Question regarding IS-IS and EIGRP redistribution

[tianye123]

Hello,

I have a question regarding the redistribution of EIGRP routes into IS-IS.

I have to connect 2 networks, one is using EIGRP as routing protocol, the other one IS-IS.

This is the (simplified) setup.

network 192.168.1.0/24 --- Router #1 (EIGRP) ------ Router #2 (EIGRP, IS-IS) ------ Router #3 (IS-IS) --- 192.168.2.0/24

Router #2 is acting as gateway  between the 2 networks. Redistribution of the IS-IS routes into EIGRP is working fine.

When configuring the redistribution of EIGRP into IS-IS a strange thing happens.

Router #2 starts announcing the route for 192.168.1.0/24  in IS-IS with a default metric, Router #3 is learning the routes and announcing them in IS-IS back to Router #2.

Router #2 accepts these routes and puts Router #3 as next hop in the routing table.

As a result the route for 192.168.1.0/24  flapping  between Router #1 and Router #3.

After reducing the administrative distance for EIGRP on Router #2 to 110, to make it lower than IS-IS's distance of 115, the flapping stops and the routing is stable.

Now, my question: Is this the correct way to configure this or is there a better way to do the redistribution?

Here are some code snipplets:

Router #1

=======

router eigrp 4321

network 192.168.0.0 0.0.0.255

redistribute static

redistribute connected

!

Router #2

=======

router eigrp 4321

network 192.168.1.1 0.0.0.255

redistribute isis level-1 metric 102400 1000 255 255 1500

redistribute static

distance eigrp 110 110

!

router isis

net 47.0192.0168.0001.0002.00

redistribute static ip level-1-2

redistribute eigrp 4321 level-1-2

!

Router #3

=======

router isis

net 49.0192.00168.0001.0027.00

redistribute static ip level-1-2

!

[Giuseppe Larosa]

Hello Dirk,

you are redistributing static routes everywhere what are the static routes defined on R3?

the flapping network is 192.168.0.0/24 or 192.168.1.0/24?

how can you build the EIGRP neighorship between R1 and R2?

network commands are pointing to different major networks 192.168.0.0/24 and 192.168.1.0/24

if the route has been a D EX route because redistributed by R1 into EIGRP, it is seen by R2 with default AD 170.

if R3 advertises a route to the network in ISIS R2 will prefer that route over the EIGRP D EX.

by reducing the AD of EIGRP external routes on R2 you have fixed the flapping.

What is still unclear is why R3 starts to generate a route for the same network just after R2 injects the same route in ISIS.

R3 is probably still advertising that route but it is not installed in R2 routing table.

You need to look at ISIS database to verify this

show isis database

it has several options you need to verify that R3 LSP has an entry for the network that was flapping.

my guess is that R3 has a static route for that subnet.

The next-hop used in that static route may be the key of this behaviour.

The next-hop may be in the EIGRP domain and becomes usable only when R2 starts to redistribute from EIGRP into ISIS.

Hope to help

Giuseppe

[lgijssel]

You should always be careful when redistributing mutually between two routing protocols.

Use filters to determine what gets redistributed. Always prevent routes from being learned twice, i.e once via protocol A and as redistributed route via protocol B.

The following link gives some general directions on how to handle the tricky job of configuring redistribution:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009487e.shtml#ospf

In addition to this, try to reduce the use of redistribute static / connected. This can also have unintended side effects.

regards,

Leo