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BGP load sharing question 11 years 1 month ago #34439

  • sam4sg
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As I understand in BGP load sharing, outgoing load sharing is done using weight and incoming load sharing is done using router map and prepend.

Right now, I have a single ISP and therefore no load sharing.
To the ISP, the BGP configuration is ( this config was sent by the ISP to be done in ours)


router bgp 100
no synchronization
bgp log-neighbor-changes
network mask
neighbor remote-as 200
neighbor description eBGP peering with ISP Router
neighbor version 4
neighbor send-community
neighbor prefix-list AS200-IN in
neighbor prefix-list AS200-OUT out
no auto-summary

ip prefix-list AS200-IN seq 10 permit
ip prefix-list AS200-IN seq 100 deny le 32

ip prefix-list AS200-OUT seq 10 permit
ip prefix-list AS200-OUT seq 100 deny

ip classless
ip route Null0 250
ip http server
I don't have an exact 100% idea on what some of the above settings mean.

Now, we are trying to connect to another ISP (assume BGP 400, with neighbour IP:,
how do i config it such that we can load share the incoming and outgoing traffic.

We have 4 IP ranges

How to use route-map and prepend to ISP 1 so that 64.0 and 66.0 is prefered incoming via ISP1 and
65.0 and 67.0 is prefered incoming via ISP2. And also how to use the weight feature here ?

In a summary, need help such that.

both ISP are on BGP.
range 64.x and 66.x is prefered incoming via ISP1
range 65.x and 67.x is prefered incoming via ISP2
50% weight or equal outgoing traffic share via both ISPs.

Is it also possible to create a preferred route based on source or destination IP?

In all cases, if ISP1 is down, all must work with ISP2 and vice versa.

Please let me know if more info is required.

Re: BGP load sharing question 10 years 7 months ago #35737

For load sharing for outgoing traffic you can use the multipath option in BGP. With this you can use more then 1 best path.

For incomming load-sharing you have to (as you said) prepand your AS at the path you want to use at a backup.
There are more ways to accomplish this, my prefered way is with creating an access-list, match the ACL in your route-map and prepend your AS number.

Example configuration to prepand as-path 100 four times.:

access-list 10 permit
access-list 10 permit

route-map prepend_1 permit 10
match ip address 10
set as-path prepend 100 100 100 100

route-map prepend_1 permit 20
! Needed to allow the other ranges to be advertized

router bgp 100
neighbor route-map prepend_1 out

This way you influence the way you advertise your networks.
Normale your network would be reacable by just 100 (1 as_path)

now it is 100_100_100_100_100 (5 paths).

BGP will make a route decision based on a couple of things where-as AS-PATH is the first decision-maker at the most ISP's.[/i]
CCNA / CCNP / CCNA - Security / CCIP / Prince2 / Checkpoint CCSA
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