Suggestions for manipulating routes preferred direction over BGP

[pietro manicioto]

I have a bit of a conundrum I am trying to work through logically and having little luck. 

I have a site that has a core with a 1GB PTP link back to my DC core as well as a 100M MPLS link to the carrier. 

We want this site to prefer the PTP link for routing out (which it is currently), and have the MPLS has a backup link (I think the original idea was to send VOIP trafffic out the MPLS as well as one point). however the return route traffic from the internet is coming back through the MPLS circuit.

The reason for this is because we run EIGRP for our IGP ad BGP as our EGP. From our DC we redistribute Summary statements from EIGRP to BGP and then Network statements out with the summaries as well. This MPLS link though has the individual /24's for this site advertising into BGP on the MPLS as well, so the longer match is winning. As much as I hate to admit this, I was going to let this go as it has been going unnoticed for years now, but I have been getting some reports of TEAMS call quality issues and sure enough, when there is a large call the MPLS link is getting over saturated with incoming traffic causing packet drops and quality issues. 

I have found a way around this in normal working order by setting a BGP aggregate-address statement with the same summary at this remote site and then the return route traffic will work fine ( i should probably set a prepend in there as well to make sure this site never becomes preferred for the summary supernets), but this gives me no option to advertise the individual /24's if that PTP link were to go down. 

Does anyone have suggestions for how to do this and still have an automated failover? Is EEM scripting with tracks the only option? Or even a viable option? Anything else I may be missing?

I can draw something up in Visio to make this easier to injest as well if needed. 

[M02@rt37 · Accepted solution]

Hello @pietro manicioto 

Set up IP SLA on the router to track the availability of the PTP link.

ip sla 1
icmp-echo <PTP Link IP>
frequency 10
ip sla schedule 1 life forever start-time now
track 1 ip sla 1 reachability

Adjust your BGP configuration to use this track object:

router bgp <ynAS number>
neighbor <DC Core BGP Neighbor IP> remote-as <DC Core AS number>
neighbor <DC Core BGP Neighbor IP> fall-over bfd
!
address-family ipv4
network <Individual /24 Prefix>
neighbor <DC Core BGP Neighbor IP> activate
route-map ADVERTISE-CONDITIONAL out
exit-address-family
!
route-map ADVERTISE-CONDITIONAL permit 10
match track 1
set community <conditional advertisement community>

-- The conditional advertisement community is not a mandatory part but I provide it as an example of one possible approach to distinguish between the routes advertised under different conditions...

This configuration will advertise the /24s only when the IP SLA operation is successful. If the PTP link goes down, the track status changes, and BGP will stop advertising the /24s over the MPLS link.

[pietro manicioto]

Thank you. I was thinking something along this path as well, just a little different approach. I will try to get this labbed up this week sometime and test it to confirm. 

[liviu.gheorghe]

Hello @pietro manicioto ,

I'm not sure if this works in your setup without more details regarding IP addressing, but one way to achieve redundancy and have specific subnets take specific links is to announce in BGP a summary of the subnets for the remote site on both links (PTP and MPLS) and announce a specific /24 on the PTP link and another specific /24 on the MPLS.

In this setup, you have redundancy by announcing a summary on both links and specific inbound routing by way of longer prefix /24.

Hope it helps.

[pietro manicioto]

That is true, I considered trying this as well for simplicity. 

There is nothing saying I cannot change the advertisement out of the MPLS router from the DC as well, I just preferred not to. I already have alot of summary statements in there and did not want to add more if I did not need to, but this probably really is the "easy button"

[Elliot Dierksen]

Bear in mind that while you can certainly manipulate your choice of outbound routes, you can only ATTEMPT to influence the inbound route selection of routers under different control. They have and use the same means to manipulate path selection for their own reasons that may not match yours.

[pietro manicioto]

Both routers are under my control for what they advertise out. My predicament is really just the more specific routes taking over from the prior design. Its really the way it should be done, but I think someone added the PTP link at a later point and didnt think about the return route traffic. 

[MHM Cisco World]

You case is called 

Eigrp ce-pe with backdoor 

Check link below how you config this backdoor.

https://journey2theccie.wordpress.com/2020/05/05/pe-ce-routing-with-eigrp/

MHM

[pietro manicioto]

very quick initial review, I do not think this refers to what I am trying to accomplish. I will read in some more detail to see if I am missing anything. 

[MHM Cisco World]

you have two site 
you use MPLS as SP 
and there is P2P link between two site 
this P2P is called backdoor 
you should config EIGRP to prefer the path via P2P using (delay or BW, delay is better)
and config SoO to make MPLS aware of this backdoor 
no need EEM or anything else 
MHM