kamikatze
join:2007-11-02
| BGP Multihoming default-originate only setup
I'm in the process of setting up BGP sessions with two ISPs (one is Tier-1, one is Tier-not-so-one :)
What i am trying to achieve is graceful failover between the two links. I will only be receiving default-originate from both ISP.
Scenario:
---------
* Default route is 0.0.0.0 via ISP1.
* ISP1 goes down, default route gets replaced by 0.0.0.0/0 via ISP2.
* ISP1 comes back up, default route gets replaced by 0.0.0.0/0 via ISP1.
* ISP2 goes down, nobody cares.
AS1 = me
AS10 = ISP1
AS20 = ISP2
My current config looks like this:
I have a 10Mbit link with ISP1 and 1mbps with ISP2, therefore it makes sense to route all traffic through ISP1, no load balancing.
Let me understand if i got this right:
I've setup 2 route-maps like this
This indeed works and takes care of the upstream traffic.
Is this done better through neighbor weight, or is it the same thing?
The downstream path however, as far as i have read, needs to be adjusted by prepending my AS when advertising to my neighbors.
So what do i do now? :)
Would this make it a looooong long way for the internet to reach me via ISP2, so ISP1's AS would be best path?
Thank you for any insights. |
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aryoba
Premium,MVM
join:2002-08-22
| Your best bet is probably using both ISP' BGP Community, preferably with Local Preference and AS Path Prepend tuning features of both your network and theirs. This way, you can ensure the inbound and outbound traffic behaviors.
However before getting on those, you also have to confirm that your IGP network design is looking good. It is a good practice to have a good IGP network design before moving forward to BGP network design.
Are you running dynamic routing as IGP or not? Probably something like OSPF or EIGRP? If yes, then you have to make sure that the BGP and the IGP is communicating with each other properly.
In addition,
1. How large is your BGP-announced network? /24? Larger?
2. Do you announce your own network via BGP to both ISP? Or do you have your ISP announce it for you?
3. How does your BGP-announced network look like on the Internet? A check on BGP looking glass (like one in this forum's FAQ) is a must to confirm. |
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kamikatze
join:2007-11-02
| I'm running OSPF, but i'm not redistributing any routes.
1. How large is your BGP-announced network? /24? Larger?
It's a /24.
2. Do you announce your own network via BGP to both ISP? Or do you have your ISP announce it for you?
I announce it myself to both ISP.
3. How does your BGP-announced network look like on the Internet? A check on BGP looking glass (like one in this forum's FAQ) is a must to confirm.
5606 is my backup ISP (1mbps). |
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aryoba
Premium,MVM
join:2002-08-22
| said by kamikatze  :I'm running OSPF, but i'm not redistributing any routes. If there is no redistribution between BGP and OSPF, then how does OSPF know default route to the Internet? Using the default originate command?
said by kamikatze :3. How does your BGP-announced network look like on the Internet? A check on BGP looking glass (like one in this forum's FAQ) is a must to confirm.
[code] Network Next Hop Metric LocPrf Weight *> 65.106.7.139 3 0 2828 5588 5606 myAS [/code] 5606 is my backup ISP (1mbps). Assuming it is coming from Internet BGP Looking Glass, then I notice something questionable. The looking glass shows that your backup ISP is considered primary transit AS to reach your AS, which should not be happening. Using the ISP BGP Community, you should be able to tune this to make your primary ISP as the primary transit AS instead. |
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kamikatze
join:2007-11-02 | reply to kamikatze
OSPF is for internal routes only. I'll have a talk to both my ISP to make this as simple as possible.
Thanks aryoba. |
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TomS_
debugger it
Premium,MVM
join:2002-07-19
Australia
| reply to kamikatze
The easiest way to achieve it would be as pretty much mentioned before.
Advertise a longer AS path to your less preferred provider (perhaps also use one of their communities to append more of their AS to that path aswell), and give that providers default route a lower preference.
Advertise a shorter AS path to your preferred provider, and give its default route a higher preference.
This should take care of incomming and outcomming traffic going via the preferred ISP until it fails, afterwhich traffic will route in and out via the secondary ISP, and then return to the primary ISP once it is restored.
Keep in mind, though, that anyone connected to your less preferred ISP will still route traffic into your network via the link to that network, since it will likely be the preferred path from within that provider. However, anyone outside of that provider is likely to route in to you via your primary ISP where the AS path is much shorter.
Sometimes it is not always easy to achieve your desired outcomes as providers can and do do all sorts of things inside their own networks which mean that things dont behave as you would like. |
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