EIGRP problem

[joyride_us]

Hi,

here is the description of 2 routes to the same subnet : question : why does the router use the route with the highets feasible distance : the route from 173.16.2.21 has a lot lower metric...

Thank you

Zidane#sh ip eigrp topology 172.16.9.160/27

IP-EIGRP (AS 1): Topology entry for 172.16.9.160/27

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2572800

Routing Descriptor Blocks:

172.16.11.226, from Redistributed, Send flag is 0x0

Composite metric is (2572800/0), Route is External

Vector metric:

Minimum bandwidth is 1000 Kbit

Total delay is 500 microseconds

Reliability is 1/255

Load is 1/255

Minimum MTU is 1500

Hop count is 0

External data:

Originating router is 173.16.2.25 (this system)

AS number of route is 0

External protocol is RIP, external metric is 14

Administrator tag is 0 (0x00000000)

173.16.2.21 (Serial1/0:0), from 173.16.2.21, Send flag is 0x0

Composite metric is (1826048/1314048), Route is External

Vector metric:

Minimum bandwidth is 1948 Kbit

Total delay is 20000 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1500

Hop count is 1

External data:

Originating router is 173.16.2.21

AS number of route is 0

External protocol is RIP, external metric is 3

Administrator tag is 0 (0x00000000)

[Kevin Dorrell]

Both routes are external, so I guess both have an AD=170. Is the routing table entry an "EX" or an "R"? If it is an 'R', then it has an AD of 120, which wins over EIGRP externals.

It may be something to do with the fact that the second route was redistributed in this router, whereas the first one came from another router.

Kevin Dorrell

Luxembourg

[joyride_us]

Hi,

you are right, the 1st route is R with a better AD. But is it normal that the router will advertise only one of those 2 to its EIGRP neighbor (please note that this neighbor is connected to both route through the same interface)

Thank you

[Kevin Dorrell]

OK, which route gets into the routing table is determined by the AD, as we have seen.

I'm not sure what the second part of your question means, but I think it is normal that it advertises only one route for the prefix to its upstream neighbor. But it is interesting to speculate which one it will advertise. After all, neither of these routes appears in the routing table - that is another routing protocol altogether.

Probably the lower metric route - the second one. Could you do the same command sh ip eigrp topology 172.16.9.160/27 on the neighbor, and we can see if the metrics (in particular, the delay) add up.

Kevin Dorrell

Luxembourg

[joyride_us]

Hi,

well, the router installed the RIP route in the routing table and sent this one to the neighbor.

The ouput of your command is what I sent in my first message. Fascinating : although the metric of the other route (EX-EIGRP) is way lower : since the router is running RIP it self : it trusts this route more even if the router redistributes the RIP route on EIGRP as well...

[Kevin Dorrell]

In that case, I didn't understand the second part of your question. Sure, it is the RIP route that gets into the routing table, I'm not sure I know what your mean by " ... and sent this one to the neighbor ... ". Is the neighbor doing RIP as well? In that case the neighbor will prefer the RIP.

Perhaps the missing piece of the puzzle for you is that when this router advertises the EIGRP route to the neighbor (which I'm sure it does, if the neighbor speaks EIGRP too), then it flags it to say "this is an external route". The neighbor sees the flag, and gives the EIGRP route an AD=170 as well. As long as you are running RIP and EIGRP on the same router, the order of preference is (1) EIGRP internals, (2) RIP, (3) EIGRP externals. And the "external" flag propagates with the route.

Does that make sense?

Kevin Dorrell

Luxembourg

[joyride_us]

Hi,

You are right : there is a missing piece! And here it is : the rest of the network gets the information that the next hop to a given subnet is the RIP route-nexthop not the EXT-EIGRP-nexthop which I profoundly regret...I might have to just change the administartive distance of RIP route to 180 for example. (does that sound wise?). But the problem is that I cannot change the AD for a set of subnets : it seems to be a all-or-nothing...I fix a problem to create another one...

What do you think?

thank you

[Kevin Dorrell]

If you want to change the ADs, then you are a braver man than I!. ;-). But you could give it a go, and it should work. Just make sure you change them consistently in all your routers, otherwise you can end up with some horrific routing loops. AD is not propagated from router to router, (except perhaps implicitly as an internal/external flag in the particular routing protocol), so you have to enforce the consistency by hand.

The thing to remember is that the propagation of routes by each routing protocol is independent of what actually goes into (or has gone into) the routing table. Each routing protocol passes information between its routers, and keeps its own database; that has nothing to do with packet forwarding. Then each router decides which routing protocol databases it is going to use to populate its own local forwarding (routing) table, and it is there that the AD comes into play, and it is that which determines the packet forwarding..

You might be better off lowering the AD of EIGRP externals rather than raising the AD of RIP. It might be even better to limit the scope of the RIP so that it never runs on the same link as EIGRP, and you have one re-disribution point only.

I think there are techniques to change the AD selectively, but I haven't read that chapter yet! You would certainly have to do it on each and every router. Perhaps someone else can tell you about it.

Bonne chance!

Kevin Dorrell

Luxembourg