[AusNOG] IOS router selection

[Colin Stubbs]

There's a number of ways to test L1/L2, it's less about wanting to check
that a route target is at the other end, as it is about detecting
misconfigurations, spanning tree problems or cable/connection problems such
as uni-directional link.
 - The many variations of Spanning Tree Protocol
 - Uni-Directional Link Detection
 - LACP/PAgP
 - Bi-directional Forwarding Detection

Probably others that I don't know about.

CDP (and LLDP) are (insecure and unreliable) informational protocols only.
They don't tell you anything more than what *might be* at the other end of
the link and they can't do anything more than that unless you're in user
land with PoE handsets.

The original poster is talking static L3 routes on Cisco equipment. The
resulting status of object tracking from IP SLA tests against the gateway
address is the best way of handling the problem.

That all said: The **EXACT** issue that has been posed is 50% of the reason
dynamic routing was first created.

-Colin


On 5 May 2014 16:23, Joshua D'Alton <joshua at railgun.com.au> wrote:

> seems to me CDP would be used/useful for this? At least from a l2
> perspective which would be the case of outage 99% the time?
>
> sent from android
> On 05/05/2014 4:15 PM, "Chris Balmain" <chris at team.dcsi.net.au> wrote:
>
>>  Erm... meant to say ARP/ping reachability verification is *not* used
>> unless you tell it to...
>>
>>  On 05/05/14 16:12, Chris Balmain wrote:
>>
>> The route is installed in the FIB if the router has a route to the
>> gateway itself (e.g. via a connected interface in up/up status, or
>> recursively via IGP etc)
>>
>> ARP/ping reachability verification is used unless you tell it to (on
>> Cisco via "ip sla" -
>> http://www.firewall.cx/cisco-technical-knowledgebase/cisco-routers/813-cisco-router-ipsla-basic.html
>> )
>>
>> CB
>>
>>  On 05/05/14 15:58, Alex Samad - Yieldbroker wrote:
>>
>> Hi
>>
>> I am looking for some documentation that explains the way cisco behaves.
>>
>> If I have this
>>
>> IOS
>> ip route 0.0.0.0 0.0.0.0 w.x.y.z 230
>> ip route 0.0.0.0 0.0.0.0 f.g.h.i  240
>> ip route 0.0.0.0 0.0.0.0 r.s.t.u 250
>>
>> ASA
>> route internet 0.0.0.0 0.0.0.0 w.x.y.z 230
>> route internet 0.0.0.0 0.0.0.0 f.g.h.i 240
>> route internet 0.0.0.0 0.0.0.0 r.s.t.u 250
>>
>>
>> this tells me the default gateway used is w.x.y.z, unless that gateway is unavailable or dead..
>>
>> I am looking at how Cisco decides when a gateway is dead, I found documents on route selection, but nothing that specifically address dead gateways
>>
>> I presume, and from what I have seen if there is no arp its dead..
>>
>> Thanks
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