LSP size

Load isis.cfg

#IOS-XE
config replace flash:isis.cfg

#IOS-XR
configure
load bootflash:isis.cfg
commit replace
y

Enable wide metrics everywhere. Configure R1 and XR2 so that their LSP is fragmented once is it bigger than 128 bytes.

Answer

#IOS-XE
router isis
 metric-style wide

#IOS-XR
router isis 1
 add ipv4 
  metric-style wide

#R1
router isis
 lsp-mtu 128

#XR2
router isis 1
 lsp-mtu 128

Explanation

By default, an LSP is fragmented at 1500 bytes. We can control this by setting the lsp-mtu. When the router’s own LSP becomes larger than the specified MTU size, it is split into multiple LSPs.

LSP IDs are in the format <Sys ID>.<Circuit ID>-<LSP fragment>. For example, LSP R6.02-00 is R6’s pseudonode LSP on the LAN segment with circuit ID 02. For p2p circuits, the circuit ID is always 00. If a router needs to fragment its LSP, it increments the last portion. For example, R1.00-00, R1.00-01, R1.00-02, etc.

In ISIS, each LSP fragment is treated as its own LSP. It is flooded, has a seq number, etc. A router “combines” all LSP fragments from a single router in order to build the ISIS topology graph.

In our network, we should see two routers that have LSP fragments, R1 and XR2:

The TLVs are simply distributed among the multiple LSP fragments in order to fit within the LSP-mtu size.