your autonomous system and can’t really control how other autonomous systems reach you. Other AS’es might have BGP policies that route the traffic in a way you don’t intend it to go. You don’t have control over those because, its their autonomous systems after all. However, there are work arounds which allow, an autonomous system affect the other autonomous systems, one of …