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Consider the following scenario:
------ ISP1 ---- ------ ISP1 ----
192.168.101.0/24 -- SPOKE1 ------ ISP2 ---- HUB ------ ISP2 ---- SPOKE2 --- 192.168.102.0/24
------ MPLS ---- ------ MPLS ----
Each spoke has 3 connections to the HUB. A VPN is established on every connection, so each Spoke will have three VPN tunnels to the HUB.
On the HUB side, there is the following BGP configuration:
B00_FG1-HUB # show router bgp
config router bgp
set as 65001
set router-id 192.168.10.254
set ibgp-multipath enable
config neighbor-group
edit "HUB_ISP1"
set next-hop-self enable
set remote-as 65001
set route-reflector-client enable
next
edit "HUB_ISP2"
set next-hop-self enable
set remote-as 65001
set route-reflector-client enable
next
edit "HUB_MPLS"
set next-hop-self enable
set remote-as 65001
set route-reflector-client enable
next
end
config neighbor-range
edit 1
set prefix 172.16.30.0 255.255.255.0
set neighbor-group "HUB_ISP1"
next
edit 2
set prefix 172.16.31.0 255.255.255.0
set neighbor-group "HUB_ISP2"
next
edit 3
set prefix 172.16.32.0 255.255.255.0
set neighbor-group "HUB_MPLS"
next
end
config network
edit 1
set prefix 10.212.134.0 255.255.255.0
next
end
config network6
edit 1
set prefix6 ::/128
next
end
config redistribute "connected"
end
config redistribute "rip"
end
config redistribute "ospf"
end
config redistribute "static"
end
config redistribute "isis"
end
config redistribute6 "connected"
end
config redistribute6 "rip"
end
config redistribute6 "ospf"
end
config redistribute6 "static"
end
config redistribute6 "isis"
end
end
B00_FG1-HUB #
After BGP is established over the VPNs, the HUB will only select one path for each prefix on spokes:
The HUB Routing table will show the 3 paths for every route:
However, when the prefix is advertised to the other spoke, the HUB will select only the best path:
For prefix 192.168.102.0/24 there are 3 paths displayed.
'*' means the path is installed into the local RIB.
'>' means the path is selected to advertise to peers.
This will cause Spoke1 to see the same next-hop through the 3 VPN tunnels for the BGP route:
B01_FG-SPOKE1 # get router info routing-table all
...
B 192.168.102.0/24 [200/0] via 172.16.30.3 [3] (recursive is directly connected, HUB_ISP1_0), 00:27:54, [1/0]
To allow the HUB to advertise the 3 paths for the prefix to Spokes, it is necessary to enable the settings 'additional-path-select' and 'adv-additional-path' under BGP settings.
On the HUB side:
B00_FG1-HUB: config router bgp
B00_FG1-HUB (bgp) # set additional-path enable
B00_FG1-HUB (bgp) # set additional-path-select 3
Under each neighbor:
B00_FG1-HUB (bgp) # config neighbor-group
B00_FG1-HUB (neighbor-group) # edit HUB_ISP1
B00_FG1-HUB (HUB_ISP1) # set additional-path both
B00_FG1-HUB (HUB_ISP1) # set adv-additional-path 3
B00_FG1-HUB (HUB_ISP1) # next
B00_FG1-HUB (neighbor-group) # edit HUB_ISP2
B00_FG1-HUB (HUB_ISP2) # set additional-path both
B00_FG1-HUB (HUB_ISP2) # set adv-additional-path 3
B00_FG1-HUB (HUB_ISP2) # next
B00_FG1-HUB (neighbor-group) # edit HUB_MPLS
B00_FG1-HUB (HUB_MPLS) # set additional-path both
B00_FG1-HUB (HUB_MPLS) # set adv-additional-path 3
B00_FG1-HUB (HUB_MPLS) # end
B00_FG1-HUB (bgp) # end
With the above configuration, the HUB selects now the 3 paths to be advertised:
On Spoke side, enable the same parameters:
B01_FG-SPOKE1 # config router bgp
B01_FG-SPOKE1 (bgp) # set additional-path enable
B01_FG-SPOKE1 (bgp) # set additional-path-select 9
B01_FG-SPOKE1 (bgp) # config neighbor
B01_FG-SPOKE1 (neighbor) # edit 172.16.30.1
B01_FG-SPOKE1 (172.16.30.1) # set additional-path both
B01_FG-SPOKE1 (172.16.30.1) # set adv-additional-path 3
B01_FG-SPOKE1 (172.16.30.1) # next
B01_FG-SPOKE1 (neighbor) # edit 172.16.31.1
B01_FG-SPOKE1 (172.16.31.1) # set additional-path both
B01_FG-SPOKE1 (172.16.31.1) # set adv-additional-path 3
B01_FG-SPOKE1 (172.16.31.1) # next
B01_FG-SPOKE1 (neighbor) # edit 172.16.32.1
B01_FG-SPOKE1 (172.16.32.1) # set additional-path both
B01_FG-SPOKE1 (172.16.32.1) # set adv-additional-path 3
B01_FG-SPOKE1 (172.16.32.1) # end
B01_FG-SPOKE1 (bgp) # end
Verification:
Now, on the Spoke1 side, the routing-table the three paths for prefix 192.168.102.0/24:
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