4 Replies Latest reply: Sep 12, 2019 6:28 AM by Steven Davidson RSS

    Storing PMTU information


      HI all,

      I see in rfc1191:

      In general, the IP layer should associate each PMTU value that it has
      learned with a specific path. A path is identified by a source
      address, a destination address and an IP type-of-service. (Some
      implementations do not record the source address of paths; this is
      acceptable for single-homed hosts, which have only one possible
      source address.)
      Note: Some paths may be further distinguished by different
      security classifications. The details of such classifications
      are beyond the scope of this memo.
      The obvious place to store this association is as a field in the
      routing table entries. A host will not have a route for every
      possible destination, but it should be able to cache a per-host route
      for every active destination. (This requirement is already imposed
      by the need to process ICMP Redirect messages.)


      But how can I see PMTU of each active path in device.


      Thank you for your help.

        • 1. Re: Storing PMTU information

          Is there any show cli to see these values?

          • 2. Re: Storing PMTU information
            Steven Davidson

            nammon wrote:



            But how can I see PMTU of each active path in device.


            Thank you for your help.

            I don't know about any specific IOS command, off the top of my head, that will show this information but I don't think that this information would be tracked by a routing that's transiting IPv6 traffic between hosts.  This is something that would be stored by the hosts involved in the communication.  For instance, Windows stores PMTUD info:


            C:\Users\redacted>netsh interface ipv6 show destinationcache


            Interface 8: Wi-Fi


            PMTU Destination Address                          Next Hop Address

            ---- --------------------------------------------- -------------------------

            1500 2a03:2880:f027:13:face:b00c:0:2              fe80::4af7:c0ff:fe23:34b2


            I suppose that if you were to configure IPv6 on a router and then SSH to the router using IPv6 you'd likely see something about PMTU in the router for the local connection.

            • 3. Re: Storing PMTU information
              Steven Davidson

              nammon wrote:


              Is there any show cli to see these values?

              R4#show ipv6 ?

                access-list         Summary of access lists

                cef                 Cisco Express Forwarding for IPv6

                cga                 CGA features

                dhcp                IPv6 DHCP

                eigrp               Show IPv6 EIGRP

                features            IPV6 features

                general-prefix      IPv6 general prefixes

                inspect             CBAC (Context Based Access Control) information

                interface           IPv6 interface status and configuration

                lisp                Locator/ID Separation Protocol

                local               IPv6 local options

                mfib                IP multicast forwarding information base

                mld                 Multicast group membership information

                mobile              Mobile IPv6

                mobile              mobile IPv6

                mrib                Multicast Routing Information Base

                mroute              IPv6 multicast routing table

                mtu                 MTU per destination cache

                nat                 IPv6 NAT-PT information

                nd                  Show IPv6 ND related information

                neighbors           Show IPv6 neighbor cache entries

                nhrp                NHRP information

                ospf                OSPF information

                pim                 PIM information

                policy              Policy routing

                port-map            Port to Application Mapping (PAM) information

                prefix-list         List IPv6 prefix lists

                protocols           IPv6 Routing Protocols

                rip                 RIP routing protocol status

                route               Show IPv6 route table entries

                routers             Show local IPv6 routers

                rpf                 Multicast RPF information

                rsvp                RSVP show commands

                spd                 IPv6 Selective Packet Discard

                static              IPv6 static routes

                traffic             IPv6 traffic statistics

                virtual-reassembly  IPV6 Virtual Fragment Reassembly (VFR) information

                wccp                WCCP IPv6 information