Hi, I have a setup with two tables: * The GRT for all the traffic and the fullview * Another table with specific routes for GRE tunnels and OOB-ish fallback I’m not using VRFs, just `ip rules`. I’m running an IGP inside the GRE tunnels (with bird). It seems to me that Linux isn’t taking the source IP of the tunnels when looking to the MTU to use, and that the pmtudisc is triggered each time the route in the GRT is updated. (For reasons), I have the endpoint route for some tunnels in both the GRT and the specific table, as this bird output shows: bird> show route all primary 2001:4640:a14f::2/128 table all Table master6: 2001:4640:a14f::2/128 unicast [ospf_ipv6 13:45:17.496] E2 (150/26/10000) [45.91.126.96] via fe80::18f3:7bff:fe0b:ce21 on gre6 Type: OSPF-E2 univ igp_metric: 26 OSPF.metric1: 26 OSPF.metric2: 10000 OSPF.tag: 0x00000000 OSPF.router_id: 45.91.126.96 Table t6_as62240: 2001:4640:a14f::2/128 unicast [static_tinc_endpoints_ipv6 2020-12-25] * (200) via 2a0f:9440:1:49::1 on enp1s0 Type: static univ bird> The route is well pushed on all the tables: edge08-clouvider ~ # ip -s -d r g 2001:4640:a14f::2 unicast 2001:4640:a14f::2 from :: via fe80::200:5efe:b975:5804 dev gre3 table main proto bird scope global src 2a0e:f42:fffe:1::20 metric 32 users 2 age 211sec pref medium edge08-clouvider ~ # ip -s -d r g 2001:4640:a14f::2 from 2a0f:9440:1:49::2 unicast 2001:4640:a14f::2 from 2a0f:9440:1:49::2 via 2a0f:9440:1:49::1 dev enp1s0 table 62240 proto bird scope global src 2a0f:9440:1:49::2 metric 32 users 2 age 5446752sec pref medium However, a tunnel ending on 2001:4640:a14f::2 will have it’s MTU changed if the first route is updated even if the last one is actually used, e.g.: edge08-clouvider ~ # ip l sh gre6 13: gre6@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1424 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000 link/gre6 2a0f:9440:1:49::2 peer 2001:4640:a14f::2 alias Core: core02 Thus, causing the OSPF neighbor (at the other side of the tunnel) being stuck in ExStart state. My rules are very simple, I’m just looking at the source address to choose the table to use: edge08-clouvider ~ # ip -6 ru sh 0: from all lookup local 32765: from 2a0f:9440:1:49::/126 lookup 62240 32766: from all lookup main I want all the traffic to use the GRT, except when it’s sourced from an IP on the physical interface used for GRE tunnels. As the route from the table 62240 is very stable (age 5446752sec), the pmtudisc shouldn’t be triggered if the route from the GRT is updated. Putting `ip link set gre6 mtu 1448` in a cron is sufficient to work around the issue, but it’s very dirty. -- Alarig
