BGP Failover between Static and DHCP

[ngalbraith]

Hi all,

First time posting, so I apologize if I forget any info. I've ran into a big of a snag while trying to set up a secondary ISP on my network. We've been having issues with our main ISP for a while now, so we got an AT&T circuit brought in as a backup. I was expecting to be given a fiber hand-off, but instead they set up a modem and told us to connect to it with DHCP. The problem comes in because the link to our primary ISP uses BGP to a static IP address that they provided. 

Is there a way to set up our core switch so that if the link to the primary ISP goes down, it will switch over to the dhcp interface, or do I need to contact AT&T and ask for a static BGP connection?

Thanks,

[paul driver]

Hello
Having a secondary ISP link via what seems to suggest a xDSL connection should not stop you having resiliency for your network, and setting this up wouldn’t take long to accomplish however It does seem the primary ISP link is providing a totally different kind of service as to the secondary ISP link (ie: mpls service on the primary)

Can you elaborate on the actual setup of you network is?

[ngalbraith]

Hi Paul,

Thanks for your reply, and I'm happy to elaborate. Our network acts as the ISP for several tenants in our building. They all have firewalls that connect to our core switch, which currently is a Catalyst 3850. This core switch is connected to the existing ISP via BGP, and simply routes all the tenant traffic out of Gig 1/1/1 to the internet. We recently opened up a business account with AT&T, with the intent of using them as a backup in case the current ISP goes down.

Is there any specific information that you need to know?

Thank you,
Nathan

[MHM Cisco World]

config static route with dhcp as egress interface and then redistribute static route into BGP.

[paul driver]

Hello @MHM Cisco World 
Why would you need to redistribute any static into bgp ,this would mean youd be redistributing it towards the ISP bgp peer which isn’t viable correct?

[MHM Cisco World]

Core-ISP run static 
Core-AT&T run static with DHCP 

Core and other network device run BGP
as he mention he need to make BGP failover between static and DHCP, so that what I understand.
Core BGP will advertise default route to other network, 
the issue here we need BGP to detect the change of next-hop change when the default change from static ISP to DHCP.
I am right ?

[paul driver]

Hello
that other network is an ISP so they won’t be advertising any default static route in to an ISP 
The OP will have two egress points towards two ISP (BGP and probably a xDSL) each ISP or the same ISP will be aware of the public networks the OP is using they should not have to redistribute anything unless it’s an L3vpn which i am assuming it isn’t given the information already provided.

[paul driver]

Hello Nathan
Thank you for the additional detail.
If the 3850 core switch secondary link isn’t peering via BGP and given the switch doesn’t support network address translation (NAT)  I assume you will using an internal routing protocol like OSPF to dynamically advertise your tenants public networks towards AT&T...Can you confirm this?

Now regarding your resiliency, I believe the most simplistic solution in this case would be to apply conditional default static routing on the siwtch, This will point all egress traffic via the primary BGP ISP link (ISP1) and in the event of failure within/towards ISP1 occurs then this primary default static route is withdrawn, and the secondary default will be preferred and route all egress traffic via AT&T.

Example
access-list 100 permit icmp host <your local bgp peering ip > host 8.8.8.8 echo
route-map ipsla
match ip address 100
set ip next-hop <bgp peer next hop>
set interface NULL0

ip sla 10
icmp-echo 8.8.8.8 source-interface <bgp interface>
timeout 1500
threshold 3000
frequency 15
ip sla schedule 10 start now life forever

track 10 rtr10 reachability

ip local policy route-map ipsla
ip route 0.0.0.0 0.0.0.0 (ISP interface) x.x.x.x name ISP1 track 10
ip route 0.0.0.0 0.0.0.0 (ISP interface) x.x.x.x name ISP2 2