Applying Bgp As_Path Regular Expressions - Huawei Quidway NetEngine80E Configuration Manual

HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
Checking the Configurations
After you disable the BGP-IPv4 unicast address family, run the display current-
configuration configuration bgp | include default ipv4-unicast command to check the
configurations. For example:
# Display the BGP-IPv4 unicast address family configuration.
<HUAWEI> display current-configuration configuration bgp | include default ipv4-
unicast
undo default ipv4-unicast

8.25 Applying BGP AS_Path Regular Expressions

A regular expression is a string of characters that define a rule, against which other strings of
characters are matched. For example, you can define an As_Path regular expression for an
AS_Path Filter and match the AS_Path information carried in BGP routes against the expression
to filter the BGP routes.
AS_Path Components
An AS_Path consists of one or more AS_Path components. The components are expressed using
binary numbers, parentheses "( )", brackets "[ ]", braces "{ }", and spaces. The AS_Path
components are as follows:
l
l
l
l
Figure 8-7
Figure 8-7 Format of AS_Path in a BGP routing table
AS_Path: (65001 65003) [65002 65004] 100 200 300 {400 500}
AS_Path Regular Expressions
An AS_Path filter uses a regular expression to define matching rules. A regular expression
consists of the following parts:
Issue 02 (2014-09-30)
AS_Sequence: records in reverse order all the numbers of the ASs through which a route
passes from the local device to the destination.
AS_Set: records without an order all the numbers of the ASs through which a route passes
from the local device to the destination. In most cases, AS_Set is used after route
summarization because BGP speakers do not know the actual sequence of ASs through
which the specific routes pass. During route selection, a router considers that AS_Set carries
only one AS number regardless of the actual number of ASs.
AS_Confed_Sequence: records in reverse order all the numbers of the sub-ASs within a
BGP confederation through which a route passes from the local device to the destination.
AS_Confed_Set: records without an order all the numbers of the sub-ASs within a BGP
confederation through which a route passes from the local device to the destination.
shows the complete format of an AS_Path in a BGP routing table.
AS_Confed_Sequence
AS_Confed_Set
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AS_Set
AS_Sequence
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