BGP Weight Attribute Explained (Part 2)

This is another post about BGP Weight Attributes.

Objective:

1. Established EBGP peering using two methods.

2. Implement a policy for so that all networks on R4 will take the path via R3.

Topology:

R1 BGP Configurations:

R1#sh run | sec bgp
router bgp 1
bgp log-neighbor-changes
network 100.100.100.100 mask 255.255.255.255
neighbor 2.2.2.2 remote-as 2 <<<< Method # 1 to create BGP peering
neighbor 2.2.2.2 ebgp-multihop 10
neighbor 2.2.2.2 update-source Loopback0
neighbor 192.168.13.3 remote-as 2 <<< Method # 2 to create BGP peering

Static Route in R1 and R2:

R1#sh run | inc ip route
ip route 2.2.2.2 255.255.255.255 192.168.12.2

R2#sh run | inc ip route
ip route 1.1.1.1 255.255.255.255 192.168.12.1

IGP Configs on R2:

R2#sh run | sec ospf
router ospf 1
network 2.2.2.2 0.0.0.0 area 0
network 192.168.24.0 0.0.0.255 area 0

IBGP Configs on R2:

R2#sh run | sec bgp
router bgp 2
bgp log-neighbor-changes
network 192.168.12.0
neighbor 1.1.1.1 remote-as 1 <<< Method # 1 to create BGP peering
neighbor 1.1.1.1 ebgp-multihop 10
neighbor 1.1.1.1 update-source Loopback0
neighbor 3.3.3.3 remote-as 2
neighbor 3.3.3.3 update-source Loopback0
neighbor 4.4.4.4 remote-as 2
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 next-hop-self

IGP Configs on R3:

R3#sh run | sec ospf
router ospf 1
network 3.3.3.3 0.0.0.0 area 0
network 192.168.34.0 0.0.0.255 area 0

IBGP Configs on R3:

R3#sh run | sec bgp
router bgp 2
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 2
neighbor 2.2.2.2 update-source Loopback0
neighbor 4.4.4.4 remote-as 2
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 next-hop-self
neighbor 192.168.13.1 remote-as 1

IGP Configs on R4:

R4#sh run | sec ospf
router ospf 1
network 4.4.4.4 0.0.0.0 area 0
network 192.168.24.0 0.0.0.255 area 0
network 192.168.34.0 0.0.0.255 area 0

BGP Configs on R4:

R4#sh run | sec bgp
router bgp 2
bgp log-neighbor-changes
network 11.11.11.11 mask 255.255.255.255
network 22.22.22.22 mask 255.255.255.255
network 33.33.33.33 mask 255.255.255.255
network 44.44.44.44 mask 255.255.255.255
neighbor 2.2.2.2 remote-as 2
neighbor 2.2.2.2 update-source Loopback0
neighbor 3.3.3.3 remote-as 2
neighbor 3.3.3.3 update-source Loopback0

Checking OSPF Neighbors:

R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
44.44.44.44 0 FULL/ – 00:00:36 192.168.24.4 Serial3/2

R3# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
44.44.44.44 0 FULL/ – 00:00:32 192.168.34.4 Serial3/3

R4#show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           0   FULL/ –        00:00:38    192.168.34.3    Serial3/3
2.2.2.2           0   FULL/ –        00:00:32    192.168.24.2    Serial3/2

Checking BGP Neighbors:

R1#sh ip bgp summary
BGP router identifier 100.100.100.100, local AS number 1
BGP table version is 6, main routing table version 6
5 network entries using 720 bytes of memory
5 path entries using 400 bytes of memory
2/2 BGP path/bestpath attribute entries using 272 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 1416 total bytes of memory
BGP activity 5/0 prefixes, 5/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2               4            2      27      27        6    0     0                  00:20:31        2
192.168.13.3    4            2      26      27        6    0    0               00:20:27        2

R2#sh ip bgp summary
BGP router identifier 2.2.2.2, local AS number 2
BGP table version is 7, main routing table version 7
6 network entries using 864 bytes of memory
7 path entries using 560 bytes of memory
4/3 BGP path/bestpath attribute entries using 544 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 1992 total bytes of memory
BGP activity 6/0 prefixes, 7/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer InQ OutQ Up/Down State/PfxRcd
1.1.1.1         4            1      28      27        7    0    0 00:20:40        1
3.3.3.3         4            2      26      28        7    0    0 00:20:26        1
4.4.4.4         4            2      27      27        7    0    0 00:20:28        4
R2#

R3#show ip bgp summary
BGP router identifier 3.3.3.3, local AS number 2
BGP table version is 7, main routing table version 7
6 network entries using 864 bytes of memory
7 path entries using 560 bytes of memory
3/2 BGP path/bestpath attribute entries using 408 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 1856 total bytes of memory
BGP activity 6/0 prefixes, 7/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2         4            2      28      26        7    0    0 00:20:38        2
4.4.4.4         4            2      26      27        7    0    0 00:20:41        4
192.168.13.1    4            1      28      27        7    0    0 00:20:47        1

R4#show ip bgp summary
BGP router identifier 44.44.44.44, local AS number 2
BGP table version is 8, main routing table version 8
6 network entries using 864 bytes of memory
7 path entries using 560 bytes of memory
3/3 BGP path/bestpath attribute entries using 408 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 1856 total bytes of memory
BGP activity 6/0 prefixes, 7/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2         4            2      28      27        8    0    0 00:21:05        2
3.3.3.3         4            2      27      26        8    0    0 00:21:05        1

Checking the BGP Topology

R1#sh ip bgp
BGP table version is 6, local router ID is 100.100.100.100
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
*> 11.11.11.11/32   2.2.2.2                                0 2 i
*                    192.168.13.3                           0 2 i
*> 22.22.22.22/32   2.2.2.2                                0 2 i
*                    192.168.13.3                           0 2 i
*> 33.33.33.33/32   2.2.2.2                                0 2 i
*                    192.168.13.3                           0 2 i
*> 44.44.44.44/32   2.2.2.2                                0 2 i
*                    192.168.13.3                           0 2 i
r> 192.168.12.0     2.2.2.2                  0             0 2 i
r                    192.168.13.3                           0 2 i

R2#sh ip bgp
BGP table version is 9, 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, 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
*>i 11.11.11.11/32   4.4.4.4                  0    100      0 i
*>i 22.22.22.22/32   4.4.4.4                  0    100      0 i
*>i 33.33.33.33/32   4.4.4.4                  0    100      0 i
*>i 44.44.44.44/32   4.4.4.4                  0    100      0 i
* i 100.100.100.100/32
                       192.168.13.1             0    100      0 1 i
*>                   1.1.1.1                  0             0 1 i
*> 192.168.12.0     0.0.0.0                  0         32768 i

R3#show ip bgp
BGP table version is 9, 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, 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
*>i 11.11.11.11/32   4.4.4.4                  0    100      0 i
*>i 22.22.22.22/32   4.4.4.4                  0    100      0 i
*>i 33.33.33.33/32   4.4.4.4                  0    100      0 i
*>i 44.44.44.44/32   4.4.4.4                  0    100      0 i
* i 100.100.100.100/32
                       1.1.1.1                    0    100      0 1 i
*>                   192.168.13.1           0             0 1 i
*>i 192.168.12.0     2.2.2.2                  0    100      0 i

R4#show ip bgp
BGP table version is 11, local router ID is 44.44.44.44
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
*> 11.11.11.11/32   0.0.0.0                  0         32768 i
*> 22.22.22.22/32   0.0.0.0                  0         32768 i
*> 33.33.33.33/32   0.0.0.0                  0         32768 i
*> 44.44.44.44/32   0.0.0.0                  0         32768 i
* i 100.100.100.100/32
                       3.3.3.3                  0    100      0 1 i
*>i                  2.2.2.2                  0    100      0 1 i
*>i 192.168.12.0     2.2.2.2                  0    100      0 i

Let’s check the routing table on R1:

R1#show ip route bgp
Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
       E1 – OSPF external type 1, E2 – OSPF external type 2
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      11.0.0.0/32 is subnetted, 1 subnets
B        11.11.11.11 [20/0] via 2.2.2.2, 00:03:03
      22.0.0.0/32 is subnetted, 1 subnets
B        22.22.22.22 [20/0] via 2.2.2.2, 00:03:03
      33.0.0.0/32 is subnetted, 1 subnets
B        33.33.33.33 [20/0] via 2.2.2.2, 00:03:03
      44.0.0.0/32 is subnetted, 1 subnets
B        44.44.44.44 [20/0] via 2.2.2.2, 00:03:03

** As observed, the paths towards R4 network is via R2.

To satisfy the second objective, I’m creating a route-map to influence the path towards R3:

R1(config)#route-map SET-WEIGHT-R3 permit 10
R1(config-route-map)#set weigh
R1(config-route-map)#set weight 500
R1(config-route-map)#match ip address 1

** I have increased the weight to 500 ..the default weight is 0. By setting the weight to higher value than 0, then it will prefer the path with higher weight attribute”

Here’s the access-list that will be used for the route-map SET-WEIGHT-R3

R1(config)#access-list 1 permit 11.11.11.11 0.0.0.0
R1(config)#access-list 1 permit 22.22.22.22 0.0.0.0
R1(config)#access-list 1 permit 33.33.33.33 0.0.0.0
R1(config)#access-list 1 permit 44.44.44.44 0.0.0.0

Apply the route-map on R1 to R3 BGP peering.

R1(config)#router bgp 1
R1(config-router)#neighbor 192.168.13.3 route-map SET-WEIGHT-R3 in

Now, let’s check the new BGP topology after clearing BGP.

R1#clear ip bgp *
R1#
*Sep 21 22:38:43.363: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Down User reset
*Sep 21 22:38:43.363: %BGP_SESSION-5-ADJCHANGE: neighbor 2.2.2.2 IPv4 Unicast topology base removed from session User reset
*Sep 21 22:38:43.371: %BGP-5-ADJCHANGE: neighbor 192.168.13.3 Down User reset
*Sep 21 22:38:43.375: %BGP_SESSION-5-ADJCHANGE: neighbor 192.168.13.3 IPv4 Unicast topology base removed from session User reset
*Sep 21 22:38:43.871: %BGP-5-ADJCHANGE: neighbor 192.168.13.3 Up
*Sep 21 22:38:43.875: %BGP-5-ADJCHANGE: neighbor 2.2.2.2 Up

R1#sh ip bgp
BGP table version is 7, local router ID is 100.100.100.100
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
*   11.11.11.11/32   2.2.2.2                                0             2 i
*>                   192.168.13.3                         500         2 i
*   22.22.22.22/32   2.2.2.2                                0           2 i
*>                   192.168.13.3                         500             2 i
*   33.33.33.33/32   2.2.2.2                                0           2 i
*>                   192.168.13.3                         500             2 i
*   44.44.44.44/32   2.2.2.2                                0     2 i
*>                   192.168.13.3                         500             2 i
*> 100.100.100.100/32
                       0.0.0.0                  0         32768 i
r> 192.168.12.0     2.2.2.2                  0             0 2 i

*** As observed, we can see that the best path have changed to R3.****

NETWORK LESSON