04-06-2004 02:17 AM - edited 03-02-2019 02:49 PM
Network Info:
1) One Cisco router running BGP.
2) Connected with two backbone providers. (A and B)
3) /21 IP block from APNIC.
Requirements:
1) Some IP classes to use A as primary link and others to use B as primary link.
2) In case of failure of any one link all traffic redirected to the available link.
04-06-2004 02:49 AM
One solution could be:
For inbound traffic:
- Announce /21 on both links
- Announce />21 specific nets on the wanted links
This means both links can be used for all addresses but if routes are chosen by longest-match and not filtered away (because of >21 mask) the specified links will be used. You may experience some asymmetric and non-optimal routing in such a scenario, but it shouldn't be too bad (it all depends on the infrastructure of the providers).
You should also check with your providers if they accept smaller prefixes (btw: smaller prefixes WILL be filtered on many gateways around the world).
MED could have worked if you had one upstream provider with two links, but MED is not exchanged between external AS's (it's just passed from originator to neighbors).
For outbound traffic:
- Use Policy-based routing
04-06-2004 04:01 AM
Our provider does not support advertising bigger prefixes. They insist on advertising /24s.
The main concern is that user of 4 network classes /22 be routed through our Se5/1 primarily unless then link is down.
04-06-2004 06:10 AM
"Our provider does not support advertising bigger prefixes. They insist on advertising /24s."
What? That doesn't make any sense at all.... The only option, in this case, is going to be prepending the AS Path, which isn't going to gaurentee the correct routing. You could use conditional advertisement, I suppose, if you don't want _any_ traffic to come down the backup link:
http://www.cisco.com/en/US/tech/tk365/tk80/technologies_configuration_example09186a0080094309.shtml
:-)
Russ.W
04-09-2004 01:25 AM
how this can work with my senario. My current config is:
router bgp uuuuu
no synchronization
bgp log-neighbor-changes
network xxx.xx.x44.0 (/24)
network xxx.xx.x48.0 (/24)
network xxx.xx.x49.0 (/24)
network xxx.xx.x50.0 (/24)
network xxx.xx.x51.0 (/24)
neighbor xx.xxx.xxx.17 remote-as bbbbb
neighbor xx.xxx.xxx.17 route-map localonly out
neighbor xxx.xxx.xxx.189 remote-as aaaaa
neighbor xxx.xxx.xxx.189 route-map localonly out
!
ip as-path access-list 10 permit ^$
!
route-map localonly permit 10
match as-path 10
04-11-2004 02:27 PM
I am assuming that you have got x.x.44.0/21 from APNIC.
Senario as asked by you:
1. Traffic for the bellow network prefixes should come via ISP A
x.x.44.0/24
x.x.48.0/24
x.x.49.0/24
2. Traffic for the bellow network prefixes should come via ISP B
x.x.50.0/24
x.x.51.0/24
3.If ISP-A / ISP-B link fails/problem in the upstream
Traffic for all the network prefixes will come via the other working provider.(A/B)
Bellow is the sample config:
---------------------x----------------------------
router bgp uuuuu
no synchronization
bgp log-neighbor-changes
network xx.xx.44.0 (/21)
network xx.xx.44.0 (/24)
network xx.xx.48.0 (/24)
network xx.xx.49.0 (/24)
network xx.xx.50.0 (/24)
network xx.xx.51.0 (/24)
neighbor xx.xx.xx.17 remote-as bbbbb
neighbor xx.xx.xx.17 route-map B out
neighbor xx.xx.xx.189 remote-as aaaaa
neighbor xx.xx.xx.189 route-map A out
!
ip prefix-list A-prefix seq 10 permit xx.xx.44.0/24
ip prefix-list A-prefix seq 20 permit xx.xx.48.0/24
ip prefix-list A-prefix seq 30 permit xx.xx.49.0/24
ip prefix-list A-prefix seq 40 permit xx.xx.44.0/21
ip prefix-list B-prefix seq 10 permit xx.xx.50.0/24
ip prefix-list B-prefix seq 20 permit xx.xx.51.0/24
ip prefix-list B-prefix seq 30 permit xx.xx.44.0/21
!
route-map A permit 10
match ip address prefix-list A-prefix
route-map B permit 10
match ip address prefix-list B-prefix
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