when backup route works

[sivakondalarao]

Hi,

I have small query. I have routes like this in one router.

ip route 10.20.20.0 255.255.255.0 192.168.1.2 (via Router2)

ip route 10.20.20.0 255.255.255.0 192.168.2.2 10 (via Router3)

Please look into the attached diagram.

My question is link between Router4 and Router6 is broken, then will it take backup route 10.20.20.0 via 192.168.2.2 (i.e, Router3)

Regards

SKRAO

[chrihussey]

With the static routing as long as the link is up between Router 1 & 2, it will send packets that way regardless if the connection between 4 & 6 fails.

[sivakondalarao]

If I enable SAA in between Router1 and Router8 in two directions that is via Router2 and Router3.

Router8 will not respond to Router1 via Router2.It will respond via Router3 only.

Using this technique we can delete route to 10.20.20.0/24 via 192.168.1.2 in Router1 routing table.

So there will be only one active route via 192.168.2.2 via Router3.

So invoking backup route is possible in this way.

Is it right or wrong? Some one suggested me this.

I didn't check it practically.

Required your suggestions.

Regards

SKRAO

[chrihussey]

SAA is nothing more than a service level / performance monitoring tool. It does not control your routing in any way.

The best option is to enable a dynamic routing protocol (OSPF, EIGRP, RIP, etc) to dynamically re-route traffic if a link were to fail.

[ilya.varlashkin]

SAA is only part of the solution. SAA monitors some target (for example, ping some IP address not necessarily next-hop). The other part of the solution is to read status of that monitor object (this process is called tracking) and install a route if status is OK. In reverse that also means that if after route has been installed the check fails (e.g. SAA didn't receive reply) then the route will be removed. Cisco refers to this method as 'reliable static routing'. The solution is very simple and yet it's powerful. As a monitoring target it's better to specify not just next-hop router, but something more meaningful for determining whether link is usable.

There is one catch using reliable static routes, or routes with tracking - if the route is down, how does SAA reach target? Looks like it's gonna be down all the time unless route is installed. And if route is installed what's the point of tracking? Similar to chicken and egg issue. To resolve this loop, SAA tracking is often implemented together with policy-routing that will ensure that probe packets are always sent via particular interface.

We use 'reliable static routing' all the time when more than one link provisioned to a site, unless specific situation demands dynamically learn routing information. Only then we'll put a dynamic routing protocol. We use ordinary static routes only in situations where there is only one line to a site (what's point to monitor it - if it doesn't work we haven't got anything else anyway, so no complexety).

Here is document on CCO providing more information on how reliable static routing works and sample configurations - http://www.cisco.com/en/US/products/sw/iosswrel/ps5413/products_feature_guide09186a00801d862d.html

Let me know if you need more help on this.

[gpulos]

yes,

in the diagram you have provided, if the link between rtr4 & rtr6 goes down, then the only path available to 10.20.20.0/24 is via 192.168.2.2

the problem is that rtr1 still knows of 10.20.20.0/24 via 192.168.1.2 is a better cost then 192.168.2.2. (this is due to your staticRoute via 192.168.1.2)

this happens because rtr1 has no idea that 10.20.20.0/24 is not reachable via 192.168.1.2. even if the link between 4 & 6 goes down, rtr1 knows to get to 10.20.20.0/24 it must send the packet to 192.168.1.2.

if you used a routing protocol through 1, 2, 4 & 6 at least with a floatingStaticRoute pointing to 192.168.2.2, then you could acheive a link outage between 4 & 6 and the routers would adjust their tables via routingProtocol and the floatingStaticRoute (that has the higher cost) would be used.

it may be best to implement a basic eigrp or even rip to get basic link status/routing updates.

(of course, if you use a routing protocol such as eigrp or even rip throughout all routers, there should be no need for a floatingStaticRoute with your design)

see these links for more routing protocol info:

eigrp - http://www.cisco.com/en/US/tech/tk365/tk207/tsd_technology_support_sub-protocol_home.html

igrp - http://www.cisco.com/en/US/tech/tk365/tk352/tsd_technology_support_sub-protocol_home.html

rip - http://www.cisco.com/en/US/tech/tk365/tk554/tsd_technology_support_sub-protocol_home.html