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.

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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

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