Weight attribute - BGP

Below are the characteristics of weight:-

1.       Manipulate Outgoing traffic

2.       Cisco Proprietary

3.       Only applicable when two exit points are on single router.

4.       Never advertise out from router and will remain local to router.

5.       Higher is preferred

6.       Range is 0 – 65535.

7.       When using Route-map to manipulate weight then it cannot be applied in Outgoing direction as shown below. It has to be in inbound direction.

          

            R1(config-router)#neighbor 13.13.13.3 route-map SET_WEIGHT out

                % "SET_WEIGHT" used as BGP outbound route-map, set weight not supported

In below configuration example, R1 is having two paths to reach to 20.1.1.x subnet and best path is from R2 i.e. 12.12.12.2. We want to change the outgoing traffic from R1 to R3 by increasing the weight of R3 neighbor but remember it will not manipulate the incoming traffic. It will change the outgoing traffic directions only,

Weight will change the path for 20.1.1.x from R2 to R3.

R1:-

interface Loopback0

 ip address 1.1.1.1 255.255.255.255

interface FastEthernet0/0

 description " Connected to R2"

 ip address 12.12.12.1 255.255.255.0

no shut

interface FastEthernet0/1

 description " Connected to R3"

 ip address 13.13.13.1 255.255.255.0

 no shut

interface FastEthernet2/0

 description " Connected to PC1"

 ip address 10.1.1.1 255.255.255.0

no shut

router bgp 1

 no synchronization

 bgp log-neighbor-changes

 network 10.1.1.0 mask 255.255.255.0

 neighbor 12.12.12.2 remote-as 1

 neighbor 13.13.13.3 remote-as 1

Without Weight configured.

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, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 10.1.1.0/24      0.0.0.0                  0         32768 i

* i20.1.1.0/24      13.13.13.3               0    100      0 2 i

*>i                 12.12.12.2               0    100      0 2 I  \\Route Via R2 is preferred without weight

With Weight configured.

Weight can be set using any of the below methods:-

Option A:-

router bgp 1

neighbor 13.13.13.3 weight 100

OR

Option B:-  It is more flexible as in here we can apply weight on specific routes as well by calling an Prefix-list or access-list that matches the routes under Route map.

router bgp 1

neighbor 13.13.13.3 route-map SET_WEIGHT in

route-map  SET_WEIGHT

set weight 100

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, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 10.1.1.0/24      0.0.0.0                  0         32768 i

*>i20.1.1.0/24      13.13.13.3               0    100    100 2 I   \\Above configured weight has changed the path

* i                 12.12.12.2               0    100      0 2 i

R2:-

interface Loopback0

 ip address 2.2.2.2 255.255.255.255

interface FastEthernet0/0

 description " Connected to R1"

 ip address 12.12.12.2 255.255.255.0

no shut

interface FastEthernet1/0

 description " Connected to R5"

 ip address 25.25.25.2 255.255.255.0

 no shut

router bgp 1

 no synchronization

 bgp router-id 2.2.2.2

 bgp log-neighbor-changes

 neighbor 12.12.12.1 remote-as 1

 neighbor 12.12.12.1 next-hop-self

 neighbor 25.25.25.5 remote-as 2

R3:-

interface Loopback0

 ip address 3.3.3.3 255.255.255.255

interface FastEthernet0/1

description " Connected to R1"

 ip address 13.13.13.3 255.255.255.0

no shut

interface FastEthernet1/0

description " Connected to R4"

 ip address 34.34.34.3 255.255.255.0

no shut

router bgp 1

 no synchronization

 bgp router-id 3.3.3.3

 bgp log-neighbor-changes

 neighbor 13.13.13.1 remote-as 1

 neighbor 13.13.13.1 next-hop-self

 neighbor 34.34.34.4 remote-as 2

 no auto-summary

!

R4:-

interface FastEthernet0/1

description " Connected to R6"

 ip address 46.46.46.4 255.255.255.0

no shut

interface FastEthernet1/0

description " Connected to R3"

 ip address 34.34.34.4 255.255.255.0

no shut

router bgp 2

 no synchronization

 bgp log-neighbor-changes

 neighbor 34.34.34.3 remote-as 1

 neighbor 46.46.46.6 remote-as 2

 neighbor 46.46.46.6 next-hop-self

 no auto-summary

!

R5:-

interface Loopback0

 ip address 5.5.5.5 255.255.255.255

interface FastEthernet0/0

description " Connected to R6"

 ip address 56.56.56.5 255.255.255.0

no shut

interface FastEthernet1/0

description " Connected to R2"

 ip address 25.25.25.5 255.255.255.0

no shut

router bgp 2

 no synchronization

 bgp log-neighbor-changes

 neighbor 25.25.25.2 remote-as 1

 neighbor 56.56.56.6 remote-as 2

 neighbor 56.56.56.6 next-hop-self

 no auto-summary

R6:-

interface Loopback0

 ip address 6.6.6.6 255.255.255.255

interface FastEthernet0/0

description " Connected to R5"

 ip address 56.56.56.6 255.255.255.0

no shut

interface FastEthernet0/1

description " Connected to R4"

 ip address 46.46.46.6 255.255.255.0

no shut

interface FastEthernet2/0

description " Connected to PC2"

 ip address 20.1.1.1 255.255.255.0

 no shut

router bgp 2

 no synchronization

 bgp log-neighbor-changes

 network 20.1.1.0 mask 255.255.255.0

 neighbor 46.46.46.4 remote-as 2

 neighbor 56.56.56.5 remote-as 2

 no auto-summary

PC1:-

PC1> show ip

NAME        : PC1[1]

IP/MASK     : 10.1.1.10/24

GATEWAY     : 10.1.1.1

DNS         :

MAC         : 00:50:79:66:68:00

LPORT       : 10003

RHOST:PORT  : 127.0.0.1:10002

MTU:        : 1500

PC2:-

PC2> show ip

NAME        : PC2[1]

IP/MASK     : 20.1.1.10/24

GATEWAY     : 20.1.1.1

DNS         :

MAC         : 00:50:79:66:68:00

LPORT       : 10003

RHOST:PORT  : 127.0.0.1:10002

MTU:        : 1500

Verification:-

R2:-

R2#sh ip bgp summ

BGP router identifier 2.2.2.2, local AS number 1

BGP table version is 3, main routing table version 3

2 network entries using 256 bytes of memory

2 path entries using 104 bytes of memory

2/2 BGP path/bestpath attribute entries using 248 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 632 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

12.12.12.1      4            1     316     317        3    0    0 04:41:15        1

25.25.25.5      4            2     312     313        3    0    0 04:41:10        1

R2#sh ip bgp

BGP table version is 3, local router ID is 2.2.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i10.1.1.0/24      12.12.12.1               0    100      0 i

*> 20.1.1.0/24      25.25.25.5                             0 2 i

R3:-

R3#sh ip bgp summ

BGP router identifier 3.3.3.3, local AS number 1

BGP table version is 3, main routing table version 3

2 network entries using 256 bytes of memory

2 path entries using 104 bytes of memory

2/2 BGP path/bestpath attribute entries using 248 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 632 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

13.13.13.1      4            1     318     314        3    0    0 04:41:25        1

34.34.34.4      4            2     312     316        3    0    0 04:41:21        1

R3#sh ip bgp

BGP table version is 3, local router ID is 3.3.3.3

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i10.1.1.0/24      13.13.13.1               0    100      0 i

*> 20.1.1.0/24      34.34.34.4                             0 2 i

R4:-

R4#sh ip bgp summ

BGP router identifier 4.4.4.4, local AS number 2

BGP table version is 4, main routing table version 4

2 network entries using 256 bytes of memory

2 path entries using 104 bytes of memory

2/2 BGP path/bestpath attribute entries using 248 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 632 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

34.34.34.3      4            1     323     319        4    0    0 04:47:19        1

46.46.46.6      4            2     321     320        4    0    0 04:47:20        1

R4#sh ip bgp

BGP table version is 4, local router ID is 4.4.4.4

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 10.1.1.0/24      34.34.34.3                             0 1 i

*>i20.1.1.0/24      46.46.46.6               0    100      0 i

R4#

R5:-

R5#sh ip bgp summ

BGP router identifier 5.5.5.5, local AS number 2

BGP table version is 3, main routing table version 3

2 network entries using 256 bytes of memory

2 path entries using 104 bytes of memory

2/2 BGP path/bestpath attribute entries using 248 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 632 total bytes of memory

BGP activity 2/0 prefixes, 2/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

25.25.25.2      4            1     321     320        3    0    0 04:47:42        1

56.56.56.6      4            2     319     320        3    0    0 04:47:43        1

R5#sh ip bgp

BGP table version is 3, local router ID is 5.5.5.5

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*> 10.1.1.0/24      25.25.25.2                             0 1 i

*>i20.1.1.0/24      56.56.56.6               0    100      0 i

R5#

R6:-

R6#sh ip bgp summ

BGP router identifier 6.6.6.6, local AS number 2

BGP table version is 4, main routing table version 4

2 network entries using 256 bytes of memory

3 path entries using 156 bytes of memory

2/2 BGP path/bestpath attribute entries using 248 bytes of memory

1 BGP AS-PATH entries using 24 bytes of memory

0 BGP route-map cache entries using 0 bytes of memory

0 BGP filter-list cache entries using 0 bytes of memory

BGP using 684 total bytes of memory

BGP activity 2/0 prefixes, 3/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd

46.46.46.4      4            2     321     322        4    0    0 04:48:27        1

56.56.56.5      4            2     320     319        4    0    0 04:48:28        1

R6#sh ip bgp

BGP table version is 4, local router ID is 6.6.6.6

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale, m multipath, b backup-path, x best-external

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i10.1.1.0/24      46.46.46.4               0    100      0 1 i

* i                 56.56.56.5               0    100      0 1 i

*> 20.1.1.0/24      0.0.0.0                  0         32768 i

R6#

PC1:-

PC1> trace 20.1.1.10

trace to 20.1.1.10, 8 hops max, press Ctrl+C to stop

 1   10.1.1.1   40.000 ms  10.000 ms  10.000 ms

 2   13.13.13.3   30.000 ms  30.000 ms  30.000 ms

 3   34.34.34.4   50.000 ms  50.000 ms  50.000 ms

 4   46.46.46.6   90.000 ms  80.000 ms  70.000 ms

 5   *20.1.1.10   150.000 ms (ICMP type:3, code:3, Destination port unreachable)

PC2:-

PC2> trace 10.1.1.10

trace to 10.1.1.10, 8 hops max, press Ctrl+C to stop

 1   20.1.1.1   10.000 ms  10.000 ms  10.000 ms

 2   46.46.46.4   30.000 ms  30.000 ms  30.000 ms

 3   34.34.34.3   40.000 ms  50.000 ms  50.000 ms

 4   13.13.13.1   60.000 ms  60.000 ms  60.000 ms

 5   *10.1.1.10   70.000 ms (ICMP type:3, code:3, Destination port unreachable)