The
below topology will be used to demonstrate this feature and the proof of
function
The
setup consists of 5 routers each participating in its own AS # with each router
advertising its loopback 0 network into BGP (each router loopback 0 interface
follow the criteria x.x.x.x/32 where x is the router number)
Checking
the BGP table for the prefixes received
R1#sh
ip bgp
BGP
table version is 6, local router ID is 1.1.1.1
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
*>
1.1.1.1/32 0.0.0.0 0 32768 i
*>
2.2.2.2/32 192.168.12.2 0 0 2 i
*>
3.3.3.3/32 192.168.13.3 0 0 3 i
*
4.4.4.4/32 192.168.13.3 0 3 4 i
*> 192.168.12.2 0 2 4 i
*
5.5.5.5/32 192.168.13.3 0 3 4 5 i
*> 192.168.12.2 0 2 4 5 i
As
we can see from the output above that R1 is learning 4.4.4.4/32 and 5.5.5.5/32 from
two different paths , it prefers the path through R2 because R2 has the lowest
BGP router ID
R1#show
ip bgp 5.5.5.5 255.255.255.255
BGP
routing table entry for 5.5.5.5/32, version 6
Paths:
(2 available, best #2, table default)
Advertised to update-groups:
1
Refresh Epoch 1
3 4 5
192.168.13.3 from 192.168.13.3 (3.3.3.3)
Origin IGP, localpref 100, valid,
external
rx pathid: 0, tx pathid: 0
Refresh Epoch 1
2 4 5
192.168.12.2 from 192.168.12.2 (2.2.2.2)
Origin IGP, localpref 100, valid,
external, best
rx pathid: 0, tx pathid: 0x0
R1#show
ip cef 5.5.5.5 255.255.255.255
5.5.5.5/32
nexthop 192.168.12.2 FastEthernet1/0
Through
FIB, only one entry is installed which is the best route to reach that
destination
Now,
let us turn on prefix-independent convergence on R1
R1
R1(config)#router
bgp 1
R1(config-router)#address-family
ipv4 unicast
R1(config-router-af)#bgp
additional-paths install
R1#sh
ip bgp
BGP
table version is 11, local router ID is 1.1.1.1
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
*>
1.1.1.1/32 0.0.0.0 0 32768 i
*>
2.2.2.2/32 192.168.12.2 0 0 2 i
*>
3.3.3.3/32 192.168.13.3 0 0 3 i
*b 4.4.4.4/32 192.168.13.3 0 3 4 i
*> 192.168.12.2 0 2 4 i
*b 5.5.5.5/32 192.168.13.3 0 3 4 5 i
*> 192.168.12.2 0 2 4 5 i
R1#sh
ip bgp 5.5.5.5 255.255.255.255
BGP
routing table entry for 5.5.5.5/32, version 11
Paths:
(2 available, best #2, table default)
Additional-path-install
Advertised to update-groups:
1
Refresh Epoch 1
3 4 5
192.168.13.3 from 192.168.13.3 (3.3.3.3)
Origin IGP, localpref 100, valid,
external, backup/repair, recursive-via-connected
rx pathid: 0, tx pathid: 0
Refresh Epoch 1
2 4 5
192.168.12.2 from 192.168.12.2 (2.2.2.2)
Origin IGP, localpref 100, valid,
external, best , recursive-via-connected
rx pathid: 0, tx pathid: 0x0
R1#sh
ip cef 5.5.5.5 255.255.255.255 detail
5.5.5.5/32,
epoch 0, flags rib only nolabel, rib defined all labels
recursive via 192.168.12.2
attached to FastEthernet1/0
recursive via
192.168.13.3, repair
attached to FastEthernet1/1
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