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Beginner

Multihoming - Null Routes and iBGP

Consider a multihoming scenario with two routers (R1 and R2) which are peering with each other over an iBGP session.

 

Both routers are in turn peering with different ISPs (ISP1 and ISP2) over eBGP.

 

The subnets being advertised are (ignore use of private addresses):

 

192.186.0.0/24

192.168.1.0/24

192.168.2.0/24

192.168.3.0/24

 

In my experience the network is not correctly advertised to the eBGP peer to the ISP (for example R1 to ISP1) when the route is not injected into the routing table via a null route state for example:

ip route 192.168.0.0 255.255.255.0 Null0

 

For R2 to correctly advertise the subnets to ISP2 will a null route statement have to exist - or will learning the routes via iBGP via R1 be sufficient?


Thank you
 

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Hall of Fame Guru

You only need the static routes if the route is not in the IP routing table from some other means eg. EIGRP or OSPF etc.

If that is the case then yes if you configure it on R1 then R2 will learn the route via IBGP and advertise to other EBGP peers.

The obvious drawback to this is if R1 goes down then R2 will no longer receive it and therefore will not advertise it so now that IP subnet is unreachable.

However that may not be an issue for you depending on your topology.

Jon

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Hall of Fame Guru

You only need the static routes if the route is not in the IP routing table from some other means eg. EIGRP or OSPF etc.

If that is the case then yes if you configure it on R1 then R2 will learn the route via IBGP and advertise to other EBGP peers.

The obvious drawback to this is if R1 goes down then R2 will no longer receive it and therefore will not advertise it so now that IP subnet is unreachable.

However that may not be an issue for you depending on your topology.

Jon

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Participant

Are you running any IGP between the downstream switches to internet edge routers? Also it is safe to use higher AD value for the null route.

-Sathvik

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Beginner

Hi Jon,

 

Thank you for your advice - my only question is how would I protect against the subnets becoming unreachable in case R1 was to go down.

 

Would a floating static null route on R2 be required?

The context is a L2 network up the distribution/collapsed core, each L2 access switch connects into each L3 distribution switch (R1 and R2). In the event R1 goes down, an access switch will unblock its second link going to R2 - and the traffic should flow as R2 is now the root bridge in the topology. The SVIs are still reachable as this is a HSRP VLAN interface.

 

All that would be needed is for the routes to be injected into the routing table?

 

What if I use loopback interfaces for iBGP will this keep the routes in the routing table, or will they be lost also despite the session staying up?

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So are you going to advertise the routes from R2 using AS prepending so that R1 is the preferred router as long as it is up ?

I am assuming you are.

If so a floating static would work but then so would a normal static.

I can't help feeling I am not answering your question properly.

Is there something I am not understanding about your setup ?

Jon

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I am conscious that if R1 goes down down, the routes learned via iBGP on R2 would be removed from the table making the networks unreachable from outside the network.

 

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I see what you are saying now I think.

M apologies, I assumed you had network statements for those subnets under your BGP configuration on R2 but actually I suspect you haven't ie R2 is simply advertising the networks to it's peer that it learns from IBGP.

Is that correct ?

Jon

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Hi Jon,

 

Both have network statements but would these be superfluous on R2 since it is learning them via iBGP from R1?

Most of the BGP Multihoming Cisco documentation I have come across has both routers with network statements even though they are learnt via iBGP.

 

Thank you for your help so far Jon.

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Okay are you influencing which ISP should be used to get to those networks at the moment or do you not care ie. traffic can come in via either ISP ?

If you don't care then a static or floating static would be fine.

Jon

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Hi Jon,

 

I think there might be some misunderstanding I probably didn't explain it too well.

 

R1 and R2 are peering with two seperate peers and all routes from R2 are preferred using Local Preference over the iBGP session between them. AS Path preprending also takes place on R1 to prefer R2 inbound.

 

At the moment however, R1 advertises the networks as it is currently the primary router, the null routes are what installs the routes into the BGP table to allow them to be advertised. My question was that if R1 was to go down, would those routes still be kept in the routing table on R2?

 

Thanks,

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Okay, thanks for clarifying the situation.

If R1 goes down then it no longer advertises those routes to R2 via IBGP so R2 no longer advertises them even though it has network statements.

If both routers are meant to be advertising those routes and you are using AS prepending to influence which router is use to influence inbound traffic then using static routes on both routers will work fine.

Jon

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Thank you Jon -  to clarify, are you using the terms null route and static route interchangeably? 

 

Ie, a floating static route with an AD higher than routes learned from iBGP that will be used when the routes learnt via iBGP are removed from the table?

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Sorry I should have been clearer.

Yes I am using them interchangeably.

Jon

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