BGP Weight Lab 1

At BGP best path selection algorithm the weight attribute is first.
The default weight for learned routes is 0 and the default weight for a locally originated route is 32768. Weight attribute is preferred large number weight. Only effect on outgoing traffic.
 

Figure 1.1

In this scenario by default R1 will choose AS200 to reach AS500. We want to use AS300 path to reach to AS500. So let play with weight. We have  two options using to Weight

1. Set the weight attribute on the neighbor (That will affect all routes learn from that neighbor)
2. Using Route map can specific routes to have the weight set.

Configuration

Weight attribute on neighbor

R1
router bgp 140
 no synchronization
 bgp log-neighbor-changes
 network 1.1.1.0 mask 255.255.255.0
 neighbor 10.1.12.2 remote-as 200
 neighbor 10.1.14.4 remote-as 140
 neighbor 10.1.14.4 next-hop-self
 neighbor 10.1.14.4 weight 100
 no auto-summary

R2
 router bgp 200
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.1.12.1 remote-as 140
 neighbor 10.1.25.5 remote-as 500
 no auto-summary

R3
router bgp 300
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.1.34.4 remote-as 140
 neighbor 10.1.35.5 remote-as 500
 no auto-summary


R4
router bgp 140
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.1.14.1 remote-as 140
 neighbor 10.1.14.1 next-hop-self
 neighbor 10.1.34.3 remote-as 300
 no auto-summary

R5
router bgp 500
 no synchronization
 bgp log-neighbor-changes
 network 5.5.5.0 mask 255.255.255.0
 neighbor 10.1.25.2 remote-as 200
 neighbor 10.1.35.3 remote-as 300
 no auto-summary

Verification

R1
Before
R1#sh ip bgp
BGP table version is 3, 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
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric  LocPrf  Weight  Path
*> 1.1.1.0/24       0.0.0.0                  0                32768   i
* i5.5.5.0/24       10.1.14.4                0     100       0         300 500 i
*>                  10.1.12.2                                      0         200 500 i

After

R1#sh ip bgp
BGP table version is 7, 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
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric   LocPrf   Weight  Path
*> 1.1.1.0/24       0.0.0.0                  0                  32768    i
*>i5.5.5.0/24       10.1.14.4               0    100        100        300 500 i
*                         10.1.12.2                                  0           200 500 i

R1#trace 5.5.5.5 source 1.1.1.1
Type escape sequence to abort.
Tracing the route to 5.5.5.5
  1 10.1.14.4 12 msec 52 msec 12 msec
  2 10.1.34.3 12 msec 24 msec 20 msec
  3 10.1.35.5 44 msec 20 msec 24 msec

R5#trace 1.1.1.1 so lo0
Type escape sequence to abort.
Tracing the route to 1.1.1.1
  1 10.1.25.2 8 msec 8 msec 12 msec
  2 10.1.12.1 32 msec 24 msec 44 msec

BGP weight attribute is not share to neighbor.

R4#sh ip bgp
BGP table version is 10, local router ID is 10.1.34.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf   Weight  Path
*>i1.1.1.0/24       10.1.14.1                0    100      0         i
*> 5.5.5.0/24       10.1.34.3                                 0         300 500 i

Weight attribute with Route map

Let add the network 55.55.55.0/24 network at R5. So we can compare 5.5.5.0/24 network and 55.55.55.0/24 network.

First remove the previous configure.

Configuration

R1
router bgp 140
 no neighbor 10.1.14.4 weight 100

Clear ip bgp *
Be careful this command effect all routes.

access-list 1 permit 5.5.5.0 0.0.0.255

route-map R5_Weight permit 10
 match ip address 1
 set weight 200

router bgp 140
neighbor 10.1.14.4 route-map R5_Weight in

Verification

R1#sh ip bgp
BGP table version is 4, 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
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric  LocPrf  Weight  Path
*> 1.1.1.0/24       0.0.0.0                  0                32768  i
*  5.5.5.0/24       10.1.12.2                                  0        200 500 i
*>i                 10.1.14.4                0    100          200     300 500 i
*> 55.55.55.0/24    10.1.12.2                              0        200 500 i

R1#traceroute 5.5.5.5 source 1.1.1.1
Type escape sequence to abort.
Tracing the route to 5.5.5.5
  1 10.1.14.4 16 msec 36 msec 44 msec
  2 10.1.34.3 20 msec 24 msec 20 msec
  3 10.1.35.5 44 msec 24 msec 44 msec

R1#traceroute 55.55.55.55 source 1.1.1.1
Type escape sequence to abort.
Tracing the route to 55.55.55.55
  1 10.1.12.2 12 msec 32 msec 44 msec
  2 10.1.25.5 88 msec 64 msec 24 msec