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Overview:
The weight attribute is a Cisco attribute and is local the the router. The weight is the first choice in the best path selection process. It is a value that only makes sense to the specific router and which is not propagated or carried through any of the route updates. A weight can be a number from 0 to 65535. Paths that the router originates have a weight of 32768 by default and other paths have a weight of zero.
Our Diagram:
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Network traffic BEFORE SETTING WEIGHT:
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R1 config
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router bgp 1
no synchronization
bgp router-id 1.1.1.1
network 1.1.1.1 mask 255.255.255.255
network 10.1.1.1 mask 255.255.255.255
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 <=== Have to do this else cannot ping 3.3.3.3 as next-hop wrong
no auto-summary
!
ip classless
ip route 2.2.2.2 255.255.255.255 10.1.1.2
no ip http server
!
route-map setnexthop permit 10 <=== Have to do this else cannot ping 3.3.3.3 as next-hop wrong
set ip next-hop peer-address
!
!
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R2 config
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router bgp 1
no synchronization
bgp router-id 2.2.2.2
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
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R3 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
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
!
route-map setnexthop permit 10
set ip next-hop peer-address
!
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R4 config
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router bgp 2
no synchronization
bgp router-id 4.4.4.4
network 4.4.4.4 mask 255.255.255.255
network 10.1.2.0 mask 255.255.255.0
network 10.1.5.0 mask 255.255.255.0
network 10.1.5.0 mask 255.255.255.0
neighbor 2.2.2.2 ebgp-multihop 255
neighbor 2.2.2.2 update-source Loopback0
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 next-hop-self
neighbor 5.5.5.5 route-map setnexthop in
no auto-summary
!
no ip http server
ip classless
ip route 2.2.2.2 255.255.255.255 10.1.2.1
ip route 5.5.5.5 255.255.255.255 10.1.5.2
!
route-map setnexthop permit 10
set ip next-hop peer-address
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R5 config
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router bgp 2
no synchronization
bgp router-id 5.5.5.5
bgp cluster-id 167839233
network 5.5.5.5 mask 255.255.255.255
network 10.1.4.0 mask 255.255.255.0
network 10.1.5.0 mask 255.255.255.0
network 10.1.6.0 mask 255.255.255.0
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 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-reflector-client
neighbor 4.4.4.4 next-hop-self
neighbor 6.6.6.6 remote-as 2
neighbor 6.6.6.6 ebgp-multihop 255
neighbor 6.6.6.6 update-source Loopback0
neighbor 6.6.6.6 route-reflector-client
neighbor 6.6.6.6 next-hop-self
no auto-summary
!
no ip http server
ip classless
ip route 3.3.3.3 255.255.255.255 10.1.4.1
ip route 4.4.4.4 255.255.255.255 10.1.5.1
ip route 6.6.6.6 255.255.255.255 10.1.6.2
!
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R6 config
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router bgp 2
no synchronization
bgp router-id 6.6.6.6
network 6.6.6.6 mask 255.255.255.255
network 10.1.6.0 mask 255.255.255.0
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 setnexthop in
no auto-summary
!
ip classless
ip route 5.5.5.5 255.255.255.255 10.1.6.1
no ip http server
!
access-list 1 permit any
route-map setnexthop permit 10
match ip address 1
set ip next-hop peer-address
!
route-map setnexthop permit 20
!
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OUTPUT from Trace Tests:
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R1 output:
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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 20 msec 20 msec
2 10.1.2.2 32 msec 32 msec 28 msec
3 10.1.5.2 [AS 2] 48 msec 44 msec 44 msec
4 10.1.6.2 [AS 2] 44 msec * 44 msec
r1#
NOTICE: Path taken by traffic is R2 ==> R4 ==> R5 ==> R6
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R6 output:
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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 0 msec 4 msec 0 msec
2 10.1.4.1 20 msec 16 msec 20 msec
3 10.1.3.1 [AS 1] 36 msec 36 msec 32 msec
4 10.1.1.1 [AS 1] 52 msec * 48 msec
r6#
NOTICE: Path taken by traffic is R5 ==> R3 ==> R2 ==> R1
Therefore traffic is taking different routes (asynchronous routing) - we want to force all traffic to use R2 when leaving AS1, and use R4 coming back
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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 10.1.4.2 16 msec 20 msec 20 msec
2 10.1.6.2 [AS 2] 20 msec * 20 msec
r3#
NOTICE: R3 is going directly to R5 then R6 - we want to change this.
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NOTICE weights:
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R2 output:
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R3 output:
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Method 1: CHANGE WEIGHTS ON R3
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In this method we set the weight for all routes to neighbors
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R3 Changes:
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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 weight 100 <==== Notice, weight changed to 100
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
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
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R3 Output
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Notice: Weights have changed for 2.2.2.2 to 100 whereas before it was 0.
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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 16 msec 20 msec
2 10.1.2.2 36 msec 28 msec 32 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#
Notice - R3 now sends traffic via R2 instead of direct to the other AS2
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CHANGE WEIGHTS ON R5: Second Method
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In this method we set the weight using as path and filter lists
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R5 output before routing changes
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R5 config changes:
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router bgp 2
no synchronization
bgp router-id 5.5.5.5
bgp cluster-id 167839233
network 5.5.5.5 mask 255.255.255.255
network 10.1.4.0 mask 255.255.255.0
network 10.1.5.0 mask 255.255.255.0
network 10.1.6.0 mask 255.255.255.0
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 filter-list 1 weight 100
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-reflector-client
neighbor 4.4.4.4 next-hop-self
neighbor 6.6.6.6 remote-as 2
neighbor 6.6.6.6 ebgp-multihop 255
neighbor 6.6.6.6 update-source Loopback0
neighbor 6.6.6.6 route-reflector-client
neighbor 6.6.6.6 next-hop-self
no auto-summary
!
no ip http server
ip classless
ip route 3.3.3.3 255.255.255.255 10.1.4.1
ip route 4.4.4.4 255.255.255.255 10.1.5.1
ip route 6.6.6.6 255.255.255.255 10.1.6.2
!
ip as-path access-list 1 permit ^2$
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CHANGE WEIGHTS ON R3: Third Method
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In this method we set the weight using a route map
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R3 outputs:
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R3 config changes:
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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 setweightin in <===== Point to a route-map
neighbor 5.5.5.5 remote-as 2
neighbor 5.5.5.5 ebgp-multihop 255
neighbor 5.5.5.5 update-source Loopback0
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$ <=====AS-Path access-list
!
route-map setweightin permit 10 <===== Point to a route-map, notice - you can put multiple set statements in
match as-path 1
set ip next-hop peer-address
set weight 100
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R3 outputs
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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 20 msec 16 msec 16 msec
2 10.1.2.2 [AS 2] 32 msec 32 msec 28 msec
3 10.1.5.2 [AS 2] 28 msec 24 msec 28 msec
4 10.1.6.2 [AS 2] 28 msec * 24 msec
r3#
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