maximum ip routes that can be handled by isis and ospf

[shreerampardhy]

Hello All,

Is there any limitation on the number of ip routes that isis can handle? I was going through one of the documentations which mentioned that the limit is 30K routes. This is due to the max number of fragments ( 256 ) with each containing not more than 121 routes.?

Do the following commands  help to change this number?

lsp-mtu

and clns-mtu

Has anyone seen any such limitation for OSPF ?

Regards,

Shreeram

[Muhammad Thanveer]

Hello Shreeram,

Hmm Qestion is quit logical, But what can I say is, believe me OSPF can handle lot of routes. It completly depends upon your Routers Processor and Memory.

In no I can say more than 6000 routes (reference https://supportforums.cisco.com/thread/10496)

For OSPF not to suffer, we must follow some rules when we make areas, it is because it is a linkstate algorithm, below are the same

for more information you can browse here

http://www.ciscopress.com/articles/article.asp?p=1763921&seqNum=6

Factors Influencing OSPF Scalability

Scaling is determined by the utilization of three router resources: memory, CPU, and interface bandwidth. The workload that OSPF imposes on a router depends on these factors:

• Number of adjacent neighbors for any one router: OSPF floods all link-state changes to all routers in an area. Routers with many neighbors have the most work to do when link-state changes occur. In general, any one router should have no more than 60 neighbors.
  
• Number of adjacent routers in an area: OSPF uses a CPU-intensive algorithm. The number of calculations that must be performed given n link-state packets is proportional to n log n. As a result, the larger and more unstable the area, the greater the likelihood for performance problems associated with routing protocol recalculation. Generally, an area should have no more than 50 routers. Areas that suffer with unstable links should be smaller.
  
• Number of areas supported by any one router: A router must run the link-state algorithm for each link-state change that occurs for every area in which the router resides. Every ABR is in at least two areas (the backbone and one adjacent area). In general, to maximize stability, one router should not be in more than three areas.
  
• Designated router (DR) selection: In general, the DR and backup designated router (BDR) on a multiaccess link (for example, Ethernet) have the most OSPF work to do. It is a good idea to select routers that are not already heavily loaded with CPU-intensive activities to be the DR and BDR. In addition, it is generally not a good idea to select the same router to be the DR on many multiaccess links simultaneously.
  

Please rate the helpful posts

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

[Muhammad Thanveer]

Hello Shreeram,

In case of ISIS The LSP MTU size describes the amount of information that can be recorded in a single link state packet

I beleive it has nothing to do with your database, it is used for manipulating the MTU size

lsp-mtu size is used to configure the maximum number of bytes allowed in an LSP. If this command is not configured, ISIS uses the default size of 1500 bytes while transmitting LSPs for both POS and Gigabit Ethernet.

Under normal conditions, the default MTU size should be sufficient. However, if the MTU of a link is below 1500 bytes, the link-state packet MTU must be lowered accordingly on each router in the network. If this is not done, routing becomes unpredictable

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

[shreerampardhy]

Hello Thanveer,

Thank you very much for the reply..

I am pretty sure that ospf can handle much more than 6000 routes.. i have seen tier 2 ISPs having more than 15K - 20K core devices itself.

For the ISIS maximum limits i was checking on the below link

http://books.google.com.au/books?id=u_XOQy4cBkgC&q=The+actual+calculation+is+as+follows#v=onepage&q&f=false

i am not sure if this limit has been changed.

Regards,

Shreeram

[Muhammad Thanveer]

Hello Shreeram,

I found this

There is no default value for the maximum argument, the threshold value defaults to 75 percent.

Please look in here http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/fsiredis.html#wp1044717

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

[Giuseppe Larosa]

Hello Shreeram,

>> i have seen tier 2 ISPs having more than 15K - 20K core devices itself.

Using a single IGP domain on 15000 routers ?

There are some poorly designed SP networks with a lot of external routes in OSPF DB, but this creates unique challenges to the implementation  of hardware based forwarding systems as they are not optimized for so many OSPF routes.

I would say less then 10,000 routes to be safe with minimal external routes and using iBGP for all services and only the IGP for the infrastructure ( PE loopbacks).

In any case even 50000 routes are far less then the 420000 routes of a full internet table.

Hope to help

Giuseppe

[Muhammad Thanveer]

Hello Giuseppe Larosa,

Thanks for your reply.

"I would say less then 10,000 routes to be safe with minimal external routes and using iBGP for all services and only the IGP for the infrastructure ( PE loopbacks).
In any case even 50000 routes are far less then the 420000 routes of a full internet table"

"less than 10000 routes"   is it in the case of both ospf and isis or for any IGP (Assuming a large enterprise with EIGRP)

"only the IGP for the infrastructure ( PE loopbacks)." -- I am poor in english can you please elaborate this? I would appreciate if you consider the above comment to elaborate.

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

[Alessio Andreoli]

Hi T.,
IGP like OSPF IS-IS and EIGRP can manage many routes and in theory , as our Jeff Doyle proves, the entire Internet. The problem remains the design. In the current real world scenario it is just not feasible to let an IGP managing 15k routes and less likely 15k routers. Generally, big companies like DELL and I believe AT&T implement BGP as internal routing protocol tuning the convergence.


Back to your question,
One of the IGP (lsa based) could do the job.


Hope this helps
Alessio

Sent from Cisco Technical Support iPad App

[Muhammad Thanveer]

Thanks Alessio,

It was helpful.

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

[shreerampardhy]

Hello ALl,

Thank you very much for all the inputs.  Just while we are on the topic, for OSPF, there is theorotical limit for the router and network LSA. Didnt find any other limitations at the protocol level ( apart from the obvious limitations of the devices / links ).

Since the type 1 and type 2 LSAs have the length field of 2 bytes, they cannot be more than 65535 bytes. Converting this to the LSAs .. we cannot have more than 5000 type 1 / 2 Lsa generated by any given device. In other words 5000 prefixes on a single box. I dont think this figure will be ever achieved though.The above calculations are true when we have the theorotiacal IP MTU of 65K.

Having said this, the max interface MTU that i have seen is 9k. The pratical limitations would come down below 900 prefixes.

For ISIS the limit is of the LSP size is 1492 bytes. This is due to the originatingLxLSPBufferSize. Converting this to prefixes, it can support a max of 30K prefixes. This is when we dont have any other TLV ( TE, CLNS IPV6 etc) apart from the IPv4 TLV.

There are rfcs which recommend ways to extend this limit. ( RFC5311 , RFC3786)

Any further information is always welcome

[Muhammad Thanveer]

good info

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

[Giuseppe Larosa]

Hello Shreeram,

you have made interesting considerations, however I think the real limits come from other factors looking at the whole picture of the IGP routing domain.

Even if a single OSPF node would be limited to 900 routes per area the overall limit per domain can be greater as other colleagues have mentioned in this thread.

Modern designs combine IGP and MP iBGP so the maximum number of routes in the IGP is not so important as it does not create a blocking condition for network growth

Hope to help

Giuseppe

[Giuseppe Larosa]

Hello Alessio,

>> IGP like OSPF IS-IS and EIGRP can manage many routes and in theory , as our Jeff Doyle proves, the entire Internet.

I modestly disagree IGPs are not thought to be used with 420000 or more routes. I can agree that modern cpus have much more resources then the cpu used when the IGP protocols have been introduced, so the rule of thumb of staying under 10000 routes is actually not a strict limit and you can go futher but not so far.

I have been informed of an italian service provider that has 50000 external routes in their backbone OSPF process.

Do you work for them ?

Hope to help

Giuseppe

[Giuseppe Larosa]

Hello Thanveer,

I'm sorry for my late answer.

Less then 10000 routes is just a reasonable threshold.for an IGP, I would say if the number of IGP routes is bigger you should consider to move service related prefixes to iBGP that is to review your design.

>> only the IGP for the infrastructure ( PE loopbacks)." --

The IGP should be used only for the infrastructure in a modern SP network with all service related IP prefixes advertised in MP iBGP sessions. The IGP should only provide enough information to allow BGP sessions to be setup.

we need also to consider that in the MPLS environment we need the route detail of remote PE loopbacks we cannot use route aggregation of remote PE loopbacks otherwise the check of the IGP for LDP label binding validation will fail and no end to end LSP can be created.

However,

this design allows for great scalability and is commonly used by SP.

Also some big enterprise have moved to the MPLS MP BGP routing model to provide MPLS services to different user groups like the use of L3 VPNs. Multicast based VPN (draft Rosen with mGRE for forwarding)

EIGRP might be used as IGP if no MPLS TE is used, but the trend is usually to move to OSPF or ISIS when MPLS is introduced in order to be ready to MPLS TE in the future.

Hope to help

Giuseppe

[Muhammad Thanveer]

Thanks Giuseppe Larosa

Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."