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.
Content Copyright Sean Walberg
Redistribution involving IS-IS