Connect two Backbone area via IS-IS

[Tariqemad]

Hi all ,, I have (4) Edge router .. The Network routing between ( R1 --> R3  ) OSPF on Backbone Area (i.e Area 0 ) and the Network routing between ( R4 --> R2  ) OSPF on Backbone Area (i.e Area 0 ) , Also the Network routing between ( R1 --> R2  ) IS-IS ..  My goal to connect the ( Area 0 for R3  with Area 0 for R2 ) through IS-IS Network .. Is this possible .. and if yes how we can do this .. 

below in the attachment you will fine my idea topology ,, Thanks in advance 

[Moaz.Elzhrawey]

Hi,

Good question,

Unfortunately, you can't.

But, there is a workaround to have the partitioned Area 0 get connected, which by doing the following: 

1- Create Interface Loopback on R1 and R2, then advertise them in OSPF

2- Enable OSPF to IS-IS and IS-IS redistribution (Mutual redistribution) on R1 and R2.

By doing so, you would have Loopback reachability on both R1 and R2

3- Create GRE tunnel interface on both R1 and R2 using those loopbacks created in step 1 as a source-interface.

4- Assign those tunnel interfaces to OSPF area 0.

So, finally you will have full OSPF running on top of IS-IS which will be considered as an Underlay (just a carrier) and the OSPF area 0 will be the Overlay.

*** Please rate this as "Answer" if it solved your problem.***

[Tariqemad]

Thanks for your replay ,, but i will face problems here 

1- the MTU issue ( the GRE accept not more 1500 )

2- heavy traffic ( the traffic may be exceeded 100G )

3- If i have connection between  ( R3 & R4 ) on Area 0 ( here i will face problem between the interior and exterior routing ) So all the traffic will go through the interior routing  because i will have redistribute from the ISIS side  ,, Is i'm Right .. 

So , Any advice to solve these problems .. Thanks in advance ..

[Moaz.Elzhrawey]

Hi,

1- MTU wouldn't your biggest problem, becz the overhead is affordable unless you having a critical apps which considering the fragmentation is the first enemy.

2- if you have such amount of traffic, this means you're not running ISR routers, ASR at minimum, if so, drop the GRE tunnel option and go with VXLAN.

3- to deal with the issue of using interior or redist routes, you can use filtering easily and rely only on one of routes either the local or redist.

[Moaz.Elzhrawey]

Hi,

Regarding choosing the IS-IS routes over the redist routers, so you can use the following commands to change the AD of the redist routes to whatever you need to prefer IS-IS over External, or the opoosit, 

Router(config)#router ospf 1	Starts the OSPF routing process
Router(config-router)#distance ospf intra-area 105 inter-area 105 external 125	Changes the AD to 105 for intra-area and interarea routes, and changes the AD to 125 for external routes

[Tariqemad]

Many Thanks ,,, I will test it ,,, :)