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Overview:
Local preference is an indication to the AS about which path is preferred to exit the AS in order to reach a certain network. A path with a higher local preference is more preferred. The default value for local preference is 100.
The metric attribute which is also called Multi_exit_discriminator (MED) is a hint to external neighbors about the preferred path into an AS. This is a dynamic way to influence another AS on which way to choose in order to reach a certain route given that we have multiple entry points into that AS. A lower value of a metric is more preferred.
Unlike local preference, metric is exchanged between ASs. A metric is carried into an AS but does not leave the AS.
Our Diagram:
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CHANGE LOCAL PREFERENCE ON R2
*****************************************************************
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R2 config
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router bgp 1
no synchronization
bgp router-id 2.2.2.2
bgp default local-preference 500
network 2.2.2.2 mask 255.255.255.255
network 10.1.1.0 mask 255.255.255.0
network 10.1.2.0 mask 255.255.255.0
network 10.1.3.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 1
neighbor 1.1.1.1 ebgp-multihop 255
neighbor 1.1.1.1 update-source Loopback0
neighbor 1.1.1.1 route-reflector-client
neighbor 3.3.3.3 remote-as 1
neighbor 3.3.3.3 ebgp-multihop 255
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 route-reflector-client
neighbor 4.4.4.4 remote-as 2
neighbor 4.4.4.4 ebgp-multihop 255
neighbor 4.4.4.4 update-source Loopback0
no auto-summary
!
ip classless
ip route 1.1.1.1 255.255.255.255 10.1.1.1
ip route 3.3.3.3 255.255.255.255 10.1.3.2
ip route 4.4.4.4 255.255.255.255 10.1.2.2
r6#traceroute 1.1.1.1
Type escape sequence to abort.
Tracing the route to 1.1.1.1
1 10.1.6.1 4 msec 0 msec 4 msec
2 10.1.4.1 128 msec 36 msec 36 msec
3 10.1.3.1 [AS 1] 36 msec 32 msec 36 msec
4 10.1.1.1 [AS 1] 48 msec * 48 msec
r6#
r1#traceroute 6.6.6.6
Type escape sequence to abort.
Tracing the route to 6.6.6.6
1 10.1.1.2 16 msec 16 msec 16 msec
2 10.1.2.2 32 msec 32 msec 32 msec
3 10.1.5.2 [AS 2] 48 msec 48 msec 44 msec
4 10.1.6.2 [AS 2] 44 msec * 40 msec
r1#
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R3 Output:
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r3#traceroute 6.6.6.6
Type escape sequence to abort.
Tracing the route to 6.6.6.6
1 10.1.3.1 16 msec 20 msec 20 msec
2 10.1.2.2 36 msec 32 msec 28 msec
3 10.1.5.2 [AS 2] 28 msec 28 msec 28 msec
4 10.1.6.2 [AS 2] 28 msec * 24 msec
r3#
*****************************************************************
CHANGE MED
*****************************************************************
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R6 Return traffic:
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r6#traceroute 1.1.1.1
Type escape sequence to abort.
Tracing the route to 1.1.1.1
1 10.1.6.1 4 msec 0 msec 4 msec
2 10.1.4.1 128 msec 36 msec 36 msec
3 10.1.3.1 [AS 1] 36 msec 32 msec 36 msec
4 10.1.1.1 [AS 1] 48 msec * 48 msec
r6#
================================================
R2 Change med - config:
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router bgp 1
no synchronization
bgp router-id 2.2.2.2
bgp default local-preference 500
network 2.2.2.2 mask 255.255.255.255
network 10.1.1.0 mask 255.255.255.0
network 10.1.2.0 mask 255.255.255.0
network 10.1.3.0 mask 255.255.255.0
neighbor 1.1.1.1 remote-as 1
neighbor 1.1.1.1 ebgp-multihop 255
neighbor 1.1.1.1 update-source Loopback0
neighbor 1.1.1.1 route-reflector-client
neighbor 3.3.3.3 remote-as 1
neighbor 3.3.3.3 ebgp-multihop 255
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 route-reflector-client
neighbor 4.4.4.4 remote-as 2
neighbor 4.4.4.4 ebgp-multihop 255
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 route-map setmetricout out <=== Change metric with route-map
no auto-summary
!
ip classless
ip route 1.1.1.1 255.255.255.255 10.1.1.1
ip route 3.3.3.3 255.255.255.255 10.1.3.2
ip route 4.4.4.4 255.255.255.255 10.1.2.2
no ip http server
!
route-map setmetricout permit 10 <=== Change metric with route-map
set metric 100
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R3 Change med - config:
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!
router bgp 1
no synchronization
bgp router-id 3.3.3.3
network 3.3.3.3 mask 255.255.255.255
network 10.1.3.0 mask 255.255.255.0
network 10.1.4.0 mask 255.255.255.0
neighbor 2.2.2.2 remote-as 1
neighbor 2.2.2.2 ebgp-multihop 255
neighbor 2.2.2.2 update-source Loopback0
neighbor 2.2.2.2 route-map setnexthop in
neighbor 5.5.5.5 remote-as 2
neighbor 5.5.5.5 ebgp-multihop 255
neighbor 5.5.5.5 update-source Loopback0
neighbor 5.5.5.5 route-map setmetricout out <=== Change metric with route-map
no auto-summary
!
ip classless
ip route 2.2.2.2 255.255.255.255 10.1.3.1
ip route 5.5.5.5 255.255.255.255 10.1.4.2
no ip http server
ip as-path access-list 1 permit ^2$
!
access-list 1 permit any
route-map setmetricout permit 10 <=== Change metric with route-map
set metric 200
!
route-map setnexthop permit 10
set ip next-hop peer-address
!
route-map setweightin permit 10
match as-path 1
set ip next-hop peer-address
set weight 100
!
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R6 output:
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R6 new trace:
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r6#traceroute 1.1.1.1
Type escape sequence to abort.
Tracing the route to 1.1.1.1
1 10.1.6.1 4 msec 0 msec 0 msec
2 10.1.5.1 20 msec 20 msec 16 msec
3 10.1.2.1 36 msec 32 msec 32 msec
4 10.1.1.1 [AS 1] 52 msec * 48 msec
r6#
Notice path has now changed!!!
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R6 trace to new loop on R3
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r6#traceroute 9.9.9.9
Type escape sequence to abort.
Tracing the route to 9.9.9.9
1 10.1.6.1 4 msec 0 msec 0 msec
2 10.1.5.1 20 msec 20 msec 20 msec
3 10.1.2.1 32 msec 32 msec 36 msec
4 10.1.3.2 [AS 1] 48 msec * 48 msec
r6#
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