BGP AS Path Prepending
AS Path is the fourth BGP attribute, AS Path is well known, mandatory attribute. BGP prefer the shortest AS path to get to destination. In other words path with shortest AS path list is more desirable.
You can manipulate this by using AS path prepending . Manual manipulation of AS path length is called AS path prepending. The AS path should be extended with multiple copies of the AS number of the sender.
AS path prepending is used to:
- Ensure proper return path selection.
- . Distribution the traffic load for multi-homed customers.
Results of AS path prepending can be observed on the receiving router.
let see the configuration :-https://www.internetworks.in/2019/03/bgp-as-path-prepending.html
Topology:
Goal:
- configure the topology as per the diagram and assign the IP address as per the topology.
- configure IBGP and EBGP.
- configure AS 650014 to ensure that all routes in AS 650014 should exit router_1 to reach AS 650003 (30.0.0.0) prefix.
- configure AS 650014 to ensure that the traffic from 30.0.0.0 to 10.0.0.0 return traffic should use the same path as forwarding traffic routers_1_2_3.
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.1 YES manual up up
Serial3/0 1.1.1.1 YES manual up up
Serial3/3 4.1.1.2 YES manual up up
Loopback0 11.0.0.1 YES manual up up
R2#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 20.1.1.1 YES manual up up
Serial3/0 1.1.1.2 YES manual up up
Serial3/1 2.1.1.1 YES manual up up
Loopback0 12.0.0.1 YES manual up up
R3#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 30.1.1.1 YES manual up up
Serial3/1 2.1.1.2 YES manual up up
Serial3/2 3.1.1.1 YES manual up up
Loopback0 13.0.0.1 YES manual up up
R4#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 40.1.1.1 YES manual up up
Serial3/2 3.1.1.2 YES manual up up
Serial3/3 4.1.1.1 YES manual up up
Loopback0 14.0.0.1 YES manual up up
R1(config)#router bgp 650014
R1(config-router)#neighbor 4.1.1.1 remote-as 650014
R1(config-router)#neighbor 1.1.1.2 remote-as 650002
R1(config-router)#network 10.0.0.0
R1(config-router)#network 1.0.0.0
R1(config-router)#network 4.0.0.0
R1(config-router)#network 11.0.0.0 mask 255.255.255.0
R1(config-router)#exit
R2(config)#router bgp 650002
R2(config-router)#neighbor 1.1.1.1 remote-as 650014
*Mar 13 12:23:30.111: %BGP-5-ADJCHANGE: neighbor 1.1.1.1 Up
R2(config-router)#neighbor 2.1.1.2 remote-as 650003
R2(config-router)#network 20.0.0.0
R2(config-router)#network 1.0.0.0
R2(config-router)#network 2.0.0.0
R2(config-router)#network 12.0.0.0 mask 255.255.255.0
R2(config-router)#exit
R3(config)#router bgp 650003
R3(config-router)#neighbor 2.1.1.1 remote-as 650002
*Mar 13 12:25:42.495: %BGP-5-ADJCHANGE: neighbor 2.1.1.1 Up
R3(config-router)#neighbor 3.1.1.2 remote-as 650014
R3(config-router)#network 30.0.0.0
R3(config-router)#network 3.0.0.0
R3(config-router)#network 2.0.0.0
R3(config-router)#network 13.0.0.0 mask 255.255.255.0
R3(config-router)#exit
R4(config)#router bgp 650014
R4(config-router)#neighbor 3.1.1.1 remote-as 650003
*Mar 13 12:27:46.807: %BGP-5-ADJCHANGE: neighbor 3.1.1.1 Up
R4(config-router)#neighbor 4.1.1.2 remote-as 650014
*Mar 13 12:28:10.663: %BGP-5-ADJCHANGE: neighbor 4.1.1.2 Up
R4(config-router)#network 40.0.0.0
R4(config-router)#network 4.0.0.0
R4(config-router)#network 3.0.0.0
R4(config-router)#network 14.0.0.0 mask 255.255.255.0
R4(config-router)#exit
R1#show ip bgp summary
BGP router identifier 11.0.0.1, local AS number 650014
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
1.1.1.2 4 650002 14 16 21 0 0 00:08:00 7
4.1.1.1 4 650014 14 13 21 0 0 00:03:19 7
R2#show ip bgp summary
BGP router identifier 12.0.0.1, local AS number 650002
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
1.1.1.1 4 650014 17 15 14 0 0 00:09:06 9
2.1.1.2 4 650003 12 15 14 0 0 00:06:54 9
R3#show ip bgp summary
BGP router identifier 13.0.0.1, local AS number 650003
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
2.1.1.1 4 650002 16 13 18 0 0 00:07:58 9
3.1.1.2 4 650014 12 16 18 0 0 00:05:54 9
R4#sh ip bgp summary
BGP router identifier 14.0.0.1, local AS number 650014
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
3.1.1.1 4 650003 17 12 18 0 0 00:06:20 7
4.1.1.2 4 650014 15 17 18 0 0 00:05:56 7
R3#show ip bgp
BGP table version is 18, local router ID is 13.0.0.1
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
* 1.0.0.0 3.1.1.2 0 650014 i
*> 2.1.1.1 0 0 650002 i
*> 2.0.0.0 0.0.0.0 0 32768 i
* 2.1.1.1 0 0 650002 i
* 3.0.0.0 3.1.1.2 0 0 650014 i
*> 0.0.0.0 0 32768 i
*> 4.0.0.0 3.1.1.2 0 0 650014 i
* 2.1.1.1 0 650002 650014 i
*> 10.0.0.0 3.1.1.2 0 650014 i
* 2.1.1.1 0 650002 650014 i
*> 11.0.0.0/24 3.1.1.2 0 650014 i
* 2.1.1.1 0 650002 650014 i
* 12.0.0.0/24 3.1.1.2 0 650014 650002 i
*> 2.1.1.1 0 0 650002 i
Network Next Hop Metric LocPrf Weight Path
*> 13.0.0.0/24 0.0.0.0 0 32768 i
* 14.0.0.0/24 2.1.1.1 0 650002 650014 i
*> 3.1.1.2 0 0 650014 i
* 20.0.0.0 3.1.1.2 0 650014 650002 i
*> 2.1.1.1 0 0 650002 i
*> 30.0.0.0 0.0.0.0 0 32768 i
*> 40.0.0.0 3.1.1.2 0 0 650014 i
* 2.1.1.1 0 650002 650014 i
R1#show ip bgp
BGP table version is 21, local router ID is 11.0.0.1
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
* 1.0.0.0 1.1.1.2 0 0 650002 i
*> 0.0.0.0 0 32768 i
* i 2.0.0.0 3.1.1.1 0 100 0 650003 i
*> 1.1.1.2 0 0 650002 i
*>i 3.0.0.0 4.1.1.1 0 100 0 i
* 1.1.1.2 0 650002 650003 i
* i 4.0.0.0 4.1.1.1 0 100 0 i
*> 0.0.0.0 0 32768 i
*> 10.0.0.0 0.0.0.0 0 32768 i
*> 11.0.0.0/24 0.0.0.0 0 32768 i
*> 12.0.0.0/24 1.1.1.2 0 0 650002 i
*>i 13.0.0.0/24 3.1.1.1 0 100 0 650003 i
* 1.1.1.2 0 650002 650003 i
*>i 14.0.0.0/24 4.1.1.1 0 100 0 i
Network Next Hop Metric LocPrf Weight Path
*> 20.0.0.0 1.1.1.2 0 0 650002 i
*>i 30.0.0.0 3.1.1.1 0 100 0 650003 i
* 1.1.1.2 0 650002 650003 i
*>i 40.0.0.0 4.1.1.1 0 100 0 i
R1#show ip bgp 30.1.1.1
BGP routing table entry for 30.0.0.0/8, version 20
Paths: (2 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 1
650003
3.1.1.1 from 4.1.1.1 (14.0.0.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
rx pathid: 0, tx pathid: 0x0
Refresh Epoch 1
650002 650003
1.1.1.2 from 1.1.1.2 (12.0.0.1)
Origin IGP, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
( by default AS_650014 EXIT VIA R4 to reach AS 650003 route (30.0.0.0) prefix because of shortest AS-PATH)
R1(config)# access-list 10 permit 30.0.0.0 0.255.255.255
R1(config)#route-map cisco permit 10
R1(config-route-map)#match ip address 10
R1(config-route-map)#set local-preference 300
R1(config-route-map)#exit
R1(config)#route-map cisco permit 20
R1(config-route-map)#exit
R1#show ip bgp 30.1.1.1
BGP routing table entry for 30.0.0.0/8, version 22
Paths: (1 available, best #1, table default)
Advertised to update-groups:
2
Refresh Epoch 2
650002 650003
1.1.1.2 from 1.1.1.2 (12.0.0.1)
Origin IGP, localpref 300, valid, external, best
rx pathid: 0, tx pathid: 0x0
( now configure return traffic)
R3#show ip bgp
BGP table version is 18, local router ID is 13.0.0.1
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
* 1.0.0.0 3.1.1.2 0 650014 i
*> 2.1.1.1 0 0 650002 i
*> 2.0.0.0 0.0.0.0 0 32768 i
* 2.1.1.1 0 0 650002 i
* 3.0.0.0 3.1.1.2 0 0 650014 i
*> 0.0.0.0 0 32768 i
*> 4.0.0.0 3.1.1.2 0 0 650014 i
* 2.1.1.1 0 650002 650014 i
*> 10.0.0.0 3.1.1.2 0 650014 i
* 2.1.1.1 0 650002 650014 i
*> 11.0.0.0/24 3.1.1.2 0 650014 i
* 2.1.1.1 0 650002 650014 i
* 12.0.0.0/24 3.1.1.2 0 650014 650002 i
*> 2.1.1.1 0 0 650002 i
Network Next Hop Metric LocPrf Weight Path
*> 13.0.0.0/24 0.0.0.0 0 32768 i
* 14.0.0.0/24 2.1.1.1 0 650002 650014 i
*> 3.1.1.2 0 0 650014 i
* 20.0.0.0 3.1.1.2 0 650014 650002 i
*> 2.1.1.1 0 0 650002 i
*> 30.0.0.0 0.0.0.0 0 32768 i
*> 40.0.0.0 3.1.1.2 0 0 650014 i
* 2.1.1.1 0 650002 650014 i
R3#show ip bgp 10.1.1.1
BGP routing table entry for 10.0.0.0/8, version 13
Paths: (2 available, best #1, table default)
Advertised to update-groups:
1
Refresh Epoch 1
650014
3.1.1.2 from 3.1.1.2 (14.0.0.1)
Origin IGP, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Refresh Epoch 2
650002 650014
2.1.1.1 from 2.1.1.1 (12.0.0.1)
Origin IGP, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
R4(config)#access-list 10 permit 10.0.0.0 0.255.255.255
R4(config)#route-map cisco permit 10
R4(config-route-map)#match ip address 10
R4(config-route-map)#set as-path prepend 650014 650014 650014 650014
R4(config-route-map)#exit
R4(config)#route-map cisco permit 20
R4(config-route-map)#exit
R4(config)#router bgp 650014
R4(config-router)#nei
R4(config-router)#neighbor 3.1.1.1 rou
R4(config-router)#neighbor 3.1.1.1 route-m
R4(config-router)#neighbor 3.1.1.1 route-map cisco out
R4(config-router)#end
R3#show ip bgp
BGP table version is 19, local router ID is 13.0.0.1
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
* 1.0.0.0 3.1.1.2 0 650014 i
*> 2.1.1.1 0 0 650002 i
*> 2.0.0.0 0.0.0.0 0 32768 i
* 2.1.1.1 0 0 650002 i
* 3.0.0.0 3.1.1.2 0 0 650014 i
*> 0.0.0.0 0 32768 i
*> 4.0.0.0 3.1.1.2 0 0 650014 i
* 2.1.1.1 0 650002 650014 i
* 10.0.0.0 3.1.1.2 0 650014 650014 650014 650014 650014 i
*> 2.1.1.1 0 650002 650014 i
*> 11.0.0.0/24 3.1.1.2 0 650014 i
* 2.1.1.1 0 650002 650014 i
* 12.0.0.0/24 3.1.1.2 0 650014 650002 i
Network Next Hop Metric LocPrf Weight Path
*> 2.1.1.1 0 0 650002 i
*> 13.0.0.0/24 0.0.0.0 0 32768 i
* 14.0.0.0/24 2.1.1.1 0 650002 650014 i
*> 3.1.1.2 0 0 650014 i
* 20.0.0.0 3.1.1.2 0 650014 650002 i
*> 2.1.1.1 0 0 650002 i
*> 30.0.0.0 0.0.0.0 0 32768 i
*> 40.0.0.0 3.1.1.2 0 0 650014 i
* 2.1.1.1 0 650002 650014 i
R3#show ip bgp 10.1.1.1
BGP routing table entry for 10.0.0.0/8, version 19
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1
Refresh Epoch 3
650014 650014 650014 650014 650014
3.1.1.2 from 3.1.1.2 (14.0.0.1)
Origin IGP, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Refresh Epoch 3
650002 650014
2.1.1.1 from 2.1.1.1 (12.0.0.1)
Origin IGP, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
R1#traceroute 30.1.1.1
Type escape sequence to abort.
Tracing the route to 30.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
1 1.1.1.2 52 msec 56 msec 28 msec
2 2.1.1.2 [AS 650002] 120 msec 140 msec 176 msec
R3#traceroute 10.1.1.1
Type escape sequence to abort.
Tracing the route to 10.1.1.1
VRF info: (vrf in name/id, vrf out name/id)
1 2.1.1.1 36 msec 44 msec 64 msec
2 1.1.1.1 [AS 650002] 112 msec 92 msec 76 msec
