Configuring Conditional Advertisement

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Oct 22nd, 2009
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We need to configure conditional advertisement between two routers running BGP v4. This will enable us to make two ISPs back up to each other.

However we configured two ISPs in two different routers. Do we need a ibgp between routers?

Diagram is duly attached.

We have tried the configuration in our lab, the confirguration is attached with GNS.txt file name.

Unfortunately the configuration did not work for us. we are able to get iBGP routes but it doesnot announce the second prefix when the first ISP is down.

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slmansfield Thu, 10/22/2009 - 09:47
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I don't see the attachment of your GNS.txt file. Could you include that? thx

navinder.singh Thu, 10/22/2009 - 10:18
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Thank you!!!

I am unable to attache the file, however, it is the same as attached the Net1.gif file, in which 2 routers were configured as ISPs and 2 were ibgp routers.

Giuseppe Larosa Thu, 10/22/2009 - 10:14
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Hello Navinder,

>> However we configured two ISPs in two different routers. Do we need a ibgp between routers?

generally speaking in a scenario like yours the answer is yes.

second router has to receive BGP routes from router1 however, also router1 receives routes from router2 so an iBGP session is needed but it might not be enough.

post your current config.

there is an attach file options when you create a new post

Hope to help


slmansfield Thu, 10/22/2009 - 11:09
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After reviewing the diagram (which I'm having a hard time reading, so I apologize if I am mis-interpretting something), it looks like the outbound traffic is directed via HSRP between the two gateways, GW1 and GW2.

If you want to manage inbound traffic so it takes a preferred path through one ISP and fails over to the other ISP, I think you could just configure a prepended AS path for the network(s) you are advertising to the Internet over the less preferred ISP path. I don't think you need to make GW1 and GW2 IBGP peers to accomplish this.

navinder.singh Thu, 10/22/2009 - 19:33
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Thanks Giuseppe!!

Please find the configuration below

router bgp 24471

no synchronization

bgp log-neighbor-changes



neighbor remote-as 17465

neighbor ebgp-multihop 5

neighbor update-source Loopback0

neighbor soft-reconfiguration inbound

neighbor route-map ustr_in in

neighbor route-map ustr_out out

neighbor advertise-map ISP2_out non-exist-map ISP2_down

neighbor remote-as 24471

neighbor update-source Loopback0

neighbor next-hop-self

neighbor soft-reconfiguration inbound

no auto-summary

ip as-path access-list 100 permit ^17465_[0-9]*$



ip prefix-list ISP2_in seq 5 permit


ip prefix-list ustr seq 10 permit


ip prefix-list ustr1 seq 5 permit


ip prefix-list ustr2 seq 5 permit


ip prefix-list ustr3 seq 5 permit

route-map ustr_out permit 10

match ip address prefix-list ustr3


route-map ISP2_down permit 10

match ip address prefix-list ISP2_in


route-map ISP2_out permit 10

match ip address prefix-list ustr2


route-map ustr_in permit 10

match ip address 100

slmansfield Mon, 10/26/2009 - 11:32
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I think the issue preventing the conditional advertisement from working is the route-map to the same neighbor. That route-map is only advertising When the watched route is lost, this route-map still only allows the advertisment of

If you either remove the route-map or adjust it to include you will find that the conditional advertisement works.


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