IGP on backup router to same peers?


I currently have a situation where I have 1 router connected to 2 peers (ISP primary and backup routers). I want to add a second router as a backup in case the my primary fails. I plan on using bgp local pref to set local pref higher on the backup then the primary. My question is since both routers are peering to the same ISP routers should I still do IGP between the two? Hope this makes sense.

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First, you should set local preference higher on the primary router, not the secondary.

Because of the two local routers has to speak iBGP with each other, you usually have an IGP on the internal network for the loopback-addresses to find each other. The recommended way to set up iBGP is with peering between loopback addresses.
I would recommend OSPF if you don't have any other protocol already.
Or if your routers are Cisco, then EIGRP is a good choice.

But if you don't want to use an IGP and you only have two BGP-routers that resides in the same vlan, then you could of course use the address in the local vlan as the peer address for iBGP.

Yes, you still need an IGP, the connector net to the ISP needs to be added into the IGP else when there is a failure on one of the ISP links it will blackhole traffic. You say WHY?
Becuase BGP does not modify next hop, So when one ISP link fails, and any internal traffic that routes to that router with the failed ISP link, that router will not have a route to the BGP next hop thus all BGP routes that are recceived via iBGP (from the remianing peer with the ISP link still up) will be removed from the routing table. Thats why you need an IGP, to maintain BGP next hop reachability. The other way would be to modify the BGP next hop attribute via next hop self.

hope this helps,

harbor235 ;}

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harbor235 is right about the next-hop-self. It is absolutely necessary if you choose to go without a IGP with the LAN-addresses as peers int the iBGP.
next-hop-self is also an option even when you use IGP. When you connect to an ISP you sometimes don't want to have the link-address in the BGP-table and therefore use next-hop-self. But that's maybe not important here. It is more common when you connect to several ISPs and learn the entire BGP-table in the Internet.
I still think that the two options I told you before is the way to go.
*Configure a loopback address in each router.
*Configure EIGRP or OSPF (or why not RIP2) depending on the router brand.
*Be sure that the IGP contains information about the loopback addresses and the link neworks to the ISP.
*Configure iBGP with loopback addresses as peers.
*Use local preference to tell your routers the preferred way out.
*Tell your ISP which link they should use primarily for traffic to you.
pakitlossAuthor Commented:
Yes Torvir. That is what I meant.... thanks for correcting me.
pakitlossAuthor Commented:
Thanks Harbor235,

That is exactly why I asked the question. I was worried how those links would act in that type of scenario. Maybe I am just not looking in the right place but I could not find any examples of this type of setup. So Friday I set up my layout in GNS3 and ran these type of scenarios. What an awesome piece of software.


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