Hi,
In the post ISIS IPv6 feature, we talked about the parameters for command IPv6 enable topology. In this post, I’m going to introduce an example to explain differences between those two paramters.
As we know, ISIS could support both IPv4 and IPv6, and with the IPv6 becoming more and more widely used, a new requirement comes out, which is to run the IPv4 and IPv6 simultaneously on the same physical network.
As the figure illustrates below, the link between R2 and R4 doesn’t support IPv6, and the ISIS cost displays in the figure.
On R4, import the network of direct connected interface loopback 0, both IPv4 and IPv6, into ISIS process. Based on the ISIS cost painted in the figure, the IPv4 traffic path to the loopback 0 of R4 should be: R1 > R2 > R4, and since the link between the R2 and R4 doesn’t support IPv6, so the IPv6 traffic path to the loopback 0 of R4 should be: R1 > R3 > R4.
Checking the routing table on R1, we get the result as below
IPv4 routing table:
[r1]dis ip ro
Route Flags: R - relay, D - download to fib
--------------------------------------------------------------------
Routing Tables: Public
Destinations : 9 Routes : 9
Destination/Mask Proto Pre Cost Flags NextHop Interface
4.4.4.4/32 ISIS-L2 15 2 D 10.1.12.2 GigabitEthernet0/0/0
The result is as same as we predicted.
IPv6 routing table:
[r1]dis ipv ro
Routing Table : Public
Destinations : 11 Routes : 11
Destination : 2000::4 PrefixLength : 128
NextHop : FE80::5689:98FF:FEE7:52BA Preference : 15
Cost : 2 Protocol : ISIS-L2
RelayNextHop : :: TunnelID : 0x0
Interface : GigabitEthernet0/0/0 Flags : D
The result indicates the traffic path for the IPv6 flow is also: R1 > R2 > R4
As we pointed out previously, the link between R2 and R4 didn’t support IPv6, then the IPv6 traffic cannot reach to R4.
Testing result on R1 verifies the prediction.
[r1]ping ipv 2000::4
PING 2000::4 : 56 data bytes, press CTRL_C to break
Request time out
Request time out
The resason for this error is ISIS run the SPF based on the same topology for both IPv6 and IPv4, so the result calculated out keeps the same.
To prevent such error, we can adjust the command from ipv6 enable topology standard to ipv6 enable topology ipv6. In this way, ISIS will calculate the routing path for each IP version.
After modifying the parameter, the next-hop of the route changes from G0/0/0 to G0/0/1. As the routing table below shows.
<r1>dis ipv ro
Routing Table : Public
Destinations : 11 Routes : 11
Destination : 2000::4 PrefixLength : 128
NextHop : FE80::5689:98FF:FE58:4078 Preference : 15
Cost : 30 Protocol : ISIS-L2
RelayNextHop : :: TunnelID : 0x0
Interface : GigabitEthernet0/0/1 Flags : D
Also the testing result is normal
<r1>ping ipv 2000::4
PING 2000::4 : 56 data bytes, press CTRL_C to break
Reply from 2000::4
bytes=56 Sequence=1 hop limit=63 time = 40 ms
Reply from 2000::4
bytes=56 Sequence=2 hop limit=63 time = 60 ms
Reply from 2000::4
At last, what should be pointed out is the parameter changing should be applied on all the routers, rather than only the R1.
That's all for this post. Hope it could help you understand the difference between these two parameters.
Thanks.
