In this scenario, load balancing is not an option in a multihomed
environment, so you can only do load sharing. You cannot do load
balancing because BGP selects only a single best path to a destination
among the BGP routes that are learned from the different ASs. The idea
is to set a better metric for the routes in the range 1.0.0.0 to
128.0.0.0 that are learned from ISP(A) and a better metric for the rest
of the routes that are learned from ISP(B).
Network Diagram
This section uses this network setup:
Configurations
This section uses these configurations:| RouterA |
|---|
interface Serial 0
ip address 160.20.20.1 255.255.255.0
no ip route-cache
interface Serial 1
ip address 150.10.10.1 255.255.255.0
no ip route-cache
router bgp 11
neighbor 160.20.20.2 remote-as 10
neighbor 160.20.20.2 route-map UPDATES-1 in
!--- This allows only the networks up to 128.0.0.0.
neighbor 150.10.10.2 remote-as 12
neighbor 150.10.10.2 route-map UPDATES-2 in
!--- This allows anything above the 128.0.0.0 network.
auto-summary
route-map UPDATES-1 permit 10
match ip address 1
set weight 100
route-map UPDATES-1 permit 20
match ip address 2
route-map UPDATES-2 permit 10
match ip address 1
route-map UPDATES-2 permit 20
match ip address 2
set weight 100
access-list 1 permit 0.0.0.0 127.255.255.255
access-list 2 deny 0.0.0.0 127.255.255.255
access-list 2 permit any
|
| RouterB |
|---|
interface Loopback0 ip address 2.2.2.2 255.255.255.0 int loopback 1 ip address 170.16.6.5 255.255.255.0 interface Serial 0 ip address 160.20.20.2 255.255.255.0 no ip route-cache router bgp 10 neighbor 160.20.20.1 remote-as 11 network 2.0.0.0 network 170.16.0.0 auto-summary |
| RouterC |
|---|
interface Loopback0 ip address 170.16.6.6 255.255.255.0 interface Loopback1 ip address 2.2.2.1 255.255.255.0 interface Serial 1 ip address 150.10.10.2 255.255.255.0 no ip route-cache router bgp 12 neighbor 150.10.10.1 remote-as 11 network 2.0.0.0 network 170.16.0.0 auto-summary |
Verify
Use this section to confirm that your configuration works properly.The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.
The output from the show ip route command and the output from the traceroute command show that any network lower than 128.0.0.0 exits RouterA through 160.20.20.2. This route is the next hop out of the serial 0 interface. The rest of the networks exit through 150.10.10.2, which is the next hop out of the serial 1 interface.
RouterA# show ip route !--- Output suppressed. Gateway of last resort is not set B 170.16.0.0/16 [20/0] via 150.10.10.2, 00:43:43 !--- This is the next hop out through serial 1. B 2.0.0.0/8 [20/0] via 160.20.20.2, 00:43:43 !--- This is the next hop out through serial 0. 160.20.0.0/24 is subnetted, 1 subnets C 160.20.20.0 is directly connected, Serial0 150.10.0.0/24 is subnetted, 1 subnets C 150.10.10.0 is directly connected, Serial1 RouterA# show ip bgp BGP table version is 3, local router ID is 160.20.20.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path Network Next Hop Metric LocPrf Weight Path * 2.0.0.0 150.10.10.2 0 0 12 i *> 160.20.20.2 0 100 10 i * 170.16.0.0 160.20.20.2 0 0 10 i *> 150.10.10.2 0 100 12 i RouterA# traceroute 2.2.2.2 Type escape sequence to abort. Tracing the route to 2.2.2.2 1 160.20.20.2 16 msec * 16 msec RouterA# traceroute 170.16.6.6 Type escape sequence to abort. Tracing the route to 170.16.6.6 1 150.10.10.2 4 msec * 4 msec
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