BGP Prepend is the one of BGP attribute to select best
path to destination. BGP Prepend will manipulate AS-Path to select best path.
Keep in mind thet shortest AS path will selected by BGP.
AS path is route from source to destination path. Path
mentioned here is an AS number of each router. For example if you from R2 (AS2)
will going to R4 (AS4) you have 2 way. You can go via R3 (AS3) or R1(AS1).
to see which router you are passing through you can use
/ip route print details
Before Configuring
Prepend
R2 sent will use AS1 when go to 4.4.4.4/32 which is under
AS4
[admin@R2] > ip route print detail where
dst-address=4.4.4.4/32
Flags: X - disabled, A - active, D - dynamic, C -
connect, S - static, r - rip, b - bgp, o - ospf, m - mme,
B - blackhole, U - unreachable, P - prohibit
0 ADb dst-address=4.4.4.4/32 gateway=12.12.12.1
gateway-status=12.12.12.1 reachable via
ether1 distance=20
scope=40 target-scope=10 bgp-as-path="1,4" bgp-origin=incomplete received-from=R1
1 Db dst-address=4.4.4.4/32 gateway=23.23.23.2 gateway-status=23.23.23.2
reachable via ether2 distance=20
scope=40 target-scope=10 bgp-as-path="3,4" bgp-origin=incomplete received-from=R3
Tracert going via R1 (AS1)
[admin@R2] > tool traceroute 4.4.4.4
#
ADDRESS LOSS SENT LAST
AVG BEST WORST STD-DEV STATUS
1 12.12.12.1 0%
3 0.7ms 5.7
0.7 15.7 7.1
2
4.4.4.4 0% 3
2.1ms 1.7 1
2.1 0.5
Configuring BGP Prepend
BGP prepend is set on R1, because traffic going through
R1. Keep in mind that you should apply BGP prepend on link which you will not
use. We will prepend 2 times.
/routing filter
add
chain=prepend-4.4.4.4 disabled=yes prefix=4.4.4.4/32 set-bgp-prepend=2
You should apply prepend to link which is going to R2
/routing bgp peer
set 0 out-filter=prepend-4.4.4.4
[admin@R2] >
ip route print detail where dst-address=4.4.4.4/32
Flags: X -
disabled, A - active, D - dynamic, C - connect, S - static, r - rip, b - bgp, o
- ospf, m - mme,
B - blackhole, U
- unreachable, P - prohibit
0 ADb dst-address=4.4.4.4/32
gateway=23.23.23.2 gateway-status=23.23.23.2 reachable via ether2 distance=20
scope=40 target-scope=10 bgp-as-path="3,4"
bgp-origin=incomplete received-from=R3
1
Db dst-address=4.4.4.4/32
gateway=12.12.12.1 gateway-status=12.12.12.1 reachable via ether1 distance=20
scope=40 target-scope=10 bgp-as-path="1,1,4" bgp-origin=incomplete received-from=R1
See on highlighted output. Because I prepend 2 times so
the path will written 2 time (1,1,x) you can put as many as you want. Depending
your need. Let’s me test using traceroute
[admin@R2] > tool traceroute 4.4.4.4
# ADDRESS LOSS SENT LAST
AVG BEST WORST STD-DEV STATUS
1 23.23.23.2 0%
6 0.9ms 1.1
0.7 2.1 0.5
2
4.4.4.4 0% 6
1.2ms 4.3 0.9
20.5 7.3
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