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New Member

BGP Multi-homing

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.

5 REPLIES
Silver

Re: BGP Multi-homing

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

New Member

Re: BGP Multi-homing

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.

Gold

Re: BGP Multi-homing

"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

New Member

Re: BGP Multi-homing

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

New Member

Re: BGP Multi-homing

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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