IS-IS Single topology vs Multitopology

[nwekechampion]

Hi Guys,

Can anyone explain to me in simple terms what the difference is between IS-IS single topology and multitopology?

The documents available online are very confusing and conflicting, even cisco's.

I have tried to simulate both on my labs and cannot still understand the difference.

1. What is the main difference between both?
2. When would I need to use either ST or MT? (as in use-case)
3.  Which one is used for

   dual-stack

   ?
4. They say in Single topology, the

   lsdb

   is shared with

   ipv4 and ipv6

   address families. Can someone show me how to figure this out in

   show isis database

   ?
5. They say, MT has separates LSDB for both v4 and v6 --> how do I verify this on router?

Thanks 

[pieterh]

did you find this  page ? ISIS for IPv6 ⋆ IpCisco
it is just what is says, with single topology

IPv4 and IPv6

share the same topology
with multi topology there can be a different topology created for

IPv4

and another for

IPv6

NB! the topologies can also look the same (same components, same links between components)
but still it is

multi-topology

because they are separately administrated

[nwekechampion]

Hi Peter,

Could you explain this a bit further please "NB! the topologies can also look the same (same components, same links between components)
but still it is

multi-topology

because they are separately administrated"

Thanks for your response

[Joseph W. Doherty]

"When would I need to use either ST or MT? (as in use-case)"

The only use-cases that pop to mind for using a

multi-topology (for IPv4 and IPv6)

might be the two topologies are very disjoint, and SPF might have somewhat less work computing each, separately (along with possibly doing such computation faster).  Second, possibly you might cost the same link differently when using two topologies.  Another reason would be to very much control what's exchanged between the two topologies (which I've done using multiple OSPF processes on the same devices, but without the issues attended with routing between

IPv4 and iPv6

).

[pieterh · Accepted solution]

I do not understand what is not clear, did you look at the document ?
- it looks the same (left two images)

but there are differences

[nwekechampion]

Thanks Guys.

I had to lab this one to fully get it.

Regards

Champ

[nwekechampion]

Yes.. I did.
I am a bit slow sometimes..
perhaps I'd prefer to see it in action to see to fully get it.

I used the correct topology this time, and it all became very clear

Thanks again

[erastusangula]

I am also looking for the answer

[erastusangula]

ISIS for IPv4 only = Old TLVs  = narrow metrics only  = Single stack

ISIS for IPv4 and IPv6 = Old and new TLVs = narrow and wide metrics = dual stack

TLVs used to advertise reachability to IPv6 prefixes use wide metrics by default, so enabling ISIS IPv6 without enabling wide metrics require you to configure single-topology IPv6 mode to for SPF to use the same metric (old narrow metrics 1-63) for both IPv4 and IPv6. This ensure a single SPF run for both IPv4 and IPv6 DB.

•Multi-Topology IS-IS solves the restrictions of Single topology
–Two independent topology databases maintained
–IPv4 uses Multi-Topology ID (MTID) zero(0)
–New Multi-Topology ID (MTID #2) for IPv6

-Independent SPF run