Requirement:
This article discusses how to modify the BGP routing process to use the Local-Preference rule to select only one path.
Solution:In the following example, switches Rack2sw1, Rack2sw2 and Rack2sw3 are configured for BGP routing. Rack2sw1 is receiving a valid Nexthop from switches Rack2sw2 and Rack2sw3 for the prefix 10.10.10.10/32 (see output of show ip bgp below).
Rack2sw1# show ip bgp
Local AS : 1 Local Router-id : 1.1.1.1
BGP Table Version : 6
Status codes: * - valid, > - best, i - internal, e - external, s - stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Nexthop Metric LocalPref Weight AsPath
------------------ --------------- ---------- ---------- ------ ---------
* e 10.10.10.10/32 192.168.12.2 0 0 2 ?
*>e 10.10.10.10/32 192.168.12.3 0 0 3 ?
By default the maximum number of equal cost IP load sharing paths is four.
This is the reason for two entries in the routing table for the 10.10.10.10/32 prefix:
Rack2sw1# show ip route
IP Route Entries
Destination Gateway VLAN Type Sub-Type Metric Dist.
------------------ --------------- ---- --------- ---------- ---------- -----
10.10.10.10/32 192.168.12.2 1 bgp external 0 20
10.10.10.10/32 192.168.12.3 1 bgp external 0 20
The "ip load-sharing" configuration option can be configured for the values of 2-4. Having only one path for a prefix requires configuring the BGP router for path selection. The following BGP configuration is an example that does not include a path selection:
Rack2sw1# show running-config router bgp
Running configuration:
router bgp 1
enable
bgp router-id 1.1.1.1
neighbor 192.168.12.2 remote-as 2
neighbor 192.168.12.3 remote-as 3
The first three BGP path selection rules follow.
• Prefer the route with the lowest Administrative Distance.
• Prefer the route with the larger weight.
• Prefer the route with the highest LOCAL_PREF value.
Configuration:The modified switch configuration for path selection;
• Adds an “IP Prefix-List” which matches the IP address 10.10.10.10.
• Adds a “Route-Map” which sets the BGP Local-Preference for the neighbor Rack2sw1.
• The BGP configuration includes the Route-Map .
********** IP Prefix-List to match Rack2sw1 **********
Rack2sw1# show run | begin prefix
ip prefix-list "ten-ten-ten-ten" seq 5 permit 10.10.10.10 255.255.255.255 ge 32 le 32
Rack2sw1# show ip prefix-list
ip prefix-list ten-ten-ten-ten: 1 entries
5 permit 10.10.10.10/32 ge 32 le 32
********** Route-Map to set the BGP local preference **********
Rack2sw1# show running-config | begin route-map "Local-Preference" permit
route-map "Local-Preference" permit seq 10
match ip address prefix-list "ten-ten-ten-ten"
set local-preference 110
Rack2sw1# show route-map
Routemap information
route-map "Local-Preference" permit seq 10
match ip address prefix-list "ten-ten-ten-ten"
set local-preference 110
********** Modify the BGP routing process to use the route-map **********
Rack2sw1# show running-config router bgp
Running configuration:
router bgp 1
enable
bgp router-id 1.1.1.1
neighbor 192.168.12.2 remote-as 2
neighbor 192.168.12.2 route-map "Local-Preference" in <==========
neighbor 192.168.12.3 remote-as 3
********** Restart BGP neighbor **********
Rack2sw1# clear ip bgp neighbor 192.168.12.2
VerificationUpdated Routing Table Has A Single Entry For 10.10.10.10/32:
Rack2sw1# show ip route bgp
IP Route Entries
Destination Gateway VLAN Type Sub-Type Metric Dist.
------------------ --------------- ---- --------- ---------- ---------- -----
10.10.10.10/32 192.168.12.2 1 bgp external 0 170
Rack2sw1# show ip bgp 10.10.10.10 255.255.255.255
Local AS : 1 Local Router-id : 1.1.1.1
BGP Table Version : 89
Network : 10.10.10.10/32 Nexthop : 192.168.12.2
Peer : 192.168.12.2 Origin : incomplete
Metric : 0 Local Pref :
Weight : 0 Calc. Local Pref : 110
Valid : Yes Type : external
Stale : No
Best : Yes (Higher Local Preference)
AS-Path : 2
Communities :
Network : 10.10.10.10/32 Nexthop : 192.168.12.3
Peer : 192.168.12.3 Origin : incomplete
Metric : 0 Local Pref :
Weight : 0 Calc. Local Pref : 100
Valid : Yes Type : external
Stale : No
Best : No
AS-Path : 3
Communities :