BGP loadsharing

[policymap]

We have two Cisco routers (3660 & 7206) running BGP4 towards our peering partners. The problem is that there is no loadsharing between the links, because the they are both connected to the same backbone AS. Which results in BGP taking "shortest path", and always routing though link 1.
I'm announcing a /20 out on both links.

Is there any way to loadshare the trafic without breaking the /20 up?

 

[baddos]

What do you mean by connected to the same backbone AS? Do you mean that they are connected to the same ISP with two links and you want to load balance? Also, do you want to loadbalanace incoming or outgoing or both?

 

[policymap]

Sorry for beeing so unclear
Our links are connected to two different ISP's, ISP1 and ISP2. The problem arises from, that ISP2 also has their connection to ISP1. Therefore routes through ISP2 are always 1 hop longer.
I would like to loadbalance both in/out!

 

[baddos]

Ok... Try prepending the outgoing packets to isp1 then. This will make the aspaths equal, but it won't necessarily loadshare. There is a lot of variables involved downstream from those two ISPs, but it should make it load share more than you have now.

ip prefix-list mysubnets description My advertised subnets
ip prefix-list mysubnets seq 5 1.1.1.0/24
ip prefix-list mysubnets seq 10 2.2.2.0/24
!
router bgp (your as number)
neighbor xxx.xxx.xxx.xxx route-map isp1 out
!
route-map isp1 permit 10
match ip prefix-list mysubnets
set as-path prepend (your AS number)
!

 

[policymap]

thanks for the reply Baddos, but I already tried this.
and this will only route all trafic through ISP2, and not loadbalance anything. This also makes sence, because from the "outside world" routes to our network will always be seen through ISP1.

any other suggestion?

 

[baddos]

ISP2's only connection is through ISP1? I just don't see how that's possible, but you never know. If they are multihomed, but still not equal to ISP1 try prepending one additional time. Or you can try setting the same metric and weight on both routers.

I.E. Router one's route-map has:

match ip prefix-list mysubnets
set as-path prepend (your AS number)
set metric 10
set weight 10

Route two's route-map has:
match ip prefix-list mysubnets
set metric 10
set weight 10

 

[policymap]

ISP2 is in fact multihomed, but to the same AS (europe & North America), so prepend can not do the job.

one alternativ (for outbound) would perhaps be, to request to recive a community string from ISP1, so we can distinct between the destinations, and set a higher local preference.

for inbound, a alternativ would be that ISP1 sets a higher preference on our prefixes, with path "_ISP2(AS) our(AS)" on f.ex US routers.

but this of cource requires some configuration at ISP1, and this could take some time