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New Member

EIGRP and default routes

Trying to understand why a EIGRP default route does not distribute through another router with a static default route to connected router.

EXAMPLE:

=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=

RouterA is corporate office.

RouterB and RouterC are branch offices.

RouterA and RouterB both have ISP links.

Topology: RouterA --(serial link)-- RouterB --(serial link)-- RouterC

RouterA# show running-config

router eigrp 10
redistribute connected
redistribute static
network 192.168.1.0
no auto-summary

!

ip route 0.0.0.0 0.0.0.0 <to internet>

RouterB# show running-config

router eigrp 10
  redistribute connected
  network 192.168.1.0

  network 192.168.2.0
  no auto-summary

!

ip route 0.0.0.0 0.0.0.0 <to internet> track 100

RouterC# show running-config

router eigrp 10
  redistribute connected
  network 192.168.2.0
  no auto-summary

=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=~=

If I remove the default route in router B, router C can then see the default route via EIGRP. If the static default route is added in router B, router C can not see the default route via EIGRP from router A.

What is the easiest way to fix this? Through ip summary-address eigrp 10 0.0.0.0 0.0.0.0 on router B's interface to C

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: EIGRP and default routes

ryan.s.15 wrote:

Can you point me to a document possibly that states something to the effect:


"EIGRP default routes are not redistributed down stream if an upstream router has a static default route"

I understand that the default route in B will not redistribute in EIGRP, which is what I want. I'm trying to find the reasoning that there is a default route being distributed via EIGRP from A but it gets dropped at B, because C never sees it.

I appriciate all your help.

Ryan

Don't have a document link because it's basic EIGRP behaviour. An EIGRP speaking router only advertise routes that it has in it's routing table so if the route is not in the routing table it won't be advertised to an EIGRP peer. The reasoning is simple. If the route is not installed in the routing table then the router should not be advertising to any other router that it knows how to get to that network because if it did advertise it but not install it in its routing table then when packets arrived at that router for the destination network they would be dropped because the router wouldn't have a route for that network.

This is what you are seeing and is standard behaviour for distance-vector protocols. Note that OSPF behaves differently because it is a link state protocol and as such floods LSAs to all neighbors.

Jon

9 REPLIES
Hall of Fame Super Silver

Re: EIGRP and default routes

Hello Ryan,

>> What is the easiest way to fix this?

on RB use

ip route 0.0.0.0 0.0.0.0 200

instead of

ip route 0.0.0.0 0.0.0.0 track 100

in this way also RB will use the received EIGRP default route you need to make the static a floating static route or it will be preferred over EIGRP default route.

Hope to help

Giuseppe

Hall of Fame Super Blue

Re: EIGRP and default routes

giuslar wrote:

Hello Ryan,

>> What is the easiest way to fix this?

on RB use

ip route 0.0.0.0 0.0.0.0 200

instead of

ip route 0.0.0.0 0.0.0.0 track 100

in this way also RB will use the received EIGRP default route you need to make the static a floating static route or it will be preferred over EIGRP default route.

Hope to help

Giuseppe

Guiseppe

Agreed, but then B would also use A to send traffic to the internet rather than use it's local ISP connection and i'm not sure that is what Ryan wants.

Jon

Hall of Fame Super Blue

Re: EIGRP and default routes

Ryan

The problem you have is that router B will always use the locally configured default-route rather than the one received from router A and hence that is why it does not advertise router A default-route to router C.

Even if you use a summary address from B to C then this still wouldn't mean C would use A as it's route to the internet because as soon as traffic from C got to B router B would then use the locally configured static default route and send it out of the local ISP connection.

So if you want to make sure C's traffic to the internet goes via A you will have to use PBR (Policy Based Routing) unless you can somehow connect up the routers differently.

Jon

New Member

Re: EIGRP and default routes

R.A can use the its ISP as primary and R.B and R.C can use as secondary.

R.C can use R.B's ISP as it's closer geographically.

If R.C's ISP is down I want R.C and R.B to use R.A's EIGRP default route.

What appears is that I need a summary-address to advertise the default route to R.C. R.B will need a track on the default route so if ISP on R.B fails it will remove the route and then R.B and R.C can then use R.A's advertised default route.

Is there something else that would make this eaiser? Is summary-address the right way to go?

Hall of Fame Super Blue

Re: EIGRP and default routes

Ryan

You could use a summary-address on B to C if you simply want to get C's traffic to B. Then as you say track the default-route on B and if it is removed then B will install A's route and both B & C will then use A.

Jon

New Member

Re: EIGRP and default routes

jon.marshall wrote:


The problem you have is that router B will always use the locally configured default-route rather than the one received from router A and hence that is why it does not advertise router A default-route to router C.

Even though router B is not set to advertise static routes EIGRP on that router says drop the default route so router C can't see that advertisement? Seems odd that it drops it.

I wonder if it does the same thing for static route (non-default route) entries? Thus if router B had an entry for a ip route it could potential block router C from seeing it.

Hall of Fame Super Blue

Re: EIGRP and default routes

ryan.s.15 wrote:

jon.marshall wrote:


The problem you have is that router B will always use the locally configured default-route rather than the one received from router A and hence that is why it does not advertise router A default-route to router C.

Even though router B is not set to advertise static routes EIGRP on that router says drop the default route so router C can't see that advertisement? Seems odd that it drops it.

I wonder if it does the same thing for static route (non-default route) entries? Thus if router B had an entry for a ip route it could potential block router C from seeing it.

Ryan

Router B will not use router A's default route because it has it's own. But you are not redistributing B's static route into EIGRP. So B receives the default-route from A but doesn't put it into the routing table because it has it's own default-route which has a much better AD. But this local default-route is not being redistributed into EIGRP so C will never get an advertisement for this route from B.

So B is not dropping A's advertisement, it simply doesn't use it because it has a better one.

Jon

New Member

Re: EIGRP and default routes

Can you point me to a document possibly that states something to the effect:


"EIGRP default routes are not redistributed down stream if an upstream router has a static default route"

I understand that the default route in B will not redistribute in EIGRP, which is what I want. I'm trying to find the reasoning that there is a default route being distributed via EIGRP from A but it gets dropped at B, because C never sees it.

I appriciate all your help.

Hall of Fame Super Blue

Re: EIGRP and default routes

ryan.s.15 wrote:

Can you point me to a document possibly that states something to the effect:


"EIGRP default routes are not redistributed down stream if an upstream router has a static default route"

I understand that the default route in B will not redistribute in EIGRP, which is what I want. I'm trying to find the reasoning that there is a default route being distributed via EIGRP from A but it gets dropped at B, because C never sees it.

I appriciate all your help.

Ryan

Don't have a document link because it's basic EIGRP behaviour. An EIGRP speaking router only advertise routes that it has in it's routing table so if the route is not in the routing table it won't be advertised to an EIGRP peer. The reasoning is simple. If the route is not installed in the routing table then the router should not be advertising to any other router that it knows how to get to that network because if it did advertise it but not install it in its routing table then when packets arrived at that router for the destination network they would be dropped because the router wouldn't have a route for that network.

This is what you are seeing and is standard behaviour for distance-vector protocols. Note that OSPF behaves differently because it is a link state protocol and as such floods LSAs to all neighbors.

Jon

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