topic Re: ISIS SPBM MTU calculation in ExtremeSwitching (VSP/Fabric Engine)

ISIS SPBM MTU calculation

Fijs — Tue, 18 Aug 2020 18:37:59 GMT

Hi,

The docs / guides specify that in order to form an ISIS adjacency, an MTU of at least 1594 bytes is required.

Why 1594? What is missing in the below calculation?

Original Ethernet Frame:
Destination MAC address = 6 bytes

Source MAC address = 6 bytes

802.1Q header = 4 bytes (0x8100 + 802.1p field (3 bits) + CFI (1 bit) + VLAN-id (12 bits)

Ether Type (or length) = 2 bytes (e.g. 0x0800 for IP, 0x0806 for ARP)

default maximum payload of Ethernet Frame = 1500 bytes

Cyclic Redundancy Check (CRC) = 4 bytes

This Ethernet frame of 1522 bytes is then encapsulated with 22 extra bytes:

802.1ah Encapsulation:
Backbone Destination MAC address = 6 bytes

Backbone Source MAC address = 6 bytes

Backbone Ether Type = 2 bytes (0x88A8)

Backbone VLAN-id = 2 bytes

Service Type = 2 bytes (0x88E7)

Service Flags = 1 byte

Service Identifier = 3 bytes

This gives a frame size of 1544 bytes.

Re: ISIS SPBM MTU calculation

Thomas_Gfeller — Tue, 18 Aug 2020 19:18:16 GMT

Hi Fijs

I would say the remaining 50 bytes are for Fabric Extend (VXLAN) encapsualtion. If you than encaspsulate this into IPsec, there will be another 150 Bytes added, so you will reach a max. MTU of 1671 Bytes.

Regards

Thomas

Re: ISIS SPBM MTU calculation

Fijs — Tue, 18 Aug 2020 22:32:46 GMT

Thanks Thomas, this makes sense!

Re: ISIS SPBM MTU calculation

Ludovico_Steven — Thu, 20 Aug 2020 19:31:13 GMT

These are my notes..

SPB MTUs
========
MAC-in-MAC adds 22 bytes to packets; so Ethernet maximum frame size can grow worst case from 1522 (assuming worst case q-tagged packet) to 1544
64->86 to 1522->1544

Fabric Extend VXLAN encap adds 14 (18 if WAN interface q-tagged) bytes for MAC&etype + 20 bytes (IP header) + 8 bytes (UDP header) + 8 bytes (VXLAN header) = + 50 (54 if WAN interface q-tagged) bytes
86->136 to 1544->1594 on untagged connection to WAN
86->140 to 1544->1598 on q-tagged connection to WAN

Re: ISIS SPBM MTU calculation

Miguel-Angel_RO — Thu, 27 Aug 2020 04:23:07 GMT

Fijs,

It is depending of what/where do you tag/untag the packets.

Here the output of an excel when I was on a project to validate the MTU’s and throughput of the Fabric:

Mig

Re: ISIS SPBM MTU calculation

jerrygen — Wed, 11 Mar 2026 10:28:39 GMT

Hi,

The 1594-byte requirement comes from the IS-IS SPBM control packets, not just the Ethernet frame you calculated. Your math only covers the outer Ethernet + 802.1ah headers around a normal 1500-byte payload, which indeed lands around 1544 bytes. However, IS-IS uses CLNS (Connectionless Network Service) packets carried directly over Layer-2, and those packets include additional headers like the LLC header (802.2) and the IS-IS PDU fields themselves. When those protocol headers are added, the effective frame size needed on the wire increases beyond the simple Ethernet encapsulation you listed. Get it here: https://calculateurheure.fr/

In SPBM deployments vendors typically recommend 1594 bytes as a safe MTU so IS-IS LSPs and other control PDUs can be exchanged without fragmentation. The extra ~50 bytes account for the LLC header, CLNS/IS-IS protocol overhead, and implementation safety margin used by many platforms. So your 1544-byte calculation is correct for the encapsulation portion, but it doesn’t include the IS-IS control protocol overhead, which is why the documented minimum MTU is higher.

Regards