# IPv6 Single to Multi Topology

Load **isis.ipv6.st.cfg**

```
#IOS-XE
config replace flash:isis.ipv6.st.cfg

#IOS-XR
configure
load bootflash:isis.ipv6.st.cfg
commit replace
y
```

All routers have IPv6 link-local addresses enabled on their interfaces, and a loopback IPv6 address of 2001:db8::\<router num>/128.

Single-topology IPv6 has already been setup. Migrate to Multi-topology for IPv6 without breaking IPv4 or IPv6 reachability.

## Answer <a href="#ab4df0a2-e242-4e2c-bdfd-f31a4515bfdf" id="ab4df0a2-e242-4e2c-bdfd-f31a4515bfdf"></a>

In phase one, we first enable wide-metrics everywhere. This is because MT on IOS-XE requires wide metrics. We enable it using the transition method, in which both narrow and wide metrics are generated and received.

```
#R1-R6
router isis
 metric-style transition

#XR1-2
router isis 1
 add ipv4
  metric-style transition
```

Now we can remove the transition style and use only wide-metrics.

```
#R1-R6
router isis
 metric-style wide

#XR1-2
router isis 1
 add ipv4
  metric-style wide
```

I should note that when I make these changes on IOS-XE, I see no reachability issues. However, on XRv I do see 1-2 pings lost. I was pinging 1.1.1.1 from XRv2, and when enabling this on XRv1 I lost a few pings. This may be a bug in the XRv implementation, or it may just be that XR does give you a brief outage when doing this.

In phase two we enable multi-topology on all routers but with the **transition** keyword. This forces the router to continue generating the IPv6 reachability TLV which is used in single-topology.

```
#R1-R6
router isis
 add ipv6
  multi-topology transition

#XRv1-2
router isis 1
 add ipv6
  no single-topology   ! On IOS-XR there does not appear to be a transition mode
```

Note that we lost reachability on XRv neighbors with this step because we had to enable multi-topology only. There is no “transition” mode in which XRv continues generating normal ST IPv6 prefix reachability TLVs. Specifically, when making the change first on routers R1-R6, and then on XR1, XR1 loses its reachability to XR2 prefixes. But it still has reachability to all routers R1-R6. We then turn ST off on XR2, and reachability is restored.

Now that all routers are running MT, we can turn the transition mode off.

```
#R1-R6
router isis
 add ipv6
  multi-topology

! By setting just "multi-topology" itself, it overrides the "multi-topology transition"
! command that was there previously.
```

## Summary <a href="#id-153fe243-c2c9-4ddc-aafd-7d4c41361062" id="id-153fe243-c2c9-4ddc-aafd-7d4c41361062"></a>

The **transition** keyword for multi-topology is only available on IOS-XE. When you use this keyword, the router continues generating IPv6 reachability TLVs (236) in addition to the MT IPv6 reachable prefixes TLV (237). This allows other routers that are still running in ST mode to continue to work.

<https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/15-mt/irs-15-mt-book/ip6-route-multi-isis.html#GUID-9650FA27-2E6A-44A6-B0DB-09F97CEC1D20>
