will isis pseudonodes increase routing table entrys or cpu resources?

[Ma Juner]

Two days ago, I created this question as the document, that's my fault.

Thanks Peter Paluch to remind me , I have delete the old one and recreate it here.

Hello Community,

  Recently,  I have the confused issue :

    DIS generate pseudonodes and pseudonodes LSP. Pseudonodes build the neighbor to other isis routers include DIS, like the DR function in OSPF, pseudonodes is used for saving the ISIS database synchronous entrys and reducing flooding,  so I think this mechnism can have a good performance in cpu and routing table resourse.

   However, in P2P ISIS network ISIS LSA run on a different way,

   So My problem comes out:

   Pseudonodes is not the real router but it will occupy extra LSP item！

   A network IGP is constructed upon ISIS broadcast way, if I change the router-router isis neighbor link to p2p mode, will I get a better convergence

time, isis database saving or cpu resource。And if I do this， what happend to the ISIS like isis neighbor down then rebuild or other fault?

Regards.

Juner

[Peter Paluch · Accepted solution]

Hi Juner,

You are correct in your description of the purpose of a pseudonode. In technical terms, a pseudonode reduces the memory complexity of the graph representation of a multi-access networks - instead of O(N^2) links for N routers on a multi-access segment, only O(N) links are recorded in the link-state database of each router. With 10 routers on a common segment, without the pseudonode, the link-state database of each router would contain 10*9=90 links to describe this common segment. Using a pseudonode, the number of links to describe this segment is reduced to 2*10=20 (from each router to the pseudonode and vice versa). The savings are even bigger as the number of routers on a common segment increases. As a result, the SPF algorithm also needs to investigate less links when processing the link-state database. However, a pseudonode does not reduce flooding - in fact, it has nothing to do with flooding. A pseudonode is purely a topological component inside the link-state database but it does not influence how the LSPs are flooded in the network.

Changing the ISIS network type to p2p is useful only on point-to-point links, and will cause the ISIS to simply omit creation of the pseudonode and to fully synchronize to the neighbor. The advantages are:

• For a point-to-point link, the pseudonode is unnecessary and actually only complicates the link-state database. If your network consists of lots of point-to-point links, configuring them as ISIS p2p links may save you some memory in your routers' link-state databases. The extent of savings depends on how many p2p links your network has.
• The convergence time may be slightly increased. Besides the fact that the link-state database is smaller, the routers do not need to engage into DIS elections and may become fully adjacent as soon as they detect each other.

According to Jeff Doyle's book about OSPF and ISIS, the original ISIS network type was broadcast, i.e. the one using the pseudonode and electing the DIS. However, many providers may be using Ethernet links in a point-to-point fashion, and, as Jeff eloquently put it, "electing DIS and creating unnecessary pseudonodes on these point-to-point links offends our sense of network aesthetics" So an option was added to ISIS to configure its operation on such links as if the link was inherently point-to-point, i.e. without the pseudonodes and without DIS elections.

Changing the network type to p2p on an interface will result in ISIS tearing down all adjacencies on that interface and building them anew. Intermittent connectivity outages will result until the two routers resynchronize.

Please feel welcome to ask further!

Best regards,

Peter

[Ma Juner]

hi Peter,

   Thanks a lot! You give me a clear details!  You are right!

   In my network there is some point-to-point links, and I think I should do some working as to record memory usage and calculate the link-state database reduction, so that I can have more clear understanding  on my network overall performance.

   Can you give me a link to  Jeff Doyle's book about OSPF and ISIS, I want to have a deeper reading on his saying "electing DIS and creating unnecessary pseudonodes on these point-to-point links offends our sense of network aesthetics"

   Regards,

     Juner

[Peter Paluch]

Hi Juner,

The book in question is:

OSPF and IS-IS

Choosing an IGP for Large-Scale Networks

Jeff Doyle

ISBN: 0-321-16879-8

I have somewhat adapted the quotation about the pseudonodes... the exact formulation can be found on the page 117 of this book:

Ethernet interfaces, whether 10M, 100M, 1G, or 10G, are consistently cheaper than other interfaces of equivalent speed. Accordingly, Ethernet is often used as a point-to-point connection between devices in the same equipment room, building, campus, or metro area. When used to connect only two devices, a DR or DIS serves no purpose. Yet because OSPF and IS-IS classify Ethernet interfaces as broadcast by default, one of the two devices is going to be elected as a DR or DIS.

Conscientious network engineers like to keep their networks as simple, efficient, and uncluttered as possible. To us, allowing unnecessary DRs or DISs means unnecessary network traffic, unnecessary intormation in the link state databases, and unnecessary nodes on the SPF tree. If nothing else, it offends our sense of network aesthetics. Although some might argue that this is fastidiousness run amok, when you manage a very large network you find yourself constantly on the lookout for inefficiencies. And some large networks can have hundreds or even thousands of point-to-point Ethernet links. To add to the complexity, some Ethernet links carry Virtual LAN (VLAN) logical links. If OSPF or IS-IS is run over these links, DR/DIS functions can again contribute to undesirable complexity.

Some router vendors provide you with the option of changing the Ethernet interface network type for OSPF or IS-IS or both from its default broadcast to point-to-point. In addition to making the overall network simple and hence more manageable, configuring point-to-point Ethernet links to the OSPF or IS-IS point-to-point network type enables the use of IP unnumbered for address conservation.

It cannot be said better - Jeff Doyle is the best

Best regards,

Peter