Tunnel MTU size of R5.2.1 is 4 bytes smaller than R3.6.2

Summary

After upgrading SZ to R5.2.1, tunnel MTU size became smaller than R3.6.2.

Question

Is it a software defect that tunnel MTU size was smaller than previous release?

Customer Environment

R5.2.1 SZ-100

Symptoms

The customer is seeing difference of tunnel MTU size on AP between R3.6.2 and R5.1.2/R5.2.1 as below:

--- R3.6.2
gre1 Link encap:Ethernet HWaddr 36:A1:18:2E:31:70
UP BROADCAST RUNNING NOARP MULTICAST MTU:1442 Metric:1

--- R5.1.2, R5.2.1
gre1 Link encap:Ethernet HWaddr 96:C1:7E:80:A0:A1
UP BROADCAST RUNNING NOARP MULTICAST MTU:1438 Metric:1
---

Resolution

Tunnel MTU size calculation was changed after R5.1.1.
The following is tunnel MTU size when the interface MTU size is 1500.

Tunnel type (IPv4)	R5.1 – GRE MTU	R.5.1.1 – GRE MTU
eth_ipv4_rksgre	1450	1446
eth_ipv4_rksgre + Encryption	1440	1440
eth_ipv4_udp_rksgre	1442	1438
eth_ipv4_udp_rksgre +Encryption	1440	1424

AP side has one more check when sending MTU size to DP.
AP has a check and maximum size will be 1440, so actual tunnel MTU size will be 1440 even if AP CLI output shows 1446 for eth_ipv4/rksgre.
---
rkscli: get mtu gre1
gre1 MTU: 1446
Ok
---

Tunnel type (IPv4)	R5.1 – GRE MTU	R.5.1.1 – GRE MTU
eth_ipv4_rksgre	1450	1440
eth_ipv4_rksgre + Encryption	1440	1440
eth_ipv4_udp_rksgre	1442	1438
eth_ipv4_udp_rksgre +Encryption	1440	1424

Article Number:
000011560

Updated:
April 12, 2021 07:53 PM (over 3 years ago)

Tags:
Configuration, SmartCell Gateway

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