This lab and the
next several labs will focus on BGP the bestpath selection parameters used by
BGP to choose the prefer path to a given prefix. This lab will focus on the
WEIGHT attribute which is the first attribute look at by the best path algorithm.
Concepts covered
-BGP path selection
-BGP Weight
-Applying attributes
to prefixes using route-maps
Topology
GNS3 files: Link
Solution
Let's being by
setting up our basic BGP peering
R1(config)#router
bgp 10
R1(config-router)#neighbor
192.168.21.2 remote
R1(config-router)#neighbor
192.168.21.2 remote-as 20
R1(config-router)#neighbor
192.168.13.3 remote-as 30
R2(config)#router
bgp 20
R2(config-router)#neighbor
192.168.21.1 remote-as 10
R2(config-router)#neighbor
192.168.24.4 remote-as 40
R3(config)#router
bgp 30
R3(config-router)#neighbor
192.168.13.1 remote-as 10
R3(config-router)#neighbor
192.168.34.4 remote-as 40
R4(config)#router
bgp 40
R4(config-router)#neighbor
192.168.24.2 remote-as 20
R4(config-router)#neighbor
192.168.34.3 remote-as 30
Now let's advertise
our loopback addresses
R1(config)#router
bgp 10
R1(config-router)#network
11.11.11.0 mask 255.255.255.0
R2(config)#router
bgp 20
R2(config-router)#network
22.22.22.0 mask 255.255.255.0
R3(config)#router
bgp 30
R3(config-router)#network
33.33.33.0 mask 255.255.255.0
R4(config)#router
bgp 40
R4(config-router)#network
44.44.44.0 mask 255.255.255.0
To apply the WEIGHT
attribute to the AS 40 prefixes we will use an as-path access list in concert
with a route map and apply that route map to the neighbor statement for our
preferred path
R1(config)#ip
as-path access-list 1 permit _40$
R1(config)#routerrouter
bgp 10
R1(config-router)#neighbor
192.168.13.3 route-map PREFER_AS30 in
After clearing the
BGP neighbors to flush the BGP RIB we get the following results.
R1#sh ip bgp
BGP table version is
5, local router ID is 11.11.11.11
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
*>
11.11.11.0/24 0.0.0.0 0 32768 i
*>
22.22.22.0/24
192.168.21.2 0 0 20 i
*
192.168.13.3 0 30 40 20 i
*
33.33.33.0/24
192.168.21.2 0 20 40 30 i
*>
192.168.13.3 0 0 30 i
*
44.44.44.0/24 192.168.21.2 0 20 40 i
*> 192.168.13.3 1000
30 40 I
And when we trace
R1#traceroute
44.44.44.44 source loopback 10
Type escape sequence
to abort.
Tracing the route to
44.44.44.44
VRF info: (vrf in
name/id, vrf out name/id)
1 192.168.13.3
28 msec 48 msec 24 msec
2 192.168.34.4 32 msec * 72 msec
Sources:
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