Why do we connect two ISP routers back to back

[vijaymahanth]

Why do we connect two ISP routers back to back ?

can i add secondary (isp router b) route if primary router fails to ISP A

[Seb Rupik]

Hi there,

Multihoming is used to provide at the very least redundancy and at best load-balancing.

Take a look here for some example configurations. This link sounds like your setup:

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.html#anc22

cheers,

Seb.

[Joseph W. Doherty]

Could you clarify what you mean by "we connect two ISP routers back to back"?

[vijaymahanth]

We have two ISP routers to different service providers. We are using static IP not BGP.

After we connect two routers back to back and wanted to know if we can using primary static route ip xx.xxx.xx and secondary ip yy.yy.yy ? simple question.

[Seb Rupik]

Hi there,
If you are going to use static routes what you require is an IP SLA job and a secondary floating static route with a higher metric than the default AD of 1.

In the example below assume Gi0/1 is connected to ISP1 and Gi0/2 is connected to ISP2. Replace <ISP_01_RTR_IP> with the IP address with the next hop IP on the ISP1 link.

In the event that the IP SLA job can no longer receive an ICMP reply from 8.8.8.8 via Gi0/1, it will remove the primary static route and the floating static route will be installed.

!
ip sla 1
  icmp-echo <ISP_01_RTR_IP> source-interface gi0/1
!
ip sla schedule 1 life forever start-time now
!
track 1 ip sla 1 reachability
!
ip route 0.0.0.0 0.0.0.0 gi0/1 track 1
ip route 0.0.0.0 0.0.0.0 gi0/2 2
!

cheers,

Seb.

[Joseph W. Doherty]

BTW, what Seb is suggesting is very comprehensive.

If your link to your primary ISP goes down, a floating static can redirect your traffic to the secondary. However, if the link stays up, but routing "goes down" you need something like what Seb is suggesting to detect that and force the "backup" route to take over.

[vijaymahanth]

Hi there,

I understand the points, I am unable to do it since the network setup is simple.

We have two different routers to two diff. ISP.

Using statics route. 

Connected two back to back routers

Using HSRP for WAN, l3 & L2

If one router/isp fails the second one should be taking over.

ip route 192.168.1.1 primary

ip route 192.168.2.1 secondary

will this work ?

[Joseph W. Doherty]

Could you further clarify your logical and physical topology that connects to your two routers?

[vijaymahanth]

File attached. 

[Joseph W. Doherty]

(Thanks - drawing helps much.)

You have 192.168.25.0/29, a private block, shared by your L3 core switches and your routers?

What's the IP addressing from your ISP?

[vijaymahanth]

You have 192.168.25.0/29, a private block, shared by your L3 core switches and your routers? Yes

14.143.117.xx/30

220.227.125.xxx/30