static route ASR9000 next-hop ipv4 reachable

[MARCO NUEESCH]

Dear

I found the follow text in the pdf file "Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 4.3.x" / "Implementing Static Routes"

Static Route Functional Overview

Static routes are entirely user configurable and can point to a next-hop interface, next-hop IP address, or both. In Cisco IOS XR software, if an interface was specified, then the static route is installed in the Routing Information Base (RIB) if the interface is reachable. If an interface was not specified, the route is installed if the next-hop address is reachable. The only exception to this configuration is when a static route is configured with the permanent attribute, in which case it is installed in RIB regardless of reachability.

Now my question to that text is, how it works that the asr9000 prove the reachable of a next-hop ipv4 address? The next question is if he use the sate of interface, how it works when we have a layer 2 topology between the two laser 3 equipments?

Many thx for your help

Best Regards

Marco

[AARON WEINTRAUB]

Proving the reachability is a strong word.  In this case if you have a route as such:

router static address-family ipv4 unicast

10.0.0.0/8 192.168.0.1

If any route for 192.168.0.1 exists in the RIB (192.168.0.0/24, 192.168.0.1/32, 192.168.0.0/16) then the route for 10/8 will be installed.  It doesn't actually care if 192.168.0.1 is 'reachable.

As far as state of interface goes, all it cares about if the interface is up.  As you point out, a lot of layer 2 topologies can hide some information about if the other side L3 equipment is reachable.

In this particular case you are really going the long way around to avoid running a dynamic routing protocol.  "How can I make sure the other side of a link is reachable?"  The fastest, most proven answer is to use a dynamic routing protocol, since that is what they are designed for.  However, if you want to do something different, look into object tracking with ip sla /rtr using icmp echo.  That may get you what you desire.

[xthuijs]

well stated aaron! I would like to add the ability to add an IPSLA tracker on the route in case you would like to verify the reachability of the next hop.

The nasty situation with recursive routes is always that it "obscurates" (not sure if that is correct english :) the validity of the route. IPSLA can help here to verify the connectivity to that hop with an icmp ping for instance and take it out of service if there is a threshold exceeded.

Note that IPSLA requires the MGBL pie loaded and activated.

In case interested, there is a good article on this too on the forums about EOT (enhanced object tracking), in relation to ABF (access list based forwarding) but same concept (next hop tracking)

cheers!

xander