Static route looping ?

[JUANNN]

Hello,

I am labbing in CML a topology that uses static-default routes with a DHCP gateway. The feature works (the next-hop IP address gets set by a DHCP server with the default-router IP address). I am using CML 2.7.2. 

However, I am running into an issue when I configure the static route:

For some reason, the route to the next-hop dynamically assigned by DHCP on the static route appears to be a static route too, with an AD of 254, so I am assuming that is a loop? I have configured only the following static route:

The problem is that the "looping" static route that I haven't configured is not letting my traffic to be forwarded, because on the recursive lookup for the next-hop of the default route the preferred route for 192.168.1.6 is the "looping" route (as it has the longest match), but the route I need the recursive lookup to choose is the BGP one (192.168.1.4 /30 via 192.168.1.2). 

Anyone knows about this problem? I am not trying to get a solution to route my traffic, I just want to know why is this "looping" static route appearing in the RIB. Also, the topology is all P2P routers, if it helps.

Thanks, any help is appreciated.

Juan 

[Kasun Bandara]

@JUANNN hi, can you share small diagram about your topology and network ranges between routers.

[JUANNN]

Hello Kasun Bandara,

Thanks a lot for the interest. Here is the relevant part of the topology (all other links not shown are disconnected and not being used). All .n next to the routers refers to the 192.168.1.n subnets. Note that the direct connection between TGX and TGB-10 is disconnected.

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I am using FrontDoor VRFs on TGX and TGB-10, but the issue is on the "inside VRF" (l3vpn) and the overlaying VRFs are working fine, that's why I didn't mentioned. 

Please let me know if further information from the overlaying VRFs is needed. 

Thanks,

Juan

[Kasun Bandara]

@JUANNN ok. as per my understand you shared the routing table for TGX. as your default route using DHCP as next hop to default route, it will add the first line in the routing table. same time, router will add the static route with 254 AD towards DHCP next hop as default route will use this as recursive lookup route. 

same time you have BGP learning for 192.168.4.0/30 network which is again same network and that is added to the routing table too.

i am not sure how your VRFs configured, as DHCP relayed or not in middle router.

[JUANNN]

Oh, I think I see what you mean: once the static-default route is added to the RIB, a second static route to the next-hop of the default route is added too? Is that normal behavior?

Yes, the middle routing is configured to point the DHCP server to DISA_GW. 

[Kasun Bandara]

@JUANNN if my hypothesis correct, that can happen as you using BGP too..

[MHM Cisco World]

You config CE to get it IP from PE 

So PE advertise defualt route.

This dynamic defualt route is override your manual static route 

Can you confirm above 

MHM

[JUANNN]

Hello MHM,

I am not sure what PE advertise default route means. Are you saying that DISA_GW advertises a default route? I haven't configure it like that.

Thanks

Juan

[MHM Cisco World]

Are you saying that DISA_GW advertises a default route? 
Yes that what I mean 
debug dhcp server or debug ip dhcp 
to check if DHCP Server send default route or not 

MHM

[JUANNN]

Hello MHM,

Thanks for the reply. I did the debug after flapping G1 on TGX, so the DHCP process restarts:

But I don't see how is DISA_GW going to advertise a default route, if I confiured on TGX a static-default route:

Do you notice anything in the debug?

[JUANNN]

Hello, 

I tried to configure another path from TGX to TGB-10 via another provider, STARLINK, in VRF hsinet. In this case, without using BGP between TGX and STARLINK, just subinterfaces and ROAS for TGX's G4 (ip address DHCP) and STARLINK's G1 (default-router ip address). The I configured a DHCP server on STARLINK and TGX's G4 pulled an IP address from it successfully.

After that I configured a static-default route on TGX, first using:

ip route 0.0.0.0 0.0.0.0 dhcp

and again, the second static route for the next-hop of the default route was automatically created with an AD of 254, and with a next hop to istelf (looping?). Same issue that I had with the DISA scenario that I posted above.

But when I tried to use:

it worked. The second static "looping" route wasn't created, and everything worked fine.

However, I tried this command with the DISA scenario, where BGP is running between TGX and the provider router, and the second static "looping" route was still there:

- So when I have BGP running between TGX and the provider router, the static-default route via dhcp doesn't work because of the creation of the second static "looping" route to the next hop of the static default route.

_ But if I don't have BGP running between TGX and the provider router, then the static-default route via dhcp works only if I specify the interface name and dhcp on the static route command. If I only put dhcp and I don't specify the interface name, then the second static "looping" route to the next hop of the static default route is added to the routing table. 

I can't find an explanation for this behavior, anyone knows more about this issue?

Thanks, any help is appreciated.

[MHM Cisco World]

I send you PM check it

MHM