Re: How to detect availability


"Ivan Ostres" <vina.bfgerf@xxxxxxxxxxx> wrote in message
news:MPG.1d350217275b836c989689@xxxxxxxxxxxxxxxxxx
> In article <daeev6$gop$1@xxxxxxxxxxxxxxxxxxx>, no@xxxxxxx says...
>> I have the following topology:
>>
>> - R1 ---- R2 ----- R3-----Internet
>>
>> How to make that R1 knows that the link between R2 and R3 is down? This
>> is
>> about Internet connection and R1 can't receive default route via routing
>> protocol from my ISP, since some users that use R1 as default gateway are
>> routed via another router (not shown above) using routing table (default
>> route) and other users are routed to the Internet using policy routing
>> (route-map).
>>
>
> One way would be to use routing protocol and create a loopback on R1 and
> announce it in mentioned routing protocol. If route to that "network" is
> not available at R1, that should mean that R1 has no connectivity to R3.
>
>
> --
> Ivan
>
Hi Ivan

I have been trying to achieve this in my lab since this was posted and so
far have failed.

Can you expand on your idea? I have used the below topology.


R1----R2----R3---R4
| |
R5-----------

Traffic can come from 2 source ip addresses on R1 (and i'm not interested in
return traffic at this stage)
and in my lab terminate on R5 loopback address

R2 has a route map on the interface from R1 so certain source addresses
route to R5's loopback route through R3-R4-R5 other source addresses use
R2-R5 by default. (I am using OSPF over the network and have tweaked the
bandwidth on links to ensure traffic for R5 route viaR4 from R3).

What the original poster is querying and me too now is that as the routemap
on R2 uses a next hop for certain source addresses from R1. How can we tweak
the config so that traffic entering R2 from R1 does not use the route-map
when the link from R3-R4 is down. i.e. to stop this traffic wasting
bandwidth between R2-R3 just to be routed back to R2 and onward to R5.

My initial thought was to set the next hop in the route-map to an address of
the R4 Interface but as this is not strictly the next hop all I get is hits
on the route-map but the routing is ignored via the set statement.

Is there a way to detect distant link failures to stop this bandwidth
wastage.

Regards

Toby



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