netdev - Re: Route fallback issue

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Message-ID: <7e001d45-f692-7d93-0390-f0e6077bf724@cumulusnetworks.com>
Date:   Wed, 20 Jun 2018 07:48:53 -0600
From:   David Ahern <dsa@...ulusnetworks.com>
To:     Akshat Kakkar <akshat.1984@...il.com>,
        netdev <netdev@...r.kernel.org>
Cc:     cronolog+lartc <cronolog+lartc@...glemail.com>,
        lartc <lartc@...r.kernel.org>,
        Erik Auerswald <auerswal@...x-ag.uni-kl.de>
Subject: Re: Route fallback issue

On 6/20/18 2:26 AM, Akshat Kakkar wrote:
> Hi netdev community,
> 
> I have 2 interfaces
> eno1 : 192.168.1.10/24
> eno2 : 192.168.2.10/24
> 
> I added routes as
> 172.16.0.0/12 via 192.168.1.254 metric 1
> 172.16.0.0/12 via 192.168.2.254 metric 2
> 
> My intention : All traffic to 172.16.0.0/12 should go thru eno1. If
> 192.168.1.254 is not reachable (no arp entry or link down), then it
> should fall back to eno2.

See the ignore_routes_with_linkdown and fib_multipath_use_neigh sysctl
settings.


> On Wed, Jun 20, 2018 at 1:49 PM, Erik Auerswald
> <auerswal@...x-ag.uni-kl.de> wrote:
>> Hi,
>>
>> I have usually used the "replace" keyword of iproute2 for similar
>> purposes. I would suggest a script as well, run via cron unless 1 minute
>> failover times are not acceptable. The logic could be as follows:
>>
>> if ping -c1 $PRIMARY_NH >/dev/null 2>&1; then
>>   ip route replace $PREFIX via $PRIMARY_NH
>> elif ping -c1 $SECONDARY_NH >/dev/null 2>&1; then
>>   ip route replace $PREFIX via $SECONDARY_NH
>> else
>>   ip route del $PREFIX
>> fi
>>
>> Alternatively, one could look into a routing daemon that supports static
>> routing (Zebra/Quagga/FRRouting, BIRD, ...) and check if that supports
>> some form of next-hop tracking or at least removes static routes with
>> unreachable next-hops as one would expect from experience with dedicated
>> networking devices.

A feature is in the works to have fallback nexthops.


>>
>> IMHO static route handling as done by the Linux kernel does not seem
>> useful for networking devices. I have even had bad experiences with
>> Arista switches and static routing because they relied too much on the
>> Linux kernel (probably still do).

Useful how? what did not work as expected?

Do not confuse Arista's NOS with Linux's capabilities or any NOS truly
based on Linux and using a modern kernel. A lot of work has been put
into bringing Linux up to par with NOS features. If something is not
working, demonstrate the problem on the latest kernel and inquire if
someone is working on it.