Detecting when ISP is out of service

I think this is a very basic question I still cant seem to figure out. I should slap myself!

First, the scenario.

Suppose you have two ISPs, you use one of them as your active Internet link, the other is
just a backup link.

ISP1

MyRouter

ISP2(Backup)

I have two static routes, one per ISP, the backup ISP gets a static route with a worse administrative distance than the main link AD, so if the main link goes down, the secondary link takes over.

Now the question, if ISP1 experiences problems within its network or with its ISP and it
stops giving me access to the Internet, is there a way my router can tell that? In such case,
the primary link wont phisically come down, hence the secondary static route wont take over!

I was thinking of using full internet routes through BGP, but sounds a little overkill for this.

Any ideas?

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Comments

Netstudent

could you maybe bond the two T1's or whatever the carrier technology is on that link. Maybe ppp multilink. You could use both at the same time, then if one cut you off from the internet, you could still use the other pipe.

This may be kinda overboard, but maybe there is a threshhold of the number of retransmissions that can be set. After a certain number of retranmissions, the route or link can be shut down. If your access link to the ISP is up, but your route from the ISP to the internet is down, then you should still be sending out data without any acknowledgements.

Just some thoughts...

rossonieri#1

hi Cucumber,

i get it. true that backup interface only for link failure - not for remote ISP network failure detection. then you need a smarter device than the router itself - such as another "custom built" linux router behind your router that doing scheduled script "ping ISPs DNS" or other, and create route based on it.

HTH.

EdTheLad

Cucumber wrote:

Now the question, if ISP1 experiences problems within its network or with its ISP and it
stops giving me access to the Internet, is there a way my router can tell that? In such case,
the primary link wont phisically come down, hence the secondary static route wont take over!

I was thinking of using full internet routes through BGP, but sounds a little overkill for this.

Any ideas?

You should'nt need a full bgp routing table,just a bgp default route advertised by both ISPs would be enough.Another thing you could do is create tunnels to some point at the other side of the ISPs, if you lose ip connectivity to the remote tunnel end point due to ISP failure the tunnel will go down, create two static routes to the two tunnel interfaces and you should have the result to wanted.

rossonieri#1

ed,

if the OP has remote network - probably your tunnel will work.
but i think it goes too much.

HTH.

Cucumber

thanks for the answers Netstudent, Rosso and Ed, great answers by the way.

You could use both at the same time, then if one cut you off from the internet, you could still use the other pipe.

This sounds like BGP load balancing, very neat!

then you need a smarter device than the router itself - such as another "custom built" linux router behind your router that doing scheduled script "ping ISPs DNS" or other, and create route based on it.

This one is quite good, I hadnt thought about Linux at all and I am good at shell scripts!

You should'nt need a full bgp routing table,just a bgp default route advertised by both ISPs would be enough.

So the ISP would stop advertising such routes if something wrong happened within its network? Wow I really need to slap myself.

mikej412

You could also check out ip sla monitors

Cucumber

thanks Mike I will look into it, I wonder in what certification track does the SLA stuff is included

tech-airman

Cucumber wrote:

thanks Mike I will look into it, I wonder in what certification track does the SLA stuff is included

Cucumber,

SLAs were mentioned in the 640-861 DESGN exam materials that leads to the CCDA certificate.