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AS_Path attribute
The AS_Path attribute records all the AS numbers that a route passes through from the local to the destination address in vector order. When receiving a route, if the device finds that the AS number is in the AS_Path list, it will not receive the route, thus avoiding inter-AS routing loops.
When the BGP Speaker broadcasts the routes introduced by itself:
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When the BGP Speaker advertises this route to the EBGP peer, it creates an AS_Path list carrying the local AS number in the Update message.
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When the BGP Speaker advertises this route to the IBGP peer, an empty AS_Path list is created in the Update message.
When BGP Speaker propagates routes learned from Update messages of other BGP Speakers:
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When the BGP Speaker advertises this route to the EBGP peer, it will add the local AS number to the front (leftmost) of the AS_Path list. The BGP device that receives this route can know the AS it needs to pass to reach the destination address based on the AS_Path attribute. The adjacent AS number closest to the local AS is listed first, and the other AS numbers are arranged in sequence.
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When the BGP Speaker advertises this route to the IBGP peer, the AS_Path attribute related to this route will not be changed.
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Experimental topology diagram
Experimental steps
All router IP configurations
R1 IP configuration <Huawei>system-view Enter system view, return user view with Ctrl + Z. [Huawei]sysname R1 [R1]undo info-center enable Info: Information center is disabled. [R1]interface GigabitEthernet 0/0/0 [R1-GigabitEthernet0/0/0]ip address 192.168.1.1 24 [R1-GigabitEthernet0/0/0]quit [R1]interface GigabitEthernet 0/0/1 [R1-GigabitEthernet0/0/1]ip address 192.168.2.1 24 [R1-GigabitEthernet0/0/1]quit [R1]interface LoopBack 0 [R1-LoopBack0]ip address 172.16.1.1 32 [R1-LoopBack0]quit R2 IP configuration <Huawei>system-view Enter system view, return user view with Ctrl + Z. [Huawei]sysname R2 [R2]undo info-center enable Info: Information center is disabled. [R2]interface GigabitEthernet 0/0/0 [R2-GigabitEthernet0/0/0]ip address 192.168.6.2 24 [R2-GigabitEthernet0/0/0]quit [R2]interface GigabitEthernet 0/0/1 [R2-GigabitEthernet0/0/1]ip address 192.168.4.2 24 [R2-GigabitEthernet0/0/1]quit \t [R2]interface LoopBack 0 [R2-LoopBack0]ip address 172.16.2.2 32 [R2-LoopBack0]quit R3 IP configuration <Huawei>system-view Enter system view, return user view with Ctrl + Z. [Huawei]sysname R3 [R3]undo info-center enable Info: Information center is disabled. [R3]interface GigabitEthernet 0/0/0 [R3-GigabitEthernet0/0/0]ip address 192.168.3.1 24 [R3-GigabitEthernet0/0/0]quit [R3]interface GigabitEthernet 0/0/1 [R3-GigabitEthernet0/0/1]ip address 192.168.2.2 24 [R3-GigabitEthernet0/0/1]quit R4 IP configuration <Huawei>system-view Enter system view, return user view with Ctrl + Z. [Huawei]sysname R4 [R4]undo info-center enable Info: Information center is disabled. \t [R4]interface GigabitEthernet 0/0/0 [R4-GigabitEthernet0/0/0]ip address 192.168.4.1 24 [R4-GigabitEthernet0/0/0]quit [R4]interface GigabitEthernet 0/0/1 [R4-GigabitEthernet0/0/1]ip address 192.168.3.2 24 [R4-GigabitEthernet0/0/1]quit R5 IP configuration <Huawei>system-view Enter system view, return user view with Ctrl + Z. [Huawei]sysname R5 [R5]undo info-center enable Info: Information center is disabled. \t [R5]interface GigabitEthernet 0/0/0 [R5-GigabitEthernet0/0/0]ip address 192.168.1.2 24 [R5-GigabitEthernet0/0/0]quit [R5]interface g0/0/1 [R5-GigabitEthernet0/0/1]ip address 192.168.5.1 24 [R5-GigabitEthernet0/0/1]quit R6 IP configuration <Huawei>system-view Enter system view, return user view with Ctrl + Z. [Huawei]sysname R6 [R6]undo info-center enable Info: Information center is disabled. [R6]interface GigabitEthernet 0/0/0 [R6-GigabitEthernet0/0/0]ip address 192.168.5.2 24 [R6-GigabitEthernet0/0/0]quit [R6]interface GigabitEthernet 0/0/1 [R6-GigabitEthernet0/0/1]ip address 192.168.6.1 24 [R6-GigabitEthernet0/0/1]quit
All router BGP configuration
R1 BGP configuration [R1]bgp 100 create AS100 [R1-bgp]router-id 172.16.1.1 Specify router-id [R1-bgp]peer 192.168.1.2 as-number 500 specifies the physical interface neighbor, located on AS500 [R1-bgp]peer 192.168.2.2 as-number 300 specifies the physical interface neighbor, located on AS300 [R1-bgp]network 172.16.1.1 32 Inject loopback 0 into bgp100 [R1-bgp]quit R2 BGP configuration [R2]bgp 200 [R2-bgp]router-id 172.16.2.2 [R2-bgp]peer 192.168.4.1 as-number 400 [R2-bgp]peer 192.168.6.1 as-number 500 [R2-bgp]network 172.16.2.2 32 [R2-bgp]quit R3 BGP configuration [R3] bgp 300 [R3-bgp]router-id 172.16.3.3 [R3-bgp]peer 192.168.2.1 as-number 100 [R3-bgp]peer 192.168.3.2 as-number 400 [R3-bgp]quit R4 BGP configuration [R4]bgp 400 [R4-bgp]router-id 172.16.4.4 [R4-bgp]peer 192.168.3.1 as-number 300 [R4-bgp]peer 192.168.4.2 as-number 200 [R4-bgp]quit R5 BGP configuration [R5] bgp 500 [R5-bgp]router-id 172.16.5.5 [R5-bgp]peer 192.168.5.2 as-number 500 [R5-bgp]peer 192.168.5.2 next-hop-local [R5-bgp]peer 192.168.1.1 as-number 100 [R5-bgp]quit R6 BGP configuration [R6] bgp 500 [R6-bgp]router-id 172.16.6.6 [R6-bgp]peer 192.168.5.1 as-number 500 [R6-bgp]peer 192.168.5.1 next-hop-local [R6-bgp]peer 192.168.6.2 as-number 200 [R6-bgp]quit
R1 prefix list configuration
R1 prefix list configuration [R1]ip ip-prefix as_path permit 172.16.2.2 32 Write a prefix list to match the route of the R2 loopback interface [R1]route-policy as_path permit node 10 Write a routing policy named as_path Info: New Sequence of this List. [R1-route-policy]if-match ip-prefix as_path if it matches the prefix list above [R1-route-policy]apply as-path 500 500 additive will add path 500 500 [R1-route-policy]quit [R1]route-policy as_path permit node 20 Finally, write a prefix list as the end Info: New Sequence of this List. [R1-route-policy]quit [R1]bgp 100 [R1-bgp]peer 192.168.1.2 route-policy as_path import inject the above written [R1-bgp]quit
Before configuring the prefix list, the next hop 192.168.1.2 to 172.16.2.2 is the optimal route because it only passes through AS500 and AS200.
After configuration, the next hop AS_Path and two AS500s are added, and 192.168.2.2 becomes the optimal route.
R2 prefix list configuration
R2 prefix list configuration [R2]ip ip-prefix as_path permit 172.16.1.1 32 [R2]route-policy as_path permit node 10 Info: New Sequence of this List. [R2-route-policy]if-match ip-prefix as_path if it matches the prefix list written above [R2-route-policy]apply as-path 300 400 400 100 overwrite then modify the as-path path to 300 400 400 100 Warning: The AS-Path lists of routes to which this route-policy is applied will be overwritten. Continue? [Y/N]y [R2-route-policy]quit [R2]route-policy as_path permit node 20 Info: New Sequence of this List. [R2-route-policy]quit \t [R2]bgp 200 [R2-bgp]peer 192.168.6.1 route-policy as_path import inject the above written [R2-bgp]quit
Before configuring the prefix list, the optimal next hop to 172.16.1.1 is 192.168.6.1
After configuration, the AS_Path of the next hop 192.168.6.1 is modified, and the next hop 192.168.4.1 becomes the optimal route.
