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BGP Load Sharing with 2 ISP’s

20091202 | No Comments | Labels :

Problem:
Two or more separate connections to the internet from different providers provides network redundancy. Adding BGP routing with an AS (Autonomous System) number will provide network failover without IP migration. The default BGP configuration provides a single path selection. This tutorial will provide you the information to load balance or share your outbound network connections evenly. 
Tools:
Monitoring tool of bandwidth (I like MRTG)
AS number from Arin.net
Class C network address or /24 Most ISP’s will not announce routes less than a /24
Default routing from your providers at a minimum. This will work with partial routes and default routes. 





Network Diagram:






BGP ins – outs:
BGP will only select one path (the best) to install in the routing table. BGP has a command maximum-paths. The maximum-paths command will load share between equal costs routes. The problem is the AS paths are different because you have internet connections to separate providers.
Solution:
The solution is very simple we will add our AS path to the incoming routes from each ISP. The router will think the routes are from the SAME AS and will install both into routing table. We will need to install the maximum-paths 2 in the bgp configuration because bgp does not load share by default. 

I am using fast Ethernet connection because I do not have enough serial connections 

ISP A Router Configuration:
Ethernet Interface
interface FastEthernet0/0
ip address 10.1.1.1 255.255.255.252
no ip directed-broadcast
duplex auto
speed auto

BGP Configuration to send default routing 
router bgp 65535
neighbor 10.1.1.2 remote-as 27006
neighbor 10.1.1.2 default-originate
neighbor 10.1.1.2 soft-reconfiguration inbound

ISP B Router Configuration
Ethernet Interface
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.252
no ip directed-broadcast
duplex auto
speed auto

BGP Configuration to send default routing to the customer
router bgp 6500
neighbor 10.0.0.2 remote-as 27006
neighbor 10.0.0.2 default-originate
neighbor 10.0.0.2 soft-reconfiguration inbound

CPE Router Configuration

Fast Ethernet Interfaces 

interface FastEthernet0/0
description to_ISP_A
ip address 10.1.1.2 255.255.255.252
duplex auto
speed auto

interface FastEthernet0/1
description to_ISP_B
ip address 10.0.0.2 255.255.255.252
duplex auto
speed auto

BGP Configuration

router bgp 27006
bgp log-neighbor-changes
neighbor 10.0.0.1 remote-as 6500
neighbor 10.0.0.1 route-map prepend-as in
neighbor 10.1.1.1 remote-as 65535
neighbor 10.1.1.1 soft-reconfiguration inbound
neighbor 10.1.1.1 route-map prepend-as in
maximum-paths 2

Route Map for Prepending AS 

route-map prepend-as permit 10
set as-path prepend 27006



Check you work with show commands from the CPE

Make sure you both bgp peers are connected and sharing information

Show ip bgp summary

BGP router identifier 10.1.1.2, local AS number 27006
BGP table version is 2, main routing table version 2
1 network entries using 97 bytes of memory
3 path entries using 108 bytes of memory
1 multipath network entries and 2 multipath paths
4 BGP path attribute entries using 240 bytes of memory
4 BGP AS-PATH entries using 96 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 541 total bytes of memory
1 received paths for inbound soft reconfiguration
BGP activity 2/1 prefixes, 5/2 paths, scan interval 60 secs

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.0.1 4 6500 21 23 2 0 0 00:03:30 1
10.1.1.1 4 65535 22 23 2 0 0 00:03:29 1

Check to see if each peer is sending you a default route

Check the bgp table to see the routes Notice you will see you AS number prepeneded to your ISP's AS number. This is normal and needed for the configuration to work.

cpe#show ip bgp
BGP table version is 2, local router ID is 10.1.1.2
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
*> 0.0.0.0 10.1.1.1 0 27006 65535 i
* 10.0.0.1 0 27006 6500 i

cpe#show ip bgp neighbors 10.0.0.1 received-routes 
BGP table version is 4, local router ID is 10.1.1.2
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
* 0.0.0.0 10.0.0.1 0 6500 i



cpe#show ip bgp neighbors 10.1.1.1 received-routes 
BGP table version is 2, local router ID is 10.1.1.2
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
* 0.0.0.0 10.1.1.1 0 65535 i



Check to see if both routes have been installed in the routing table.

Show ip route


cpe#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
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

Gateway of last resort is 10.1.1.1 to network 0.0.0.0

10.0.0.0/30 is subnetted, 2 subnets
C 10.0.0.0 is directly connected, FastEthernet0/1
C 10.1.1.0 is directly connected, FastEthernet0/0
B* 0.0.0.0/0 [20/0] via 10.1.1.1, 00:06:18
             [20/0] via 10.0.0.1, 00:06:18




Everything looks great.  Now configure your monitoring tools to monitor your outbound traffic, you should experience almost evenly distributed traffic.



Thanks to Impact Business Solutions for use of the routers and bandwidth to set up this scenario 


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