Mesh groups
Load isis.cfg
Configure R2 so that LSPs received on Gi2.12 are only forwarded out Gi2.24. LSPs received on Gi2.23 should only be flooded out Gi2.24 as well.
Answer
Explanation
Mesh groups allow you to control ISIS flooding behavior. By default, an LSP received on any interface will be flooded out all other interfaces. This can be an inefficient use of bandwidth in large scale, highly meshed topologies. To better scale ISIS, we can configure interfaces into groups for flooding. If an interface belongs to an ISIS mesh-group, any LSP received on that interface is only flooded out other interfaces that are not in the same mesh group. You also have the option to completely block flooding on an interface using isis mesh-group blocked. No LSPs will be flooded out the interface.
To understand how mesh groups work, let’s first look at the default behavior when R2 receives an LSP from R1 on gi2.12. I shutdown lo0 on R1 to force R1 to update its LSP. We can see that R2 floods this out Gi2.23 and Gi2.24 by using debug isis update-packets. (Keep in mind these screenshots show Gi1 instead of Gi2).
Next, we configure Gi2.12 and Gi2.23 in the same mesh group and no shut the loopback on R1. Notice that now R2 only floods the LSP out Gi2.24. It doesn’t flood the LSP out Gi2.23.
Now let’s shut the loopback on R3 and verify that the LSP is flooded out only Gi2.24. This should be the case, because Gi2.12 and Gi2.23 are in the same mesh group, so an LSP received on either is not flooded out the other.
Notice above that R1 only receives the LSP when it sees that R3.00-00 LSP has changed from R2’s periodic CSNP as the DIS. R1 then explicitly requests the LSP using a PSNP. The hold time of the LSP has decreased 7 seconds in this time period (from 1198 to 1191).
Mesh-groups are dangerous to use, because as you can see above, the only thing allowing R1 to eventually receive R3’s LSP is the periodic CSNP on the LAN. Not only did this severely impact R1’s convergence time, but R1 would never even learn of the new LSP if the network type was point-to-point.
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