BGP IPv4 - Local Preference Attributes
Local Preference Attributes
The BGP (Border Gateway Protocol) IPv4 Local Preference attribute is a crucial parameter used to influence routing decisions within an Autonomous System (AS). It is a 32-bit value assigned to incoming routes by an AS border router, indicating the local preference for a particular route. Higher Local Preference values are preferred, and routers within the AS use this attribute to determine the best path when multiple routes to the same destination exist. BGP Local Preference is typically used to control outbound traffic flow, allowing administrators to influence the selection of a particular exit point for traffic leaving the AS. This attribute is powerful for implementing traffic engineering policies and prioritizing certain paths over others based on factors like link capacity, policy requirements, or service-level agreements. Careful configuration of Local Preference values enables network administrators to achieve efficient and controlled traffic distribution within their BGP IPv4 networks.
Lab:
Disclaimer
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Cisco IOS Software, Linux Software (I86BI_LINUX-ADVENTERPRISEK9-M), Version 15.2(4)M1, DEVELOPMENT TEST SOFTWARE
Task 1: Configure BGP Local Preference Attributes for Autonomous
Step 1: In the configuration mode of router configure BGP Local Preference Attributes by following command:
R2:
router bgp 65000
address-family ipv4
neighbor 11.0.1.1 route-map xyz in
exit
route-map xyz permit 10
set local-preference 5000
exit
A policy is call on Router R2 that to reach 100 network via 11.0.1.1 through AS 65000 preference is set 5000
R1:
router bgp 65100
address-family ipv4
neighbor 22.0.1.2 route-map xyz out
exit
route-map xyz permit 10
set local-preference 10000
exit
A policy is call on Router R1 that to reach 100 network via 22.0.1.2 through AS 65100 preference is set 10000
Step 2: Verify IPv4 BGP routes by following command:
R2#show ip bgp
BGP table version is 25, local router ID is 22.0.3.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 11.0.1.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
*> 11.0.2.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
*> 11.0.3.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
*> 22.0.1.0/24 0.0.0.0 0 32768 i
*> 22.0.2.0/24 0.0.0.0 0 32768 i
*> 22.0.3.0/24 0.0.0.0 0 32768 i
*>i 33.0.1.0/24 33.0.1.3 0 100 0 i
*>i 33.0.2.0/24 33.0.1.3 0 100 0 i
*>i 33.0.3.0/24 33.0.1.3 0 100 0 i
*>i 44.0.1.0/24 44.0.1.4 0 100 0 65200 i
*>i 44.0.2.0/24 44.0.1.4 0 100 0 65200 i
*>i 44.0.3.0/24 44.0.1.4 0 100 0 65200 i
*> 100.0.0.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
Local Preference is by default 100 which is seen only on IBGP neighbor Local Preference is Local to an Autonomous. It is non-transitive parameter and is stripped of when routes are moving from one AS to another AS.
R2 - Router R2 now selects 11.0.1.1 as best path because it can reach 100 network through As 65200 with Local Preference 5000, where as to reach 100 network via 44.0.1.4 through AS 65200 with Local Preference 100 Hence path is manipulated with help of Local Preference. We as an Administrator changed the decision of Router R2 to reach 100.0.0.0 network via 44.0.1.4 to path via 11.0.1.1 as best path. Therefore it shows only one path in its BGP Table and is ironically known as Sukhiram.
R3# show ip bgp
BGP table version is 25, local router ID is 33.0.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*>i 11.0.1.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
*>i 11.0.2.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
*>i 11.0.3.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
*>i 22.0.1.0/24 22.0.1.2 0 100 0 i
*>i 22.0.2.0/24 22.0.1.2 0 100 0 i
*>i 22.0.3.0/24 22.0.1.2 0 100 0 i
*> 33.0.1.0/24 0.0.0.0 0 32768 i
*> 33.0.2.0/24 0.0.0.0 0 32768 i
*> 33.0.3.0/24 0.0.0.0 0 32768 i
*> 44.0.1.0/24 44.0.1.4 0 0 65200 i
*> 44.0.2.0/24 44.0.1.4 0 0 65200 i
*> 44.0.3.0/24 44.0.1.4 0 0 65200 i
*>i 100.0.0.0/24 11.0.1.1 0 5000 0 222 22 65100 11 111 i
* 44.0.1.4 0 0 65200 i
R3 - As Local Preference is Local to an Autonomous and R3 belongs to same Autonomous, therefore Router R3 will also select best path as 11.0.1.1 but will show both the path in its BGP table and is ironically known as Dukhiram