Posted on Tuesday, 22nd November 2005 by sean

I’ve read and heard that there are some idiosyncrasities about connected routes and IS-IS when redistributing. Most recently while reading “Optimal Routing Design” and also when talking with a friend who was studying for the BCSI exam. When redistributing out of IS-IS, some routes are missing, notably connected routes. Let’s investigate.

A simple three-router network:

192.168.0.0/16 is in the EIGRP domain, and 10.0.0.0/8 is in IS-IS. Each router has a loopback or two just for fun. R1 will be doing redistribution.

R2:

router eigrp 1
 network 0.0.0.0
 no auto-summary
 no eigrp log-neighbor-changes

R1:

clns routing
interface Loopback5
 ip router isis
interface Ethernet0
 ip router isis
router eigrp 1
 network 192.168.0.0 0.0.255.255
 no auto-summary
!
router isis
 net 49.0001.0100.5000.0002.00

R3:

clns routing
interface Loopback5
 ip router isis
interface Ethernet0
 ip router isis
router isis
 net 49.0001.0100.5000.0003.00

Routing tables on R3:

     10.0.0.0/24 is subnetted, 3 subnets
i L1    10.1.1.0 [115/20] via 10.50.0.2, Ethernet0
C       10.3.3.0 is directly connected, Loopback5
C       10.50.0.0 is directly connected, Ethernet0

Routing tables on R2:

D    192.168.11.0/24 [90/40640000] via 192.168.5.1, 01:30:25, Serial0
C    192.168.5.0/24 is directly connected, Serial0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.2.2.0 is directly connected, Loopback5
C    11.0.0.0/8 is directly connected, Loopback6

On R1, redistribute mutually between IS-IS and EIGRP:

router eigrp 1
 redistribute isis level-2 metric 10000 100 255 1 1500
 network 192.168.0.0 0.0.255.255
 no auto-summary
!
router isis
 net 49.0001.0100.5000.0002.00
 redistribute eigrp 1 metric 5

Routing tables on R3:

i L2 192.168.11.0/24 [115/15] via 10.50.0.2, Ethernet0
i L2 192.168.5.0/24 [115/15] via 10.50.0.2, Ethernet0
     10.0.0.0/24 is subnetted, 4 subnets
i L2    10.2.2.0 [115/15] via 10.50.0.2, Ethernet0
i L1    10.1.1.0 [115/20] via 10.50.0.2, Ethernet0
C       10.3.3.0 is directly connected, Loopback5
C       10.50.0.0 is directly connected, Ethernet0
i L2 11.0.0.0/8 [115/15] via 10.50.0.2, Ethernet0

Hmm… Everything there seems to be in order.

Routing tables on R2:

D    192.168.11.0/24 [90/40640000] via 192.168.5.1, 01:33:23, Serial0
C    192.168.5.0/24 is directly connected, Serial0
     10.0.0.0/24 is subnetted, 2 subnets
D EX    10.3.3.0 [170/40537600] via 192.168.5.1, 00:01:50, Serial0
C       10.2.2.0 is directly connected, Loopback5
C    11.0.0.0/8 is directly connected, Loopback6

Here, we’re missing 10.1.1.0 and 10.50.0.0 which are connected routes inside IS-IS. According to this TAC case, IS-IS connected routes don’t get redistributed:

When Intermediate System-to-Intermediate System (IS-IS) is redistributed into Open Shortest Path First (OSPF), some IS-IS routes may not be redistributed successfully. Specifically, routes advertised using IS-IS on the redistributing router are not redistributed. This behavior is expected. If a directly connected network is also advertised by IS-IS, the route is entered into the local routing table as a connected route and not as an IS-IS route. While redistributing IS-IS into OSPF, these directly connected routes are not injected into OSPF.

As I’ve shown, this also happens for EIGRP. The solution is to redistribute connected into EIGRP:

r1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
r1(config)#router eigrp 1
r1(config-router)#redistribute connected 

r2#show ip route
...

D    192.168.11.0/24 [90/40640000] via 192.168.5.1, 01:43:24, Serial0
C    192.168.5.0/24 is directly connected, Serial0
     10.0.0.0/24 is subnetted, 4 subnets
D EX    10.1.1.0 [170/40640000] via 192.168.5.1, 00:00:13, Serial0
D EX    10.3.3.0 [170/40537600] via 192.168.5.1, 00:11:50, Serial0
C       10.2.2.0 is directly connected, Loopback5
D EX    10.50.0.0 [170/40537600] via 192.168.5.1, 00:00:13, Serial0
C    11.0.0.0/8 is directly connected, Loopback6

The alternative would have been to turn on EIGRP for all interfaces and mark the two 10/8 networks as passive-interfaces. This would put the routes into EIGRP as internal routes.

So there you have it. When redistributing out of IS-IS, any locally connected routes that aren’t already in the destination protocol of the redistributing router don’t get carried over.

No related posts.

Related posts brought to you by Yet Another Related Posts Plugin.

Posted in Routing | Comments (2)

2 Responses to “Redistribution involving IS-IS”

  1. Tom Says:
    December 15th, 2006 at 11:58 am

    I think your website is great. I printed off this scenario and tried in my lab at work. There is 1 small type that was driving us nuts, your line to redistribute isis routes should redistribute level 1 routes, your command only says level 2.

    Keep up the good work.

  2. stteng Says:
    June 7th, 2009 at 3:57 pm

    I think that I found a typo in R2′s router eigrp 1′s configuration. Should the statement ‘network 0.0.0.0′ be [network 10.0.0.0]?

    What is the NET of the IS-IS router? Am I missing something?

    Thanks
    stteng

Leave a Reply