ISIS for IPv6


ISIS for IPv6 (Intermediate System to Intermediate System for IPv6)

ISIS is one of the important Link-State protocols. With entrence of IPv6 to the network world, ISIS also need to uptade itself for this new routed protocol. So, ISIS for IPv6 is developed.

Basically, ISIS for IPv6 is similar to previous version of ISIS, before this enhancement. But in this enhanced version of ISIS, there are some differences for IPv6 suppor. In this article, we ll focus on these extensions.

Let’s see the import extensions of IPv6 one by one.

IPv6 Address Support

ISIS for IPv6 is the enhanced evrsion of ISIS, that support IPv6 routing. With ISIS for IPv6 enhancements, ISIS advertise both IPv6 prefixes and the previous IPv4 and OSI prefixes. In the previous version of ISIS, only IPv4 and OSI prefixes were being advetised.

Basically, address families supportde by ISIS is increased with this ISIS for IPv6 enhancements.

Multi-Topology Extension of ISIS

Formerly, ISIS uses the same topology for both IPv4 and IPv6. It assume that all the routers in the topology must support both IPv4 and IPv6. With single topology, a single SFP per level were used for both IPv4 and IPv6. The configurations of the routes might be done for both of these protocols.

With Multi-Topology extension in ISIS, this situation has changed. Now, each protocol, IPv4 and IPv6 can has different topology and according to this, they have individual SPF.


Single topology and Multi-topology has some different characteristics. What are these differences?

• In Single Topology same topology is used for both IPv4 and IPv6. In Multi-Topology, different topologies are used for each IPv4 abd IPv6.
• In Single Topology all the routers run same set of protocols(IPv6, IPv6 or both). In Multi-Topology, different protocols can be used.
• Single Topology support one SFP. Multi-Topology support individual SFP for IPv4 and IPv6.
• Single Topology is useful form Daul stack. Multi-Topology s useful for others.
• Single Topology use old TLVs. Multi-Topology use Multi-Topology TLVs.


From the single mod to multi-topology mode, all IPv6 enabled routers must enable multi-topology mode and they must be configured with “transion” command.

New Topology ID

In ISIS for IPv6, new topology ID is given to IPv6 topology. Now, the topologies are like below:

• ISO/IPv4 Topology – MT ID #0
• IPv6 Topology – MT ID #2

New TLVs for Multi – Topology

ISIS for IPv6 Multi-Topology extension provides new TLVs. These are :

• IPv6 reachability TLV ( 0xEC )

Same as IP Internal/external reachability TLVs. Includes, IPv6 routing prefix, metric , option bits.

• IPv6 interface address TLV ( 0xE8 )

These TLV contains 128-bit IPv6 address. If it is hello PDU, it carries link-local IPv6 addresses. If not, then it carries normal IPv6 addresses.

New Network Layer Protocol Identifier (NLPID)

ISIS for IPv6 provides a new Network Layer Protocol Identifier (NLPID). This new NLPID is 0x8E (142). This is advertised by IPv6 enabled routers.

Metric Configuration

Especially for multi-topology support, metric configuration is required to support extended TLVs support. For IPv6 metric we can use “isis ipv6 metric” command. But for wide metrics that are more than default 63, we need to configure the router with “metric-style wide” command.

IPv6 Routing Protocols…

IPv6 Routing Protocols – Part 1
IPv6 Routing Protocols – Part 2 (OSPFv3)
IPv6 Routing Protocols – Part 3 (EIGRP for IPv6)
IPv6 Routing Protocols – Part 4 (RIPng)
IPv6 Routing Protocols – Part 5 (ISIS for IPv6)

IPv6 Routing Protocols CONFIGURATIONs..!

Static/Default Route Configuration Example On Cisco IOS
OSPFv3 Configuration Example On Cisco IOS
EIGRP for IPv6 Configuration Example On Cisco IOS
RIPng Configuration Example On Cisco IOS
ISIS for IPv6 Configuration Example On Cisco IOS

You can find the other IPv6 articles below…


IPv6 – Part 1 (IPv6 and IPv6 Addresses)
IPv6 – Part 2 (What does IPv6 bring?)
IPv6 – Part 3 (Address Types in IPv6)
IPv6 – Part 4 (Subnetting IPv6)

IPv6 Static Route Configuration Examples…

IPv6 Static Route Configuration on Cisco
IPv6 Static Route Configuration on Juniper

IPv4 and IPv6 Headers…

IPv4 and IPv6 Headers

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