11-06-2013 02:45 PM - edited 03-04-2019 09:30 PM
My current routing table looks like this for the default route
C2921-ISP2-1# sh ip bgp 0.0.0.0
BGP routing table entry for 0.0.0.0/0, version 4
Paths: (2 available, best #1, table default)
Advertised to update-groups:
4
Refresh Epoch 1
4922 2914
204.111.0.65 from 204.111.0.65 (204.111.0.5)
Origin IGP, localpref 100, valid, external, best
Refresh Epoch 1
64512 26202
216.235.224.1 from 216.235.224.1 (216.235.224.253)
Origin IGP, localpref 100, valid, external
The BGP configuration is simple:
router bgp 64512
bgp log-neighbor-changes
bgp bestpath as-path ignore
neighbor 204.111.0.65 remote-as 4922
neighbor 204.111.0.65 ebgp-multihop 2
neighbor 204.111.0.65 update-source GigabitEthernet0/2
neighbor 216.235.224.1 remote-as 26202
neighbor 216.235.224.1 ebgp-multihop 2
neighbor 216.235.224.1 update-source GigabitEthernet0/1
neighbor 216.235.224.1 route-map as-Kinex in
The router is picking the 204.111.0.65 path because it has the lowest router ID. Same AS hops.
Can I load balance outbound flows (not packet by packet) across these two ISPs?
11-06-2013 04:01 PM
You CANT truly load balance using BGP. For your outbound connections you will need to play with your local prefence, betweem the two eBGP neighbours.
something like:
The BGP configuration is simple:
router bgp 64512
bgp log-neighbor-changes
bgp bestpath as-path ignore
neighbor 204.111.0.65 remote-as 4922
neighbor 204.111.0.65 ebgp-multihop 2
neighbor 204.111.0.65 update-source GigabitEthernet0/2
neighbor 204.111.0.65 route-map blah1 in
neighbor 216.235.224.1 remote-as 26202
neighbor 216.235.224.1 ebgp-multihop 2
neighbor 216.235.224.1 update-source GigabitEthernet0/1
neighbor 216.235.224.1 route-map blah 2 in
route-map blah1 permit 10
set local-preference 110
route-map blah2 permit 10
set local-preference 90
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11-07-2013 05:18 AM
Thanks for your response. What about the
bgp bestpath as-path multipath-relax
command? Someone just brought this (hidden?) command to my attention.
11-07-2013 05:26 AM
Hello,
yes, the multipath-relax option will work here. Be aware that you need to enable maximum-paths 2 as well. Here you may find more information about this option:
http://wiki.nil.com/Multipath_Load_Sharing_with_two_ISPs
Hope this helps,
Jose.
11-07-2013 05:56 AM
Let me suggest a slightly different approach, which is easy and works very well for sharing load for multiple outbound BGP peers. I can not tell from this thread so far whether the ISPs are advertising more than a default route. With most ISP when you run BGP with them you have a choice of whether they will advertise just default, or advertise limited routes, or will advertise the full table. If you request at least one of your ISP to advertise limited routes then there will be a group of routes in your table that are unique to that ISP and that traffic will go to that ISP. If you use local preference to prefer the default route from the other ISP then you will automatically have load sharing between the 2 ISP for outbound connections.
HTH
Rick
11-07-2013 07:30 AM
Richard, we may want to do that.
I have in the past asked ISPs for full or for default routes.
First question: 1) Is there a standard for an ISP to advertise "limited routes".
Ideally, I would think it would split the public address range with a limited number of summary advertisements and omit the private ranges. One ISP would take the high range, one ISP would take the low. However, you mentioned only one would provide me with a limited number of routes - giving me a more specific match to their service - and the other would continue with only advertising the default route.
Second question: 2) This is related to failover for the scenario above. How would that work? The customer I am working with needs complete ISP failover (K-12 customer). It would seem I would not have that if something in the link of ISP advertising the default route fails.
11-07-2013 07:53 AM
1) I am not aware of a standard for advertising limited routes. Frequently it is that the ISP will advertise routes for its directly connected customers, or perhaps it will advertise routes that have no more than 2 or 3 or x number of hops in the AS path. I have not seen it done as a low range/high range but ask your providers and perhaps they can do that if you ask.
I suggested having one ISP advertise limited routes and one advertise default because it is the most simple to understand. It is, of course, quite possible for each provider to advertise its own limited routes and would probably result in a more efficient routing decision if we can choose the shortest path to some destinations.
2) I was explicit about learning a default from 1 ISP to make the point that at least 1 ISP needs to advertise a default. It is certainly possible, and probably desirable, that both ISP advertise a default route. And if both ISP do advertise a default route then failover is taken care of.
HTH
Rick
11-07-2013 07:20 AM
Thanks, Jose. We have a window to try this command this afternoon.
11-07-2013 07:58 AM
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Another option you might wish to consider, if your device supports PfR, that will dynamically load balance. (Personally, I've found setting up equal paths with the hidden relax command, and just taking defaults, and then allows PfR to "fine tune", works very well.)
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