BGP Load Sharing with 2 ISP’s
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.
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.
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
Ethernet Interface
interface FastEthernet0/0
ip address 10.1.1.1 255.255.255.252
no ip directed-broadcast
duplex auto
speed auto
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
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
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
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 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.
If you have any comments or requests send to
my email
address admin@ccnalab.net
Next: Influencing Inbound Traffic
Cheers
Jim G
IBS Network Team
Thanks to Impact Business Solutions for use of the routers and bandwidth to set
up this scenario
Posted in
Jim,
this load sharing will be per packet or per destination…
which is the best way to control the inbound traffic between two different service provider…
The Load sharing discussed in this document is for outbound traffic only and it is based on a per packet basis. 2 connections will divide your outbound traffic by 50% on each circuit.
You can control inbound traffic by
1. Prepending your AS number to make the link less attractive
2. Some ISP will allow community strings to raise or lower your local preference within the ISP.
I can assist you in determining the best solution if you send me some details about your network to admin@ccnalab.net
Thanks for your Post
Jim G
Hi, as you recommend I added my AS path to the incoming routes from
each ISP but it didnot work, always prefer only one ISP do you have any idea why?
Regards
Hello, A couple of suggestions you should try
1. Make sure the ISP is sending the route to your router: show ip bgp
2. In your BGP configuration you need to set the maximum-paths: maximum-paths 2
3. Sometimes you have filters on your as-path or route-maps.
Post your config minus passwords, ips, as numbers or send me an email with the config attached and the show ip route command admin@ccnalab.net
Thanks
Jim Gonzalez