I'm having a weird issue. Here is the setup. We have 2 Foundry SX's.

SX01 --->  ISPA
SX02 --->  ISPB

I have a /24 from ISPA and another /24 from ISPB. Both /24's are able to be announced to each of the providers.

iBGP runs between the two SX's and eBGP is being used from each of the SX's to the respective ISP.

So, I have the following on the SX's:

ip route null0
ip route null0

I also have prefix-lists configured for each of the /24's and configured the bgp neighbors to make use of the prefix-lists.

My problem is that whenever I add the above null routes to both routers at the same time, I get connectivity issues and complete packetloss. I've tried changing the administrative distance on these static routes but with no luck. If I only add these null routes to one of the SX's and remove them from the other, then the prefixes get announced fine to the respective ISP and things work fine.

Any ideas how to fix? On cisco devices, it's as simple as adding the above commands to both edge routers so not sure why this isn't working at the moment.

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    Jan 3, 2021 at 4:30

2 Answers 2


Check that you actually have valid internal routes for these networks to override the routes to null0. If there are no more-specific or more-preferable routes for these networks, the routers will of course discard traffic destined for them.


Your anchor routes are correct, but to override those routes you'll need more-specifics in your IGP. How you go about that is really dependent on your network layout, but for an example....

Say you have a couple of different router ports with a /28 assigned to each. One router will now have more specifics and will not blackhole traffic destined to those /28s, but the other router won't know about them unless you advertise them in your IGP (OSPF, IS-IS, etc). The routers will still send traffic for unused subnets to null0 until you have a valid more-specific route to them.

You want to keep those anchor routes so BGP will be able to advertise the /24s. If you have the /24s assigned to a single port(s) and aren't subnetting them at all, then you don't need the anchor routes. Basically your routing table needs to have a valid route to the /24s (whether to a live port or the null0 static) before BGP will advertise those routes. You just need to make sure you are redistributing more specifics in your IGP so the routers know where to send valid traffic.

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