How does eigrp propagate routes?

Unanswered Question
Apr 16th, 2010
User Badges:

Hi,


I've got a little lab pod on gns3 very simple, all are routers


LAN---Branch1----FR1(Frame Relay Switch)---WAN1----switch---Core1


Branch1

LAN ip address: 10.1.1.1

WAN ip address: 193.10.25.10


WAN1:

Interface pointing towards Branch1 (multipoint link) ip address: 193.10.25.1

Interface pointing towards Core:  10.0.0.10


Core1:

Interface pointing towards WAN1:  10.0.0.1


All routers are running eigrp


All routers have "auto-summary" except for Branch 1 which has "no auto-summary" router command configured.


Branch1

I am advertising the LAN interface (ip 10.1.1.1) with the router statement "network 10.1.1.0 0.0.0.3"



When I look at WAN1's routing table I see the following:


10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks

D       10.1.1.0/24 [90/2195456] via 193.10.25.10, 00:21:09, Serial0/0.1



And when I look at Core1's routing table I see:


10.0.0.0/24 is subnetted, 4 subnets

D       10.1.1.0 [90/2221056] via 10.0.0.10, 00:29:29, FastEthernet0/0



My question is if WAN1 is configured with the "auto-summary" router command, then why is the Core1 router seeing the individual route from the Branch router, shouldn't the Core1 router route table be similar to this:


C       10.0.0.0 is directly connected, FastEthernet0/0


and nothing else about the 10.0.0.0 network?


How do the branch router routes get advertised to the core router and why aren't they being summarised by the WAN router?


Thanks in advance


Dan

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Giuseppe Larosa Fri, 04/16/2010 - 04:11
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Dan,

if the router with auto-summary is WAN1, Branch router does not send net 10.0.0.0/8 but the specific route.


WAN1 having one interface on major network 10/8 propagates the specific route to core router.


Auto-summary happens at major network boundary, if WAN1 had an interface out of net 10.0.0.0/8 it should advertise 10/8 out it.


in other terms on branch router you should see a net 10/8 sent by WAN1 as a result of auto-summary of ip subnet in 10/8 between WAN1 and core router.



if you use auto-summary on branch, WAN1 should receive net 10/8 from it on the serial link (because that link IP subnet is not under 10/8)


Hope to help

Giuseppe

dan_track Fri, 04/16/2010 - 04:27
User Badges:

Hi,


Thanks for the reponse.


I'm not following 100% on WAN1 router I do have an interface within the 10.0.0.0/8 boundary? The interface facing towards the Core1 router has the following configured: "ip address 10.0.0.10 255.255.255.0". so from what I understand from your comment it should automatically summarise all 10.x.x.x routes to 10.0.0.0/8 to Core1, but that's not what is happening in my LAB, your thoughts on this?


Additionally can you please explain why when setting autosummary on the Branch1 router I don't see any routes for 10.0.0.0/8 pointing towards the Bracnch1 router from the WAN1 router. All I see is the directly connected route to Core1 and a "Null" summary route for 10.0.0.0/8, so if I try to ping Branch1 (10.1.1.1) from Wan1 the ping fails. Even when looking into WAN1's topology table I can't see the 10. routes from Branch1, any thoughts?


You were right though about seeing summary route on the Branch1 routers:


D       10.0.0.0/8 [90/2195456] via 193.10.25.1, 01:25:36, Serial0/0.1


Many Thanks

Dan

Giuseppe Larosa Fri, 04/16/2010 - 06:46
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Dan,

with major networks I mean the old Class A, Class B, Class C classification:

10/8 is the major network of type Class A.

10.1.1.0/24 and 10.0.0.0/xx are both subnets of 10/8 and for this reason the component route 10.1.1.0/24 coming from Branch is propagated to core by WAN1.


On branch router that is behind a subnet out of 10/8 you see the effect of auto-summary on WAN1


Hope to help

Giuseppe

dan_track Fri, 04/16/2010 - 09:44
User Badges:

Hi,


Many thanks for your explanation. I had to read it about 10 times before I understood what you meant. I beleive I've got it now. Does this rule also apply to ospf, is-is and bgp?


Thanks

Dan

Giuseppe Larosa Fri, 04/16/2010 - 11:12
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Dan,

the same reasoning applies to RIPv2.


It does not apply to link state routing protocols OSPF and ISIS that never care of major networks boundaries.


For BGP there are some important differences: BGP works on defined sessions and when auto-summary is enabled (not by default anymore to be noted) classful summary routes are sent to neighbor if a component route exists.


Hope to help

Giuseppe

Actions

This Discussion