Load Balance between to Serial lines - BGP

Unanswered Question
Jul 24th, 2009

Hi Everyone

Today i have Router Cisco 3845 connected with Serial interface to my uplink ISP, there is BGP peer between the routers and i am advertise my network to the upstream provider.

i am going to have additional router with additional serial interface that will connect to the same isp on the same router.

what is the best practice of doing Load Balace so i will be able to achive load-balance on the serials.

I have this problem too.
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Laurent Aubert Fri, 07/24/2009 - 06:11

Hi Juda,

What is your topology behind your two Internet routers ?

If they are sharing the same LAN, you could use GLBP (or several HSRP groups but it's less flexible) if there are different hosts or L3 devices doing static routing


If you are running a routing protocol on the LAN, both Internet routers can announce a default route with the same metric so they will be both installed by the other routers.

You can't load-balance using BGP because you have two routers (your ISP will because both links end on the same router).



Joseph W. Doherty Fri, 07/24/2009 - 15:56

"You can't load-balance using BGP because you have two routers . . ."

True once you get to each router, however OER/PfR technology can redirect traffic to BGP peer. (NB: it works best to get a "static" load balance using one of the techniques Laurent describes, then allow OER/PfR to fine tune based on actual load. Also note, works best for outbound, although PfR can also fine tune inbound.)

juda.barnes Tue, 07/28/2009 - 09:56

Okay guys fist of all thanks for you assistance till now :)

I have a switch (only one vlan) which connect to two cisco 3800 routers,

from the prespective of the LAN i am using

GLBAP in order to load balance the uplink traffic from my network to the world.

the two routers are connected using Serial line to my ISP, my ISP own 7200 Router.

both routers are within the same AS number,

and they have BGP peer to my ISP,

Both of them advertise my network.

Since i have GLBP then all the uplink traffic is being load balance,

how ever my problem is to load balance the downlink traffic,

right now in order to find a solution i asked my ISP to insert to static routes in his router so the router is preform default load balance (per packet or per flow) because the routing table contain two entries.

how ever i am looking for another type of solution maybe with eigrp or any other suggestions will be satisfy.

please remeber my problem is that i had 1 link which has been loaded and now i have two links and i want to load balance and achive high avilability.

HEre is the figure of the network


| R1 |------------

********|-----| |

* |

**** |

* S* |------------|


* I* |------------|

* T* ^

* C* |----| |

* H* | R2 |--------------

* |----|


Laurent Aubert Tue, 07/28/2009 - 10:16


If you have direct links meaning a link failure will be detected by you and your ISP (interface will get down) then you have a good solution.

don't forget the iBGP session between your routers to re-route the traffic to the other one in case of link failure.

I don't see why you are looking for another solution.



Joseph W. Doherty Tue, 07/28/2009 - 10:28

The only Cisco solution, which I'm aware, that combines routing with dynamic load balancing, is OER/PfR technology.


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