EIGRP Redistribution Issue - Multiple Processes on same router

[kfarrington]

All, Sorry for another EIGRP posting this week :)

Please could someone help me and take a look at this routing topology in the diagram.

Site2 Router is running EIGRP process 121 - to Site 1 and EIGRP process 122 to Site3

The Site2 router will receive external EIGRP routes in its AS121 process from the RIP AS with the external data being RIP on the path from Site2 to Site1, ie RIP AS -> EIGRP AS121. On the Site2 to Site3 link, the Site2 router will receive the same route in the AS 122 process from the RIP AS but they will have external data of AS121 as the route has come from RIP AS -> EIGRP-AS121 -> EIGRP AS122. NOTE that metrics and FD/RDs are the same from Site2 via both paths.

NOW THE QUESTION :)

The Site2 router receives the RIP route from both Site1 and Site3, and both have ADs of 170 but the route from the Site1 side never make it into the RT of Site2.

Looking at the EIGRP EVENTS, it almost looks like that when a route is installed into the RT on the Site2 router, say via EIGRP 122, a route received on the Site2 router from Site1 AS 121 never make it into the RT because EIGRP says that you already have an entry in the RT with an AD which is the same OR, maybe EIGRP says, that if AS122 puts a route into the RT, a route received from another EIGRP process (with the SAME AD) will not co-exist in the RT. I am not sure about this.

Could someone help in confirming this point.

Also, If you down the AS122 Link the AS121 link is used. When you bring the AS122 link backup, EIGRP switches back to the AS122 link. WHY IS THIS?

IN SUMMARY :-

Can a router receive the same prefix from two EIGRP ASs with the same AD and FD and both be consider sucessors and would be entered into the RT.

Is this possible.

PS. This is a legacy scenario, and I would be looking to make the EIGRP AS all one AS.

Many Thx once again,

Ken

[pkhatri]

Hi Ken,

Using two EIGRP processes with the same AD is an unsupported setup.

Therefore, it's pointless to speculate on this behaviour because:

- there are no published guidelines on how this should behave (since it's unsupported)

- any behaviour you observe may change between different IOS versions

One more thing - Cisco does have doco on what happens when you have two OSPF processes on a router (with the same AD): the first process trying to install the route makes it into the routing table. This, obviously, does not concur with what happens with EIGRP, as you have observed.

Therefore, I would say: just make it one big AS or use different routing protocols and you will not have this problem.

Hope that helps - pls rate the post if it does.

Paresh

[kfarrington]

Paresh, I hope you had a good weekend fella :)

The last diagram may have had some mis information in when I was changing AS numbers in the results.

Please see new diagrams with proper AS numbers.

The solution here seems to be if you have multiple-ASs for EIGRP, EIGRP will select the topology entry with the lowest AS number as the entry to be entered into the routing table.

This is very deterministic if you down the lowest AS path link - it will switch to the higher AS path link, and when you re-introduce the lowest AS path link, EIGRP will flip back to that link.

Also, I am just wondering if this only happens when the metrics are equal, and will test this also, I would hope that metrics do come into play before the AS number tie-breaker feature, if infact it is a feature :)

Kindest regards,

Ken

[pkhatri]

Hi Ken,

Basically what I was saying was not that what you were seeing was not deterministic, but that what is true for one release may not be true for another. I'm sure that the behaviour is perfectly deterministic as far as your IOS goes...however, it still remains an unsupported configuration.

Also, why would you want metrics to be compared between two routing processes ? I know that we are comparing metrics for two routing processes, both of which happen to be EIGRP. Does that make the metric comparable ? What may be true for one EIGRP AS (in terms of how metrics are assigned) may not be true for another EIGRP AS.... So you aren't exactly comparing apples with apples. While I have not tested this myself, I would hope that the metrics do not come into play when you have unequal metrics. I wait with bated breath for your test results :-)

Paresh.

[kfarrington]

Paresh, I fully understand :)

I have just tested, and EIGRP will compare the metrics from the different ASs to select the best path to install into the RT.

I also understand you view on the design, and this is a legacy install I am dealing with, so the best plan would defo be to enlarge the AS and have one AS that covers all the devices.

It would be good if there is any documentation on the behaviour of this.

Also, as you say, different IOS versions may act differently, so until there is an official "Cisco" explanation as to the behaviour, we shall take this with a pinch of salt :)

Interesting behaviour though that we are seeing.

Please see attached.

Kindest regards,

Ken