BGP Peering and HSRP

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Sep 4th, 2007
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Just want to double check my understanding from a couple of threads I have seen on this.

I have a configuration where I am going to be peering to an ISP and the connection is coming through a switch that will connect to two routers. Want to do BGP peering from both routers and I had throught that I could use some of the majic of HSRP for the configuration. But that does not seem to be possible.


So what it looks that I have to do is configure each router to peer with the ISP using the actual interface IP, but I can favor one over the other with padding the route announcements with additional AS-Path prepend values.


Have I got that correct?

Thanks.

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Jon Marshall Tue, 09/04/2007 - 09:23
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Hi


As far as i know you are correct in that you cannot use an HSRP address for a BGP connection,, reason being that you will not be able to specify this address in the update-source option ie. you can only specify an interface.


Yes you can favour one over the other by using


1) MED - which only propogates within your ISP AS.

2) AS-Path prepending will can propogate beyond your ISP.


HTH


Jon

sundar.palaniappan Tue, 09/04/2007 - 14:21
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In addition to what Jon said if you are trying to influence what path the inbound traffic takes then you might be able to advertise more specific route(s) out to one provider and less specific route(s) out to another provider. Check with your ISP if they are OK with this.


Let's say you have a network of 172.16.0.0/22 and you want ISP1 to the preferred path for the inbound traffic then you can advertise like this;


ISP1:

Advertise 172.16.0.0/23 & 172.16.0.2.0/23


ISP2:

Advertise 172.16.0.0/22


HTH


Sundar

robert.ward Thu, 09/06/2007 - 02:39
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I have had many customers use the HSRP config for two router setups and peer BGP. If you dont specify the update-source it uses the HSRP peer address.


The ISP peers with the HSRP address and it all works fine, until it changes to backup router.


I dont like it because only the primary HSRP router has an active BGP session and once it goes down the session starts up again on the backup router and the ISP establishes a new session hence causing an outage for a period of time.


not so bad if your doing default originate but if a customer wants full routing it takes a few minutes to populate the entire table again and of course this happens every time the HSRP pair switch.

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