LSP size

• Task

Enable wide metrics everywhere. Configure XR1 and XE8 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

XR1
router isis
 lsp-mtu 128

XE8
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, XR1 and XE8:

The TLVs are simply distributed among the multiple LSP fragments in order to fit within the LSP-mtu size. Some information in the example below a split into two the first goes in XR1.00-00 and the second half goes in XR1.00-01. If the MTU is big enough, the information will be displayed in a single LSP only.