Re: Failover route

On 3/16/19 11:57 AM, Erik Auerswald wrote:
> Hi,

Hi,

> If you have an xDSL modem such that the next-hop IP address is on the 
> ISP side, using ping to probe reachability of the next-hop should just 
> work.

I want to agree with you.  Though I have disqualified ping (ICMP) as a 
reliable test in the past for a reason that I can't recall at the 
moment.  I do know that I've run into ISPs that filter ICMP or do other 
nasty things with it, rendering it unreliable.

I think that BFD-EM would avoid some of thee ICMP scheenanigans that 
I've seen before.  The /only/ thing that the upstream router needs to do 
is route a packet.  Conversely, the upstream would need to respond ICMP 
itself and not be filtered.

There's also a chance that the router might respond to ICMP but not 
actually forward packets.  Possibly during maintenance.

> You could look into using the -r and -I options of ping (from iputils 
> as packaged by Debian, in my case).

Good to know.

> If you want to check to some specific IPs or hosts behind one ISP, 
> you can use static host routes (that are not withdrawn via any daemon 
> and stay up even when the interface status goes down) pointing out the 
> specific interface to check. Of course, those IPs should not provide 
> other useful service, or you risk blackholing them on connection failures.

Yep.  Hence my previous comments about monitoring SYN / ACK and / or 
ICMP echo request / echo reply associations.

> I'd say that is more of a tradition. Routing daemons are often written 
> to mimic Cisco IOS behaviour, and there static routes towards an IP 
> that is not reachable (except for a possible default route) are removed 
> from the RIB and FIB. This is not a requirement for a routing daemon, 
> and e.g. Cisco IOS allows overriding this default behaviour.

ACK

> IIRC the Linux kernel does not remove those routes itself.

I've had the kernel remove routes when an interface id brought down.  I 
don't recall off hand if it did so on link loss (up / down condition).

> Thanks,

You're welcome.

Thank you.



-- 
Grant. . . .
unix || die