ip route 0.0.0.0/0

Unanswered Question
Nov 2nd, 2009
User Badges:

Can someone shed light on this...


Topology

----------------

3 routers: A (Corporate), B (Branch), C (Branch)


A - EIGRP w/ redistribute static & connected

B - EIGRP w/ redistribute connected

C - EIGRP w/ redistribute connected


A is connected to B and C through MLPPP. Site A and B have their own ISP. Only A currently has a 'ip route 0.0.0.0/0' static route.


--- B --- ISP

ISP --- A |

--- C --- ISP


If a default route is placed at site B how will B be able to communicate to A or C LAN network as static route has administrative distance value 1 and EIGRP is higher?

Do I have to put static routes to site A and C network in site B router? This doesn't seem right. What am I missing here?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
a.cruea1980 Mon, 11/02/2009 - 11:16
User Badges:
  • Bronze, 100 points or more

There are two things you can do.


You can add a static route and give it a higher value than your EIGRP routes. This will cause all default traffic to go out A, though.


However, when doing routing, the router looks at the best match for a destination. So for example, if your address pool at site 'A' is 172.16.0.0/12, then when 'B' or 'C' send traffic to 172.30.0.254, it'll go to router A, not to the default route (0.0.0.0/0) because 172.30.0.254 more closely matches 172.16.0.0/12 than 0.0.0.0/0.


If you do a "sh ip route eigrp", do sites 'B' and 'C' have the proper routes to 'A'?

Edison Ortiz Mon, 11/02/2009 - 11:18
User Badges:
  • Super Bronze, 10000 points or more
  • Hall of Fame,

    Founding Member

Placing a default route on B won't affect communication to A or C as those locations will advertise specific subnets from their networks.


Remember, a specific route will be preferred over the administrative distance.


Site B will use its default route for any routes not included in the routing table.


Regards


Edison

rschachtschneider Mon, 11/02/2009 - 11:32
User Badges:

Hate to be a pain but can either of you point to documentation stating this? It makes sense logically but need proof :)


Jon Marshall Tue, 11/03/2009 - 04:11
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Ryan


I would have thought the same answer from 2 experts would have been enough proof :-), or else why not just search for documentation.


No offense intended, just that this is very basic route selection information ie. how does a router choose the best path. See this link for more details -


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml


Jon

rschachtschneider Tue, 11/03/2009 - 08:04
User Badges:

The document explains which protocol is added to the routing table. I'm trying to find a document that states something to the effect that a default route is chosen last over administrative distance.


If I go off just administrative distance then my scenario won't work as site B has site C network through EIGRP with an administrative distance of 90 but the default route is static which has an administrative distance of 1. With that being said it would choose the default route over the EIGRP.


"Remember, a specific route will be preferred over the administrative distance." Can you point me to where this is described?


Logically speaking you wouldn't want this to happen. I would think the default route would be last resort if not found in routing table.


I guess what I am looking for is a logic diagram as this document

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml

doesn't tell me either.

Jon Marshall Tue, 11/03/2009 - 11:30
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Ryan


Did you read the specific part in the doc concerning prefix matching -


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml#prefix


it states -


"Which of these routes will be installed in the routing table? Since EIGRP internal routes have the best administrative distance, it's tempting to assume the first one will be installed. However, since each of these routes has a different prefix length (subnet mask), they're considered different destinations, and they will all be installed in the routing table"


ie. it is clearly stating that all routes in the example will be installed and then it goes on to explain how the forwarding decision is made and the forwarding decision is always based on the most specific match (unless of course you are using PBR).


The default-route is the least specific route so it will be used when there is no more specific route in the routing table. Doesn't matter about the AD of the route.


Where the AD does matter is if you have 2 routes that are exactly the same prefix/subnet mask, then the router will choose the one with the lower AD.


So in your scenario as long as the EIGRP learned routes are more specific than the default route, and they should be, then the router will use the EIGRP routes if it finds a match.


Jon

Edison Ortiz Tue, 11/03/2009 - 11:57
User Badges:
  • Super Bronze, 10000 points or more
  • Hall of Fame,

    Founding Member

As you are quoting me, I'll add to Jon's reply.


We are basically saying the same thing. A default route is not a specific route - as you stated - is the gateway of last resort.



I said - a specific route will be preferred over the administrative distance.


If you have an EIGRP route of


192.168.1.0/24


and a static route of


0.0.0.0/0


and you need to forward traffic towards 192.168.1.0/24, the EIGRP route will be used - for other routes, the static will be used.


I believe you misunderstood my initial reply.


Regards


Edison



Actions

This Discussion