How does a router know the MTU of a network?












3














I've done several exercises where you're given the MTU of a net or subnet, but I wonder how routers know the MTU of the nets they're connected. I've learnt that in OSPF protocol this information is excanged among routers. However, is there another way of finding that information for a router that doesn't use OSPF?



I hope someone can help me. Thank you for your responses.






routing router layer1 interface mtu







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    3














    I've done several exercises where you're given the MTU of a net or subnet, but I wonder how routers know the MTU of the nets they're connected. I've learnt that in OSPF protocol this information is excanged among routers. However, is there another way of finding that information for a router that doesn't use OSPF?



    I hope someone can help me. Thank you for your responses.






    routing router layer1 interface mtu







    share|improve this question



























      3












      3








      3







      I've done several exercises where you're given the MTU of a net or subnet, but I wonder how routers know the MTU of the nets they're connected. I've learnt that in OSPF protocol this information is excanged among routers. However, is there another way of finding that information for a router that doesn't use OSPF?



      I hope someone can help me. Thank you for your responses.






      routing router layer1 interface mtu







      share|improve this question















      I've done several exercises where you're given the MTU of a net or subnet, but I wonder how routers know the MTU of the nets they're connected. I've learnt that in OSPF protocol this information is excanged among routers. However, is there another way of finding that information for a router that doesn't use OSPF?



      I hope someone can help me. Thank you for your responses.






      routing router layer1 interface mtu




      routing router layer1 interface mtu






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 5 hours ago









      Ron Maupin♦

      62.8k1365120




      62.8k1365120










      asked 5 hours ago









      Josemi Josemi

      302




      302






















          2 Answers
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          2














          The MTU of an interface is a function of the layer-2 protocol for that interface, and a device, including a router, will inherently know the MTU of the interface based on the protocol for the interface. For example the MTU for ethernet is 1500.



          Some routers can configure the interface MTU with a configuration command. Also virtual interfaces, e.g. a tunnel, will calculate the MTU based on the MTU of the physical interface that is the source of the tunnel, minus the tunnel protocol overhead.




          For Cisco routers, you can use the show interfaces command to see the MTU for one or more interfaces. Other vendors will have similar commands.






          share|improve this answer





























            3














            Routers know because they have physical interfaces connected to the networks. Those interfaces are configured for the correct MTU.



            You are mistaken about OSPF: The protocol does not exchange MTU information.






            share|improve this answer





















            • I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
              – Ron Maupin♦
              5 hours ago










            • I don't think so, if you consult the rfc of OSPF, version 2: rfc-editor.org/pdfrfc/rfc2328.txt.pdf , in page 195 where the information database packet is described, there is a field of interface MTU.
              – Josemi
              5 hours ago










            • @Josemi, from Why Are OSPF Neighbors Stuck in Exstart/Exchange State?: "OSPF states for adjacency formation are Down, Init, Attempt, 2-way, Exstart, Exchange, Loading and Full. There can be number of reasons why the Open Shortest Path First (OSPF) neighbors are stuck in exstart/exchange state. This document focuses on an MTU mismatch between OSPF neighbors resulting in exstart/exchange state. For more details on troubleshooting OSPF refer to Troubleshooting OSPF."
              – Ron Maupin♦
              3 hours ago











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            2 Answers
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            active

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            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

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            2














            The MTU of an interface is a function of the layer-2 protocol for that interface, and a device, including a router, will inherently know the MTU of the interface based on the protocol for the interface. For example the MTU for ethernet is 1500.



            Some routers can configure the interface MTU with a configuration command. Also virtual interfaces, e.g. a tunnel, will calculate the MTU based on the MTU of the physical interface that is the source of the tunnel, minus the tunnel protocol overhead.




            For Cisco routers, you can use the show interfaces command to see the MTU for one or more interfaces. Other vendors will have similar commands.






            share|improve this answer


























              2














              The MTU of an interface is a function of the layer-2 protocol for that interface, and a device, including a router, will inherently know the MTU of the interface based on the protocol for the interface. For example the MTU for ethernet is 1500.



              Some routers can configure the interface MTU with a configuration command. Also virtual interfaces, e.g. a tunnel, will calculate the MTU based on the MTU of the physical interface that is the source of the tunnel, minus the tunnel protocol overhead.




              For Cisco routers, you can use the show interfaces command to see the MTU for one or more interfaces. Other vendors will have similar commands.






              share|improve this answer
























                2












                2








                2






                The MTU of an interface is a function of the layer-2 protocol for that interface, and a device, including a router, will inherently know the MTU of the interface based on the protocol for the interface. For example the MTU for ethernet is 1500.



                Some routers can configure the interface MTU with a configuration command. Also virtual interfaces, e.g. a tunnel, will calculate the MTU based on the MTU of the physical interface that is the source of the tunnel, minus the tunnel protocol overhead.




                For Cisco routers, you can use the show interfaces command to see the MTU for one or more interfaces. Other vendors will have similar commands.






                share|improve this answer












                The MTU of an interface is a function of the layer-2 protocol for that interface, and a device, including a router, will inherently know the MTU of the interface based on the protocol for the interface. For example the MTU for ethernet is 1500.



                Some routers can configure the interface MTU with a configuration command. Also virtual interfaces, e.g. a tunnel, will calculate the MTU based on the MTU of the physical interface that is the source of the tunnel, minus the tunnel protocol overhead.




                For Cisco routers, you can use the show interfaces command to see the MTU for one or more interfaces. Other vendors will have similar commands.







                share|improve this answer












                share|improve this answer



                share|improve this answer










                answered 5 hours ago









                Ron Maupin♦Ron Maupin

                62.8k1365120




                62.8k1365120























                    3














                    Routers know because they have physical interfaces connected to the networks. Those interfaces are configured for the correct MTU.



                    You are mistaken about OSPF: The protocol does not exchange MTU information.






                    share|improve this answer





















                    • I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
                      – Ron Maupin♦
                      5 hours ago










                    • I don't think so, if you consult the rfc of OSPF, version 2: rfc-editor.org/pdfrfc/rfc2328.txt.pdf , in page 195 where the information database packet is described, there is a field of interface MTU.
                      – Josemi
                      5 hours ago










                    • @Josemi, from Why Are OSPF Neighbors Stuck in Exstart/Exchange State?: "OSPF states for adjacency formation are Down, Init, Attempt, 2-way, Exstart, Exchange, Loading and Full. There can be number of reasons why the Open Shortest Path First (OSPF) neighbors are stuck in exstart/exchange state. This document focuses on an MTU mismatch between OSPF neighbors resulting in exstart/exchange state. For more details on troubleshooting OSPF refer to Troubleshooting OSPF."
                      – Ron Maupin♦
                      3 hours ago
















                    3














                    Routers know because they have physical interfaces connected to the networks. Those interfaces are configured for the correct MTU.



                    You are mistaken about OSPF: The protocol does not exchange MTU information.






                    share|improve this answer





















                    • I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
                      – Ron Maupin♦
                      5 hours ago










                    • I don't think so, if you consult the rfc of OSPF, version 2: rfc-editor.org/pdfrfc/rfc2328.txt.pdf , in page 195 where the information database packet is described, there is a field of interface MTU.
                      – Josemi
                      5 hours ago










                    • @Josemi, from Why Are OSPF Neighbors Stuck in Exstart/Exchange State?: "OSPF states for adjacency formation are Down, Init, Attempt, 2-way, Exstart, Exchange, Loading and Full. There can be number of reasons why the Open Shortest Path First (OSPF) neighbors are stuck in exstart/exchange state. This document focuses on an MTU mismatch between OSPF neighbors resulting in exstart/exchange state. For more details on troubleshooting OSPF refer to Troubleshooting OSPF."
                      – Ron Maupin♦
                      3 hours ago














                    3












                    3








                    3






                    Routers know because they have physical interfaces connected to the networks. Those interfaces are configured for the correct MTU.



                    You are mistaken about OSPF: The protocol does not exchange MTU information.






                    share|improve this answer












                    Routers know because they have physical interfaces connected to the networks. Those interfaces are configured for the correct MTU.



                    You are mistaken about OSPF: The protocol does not exchange MTU information.







                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered 5 hours ago









                    Ron TrunkRon Trunk

                    34.9k23272




                    34.9k23272












                    • I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
                      – Ron Maupin♦
                      5 hours ago










                    • I don't think so, if you consult the rfc of OSPF, version 2: rfc-editor.org/pdfrfc/rfc2328.txt.pdf , in page 195 where the information database packet is described, there is a field of interface MTU.
                      – Josemi
                      5 hours ago










                    • @Josemi, from Why Are OSPF Neighbors Stuck in Exstart/Exchange State?: "OSPF states for adjacency formation are Down, Init, Attempt, 2-way, Exstart, Exchange, Loading and Full. There can be number of reasons why the Open Shortest Path First (OSPF) neighbors are stuck in exstart/exchange state. This document focuses on an MTU mismatch between OSPF neighbors resulting in exstart/exchange state. For more details on troubleshooting OSPF refer to Troubleshooting OSPF."
                      – Ron Maupin♦
                      3 hours ago


















                    • I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
                      – Ron Maupin♦
                      5 hours ago










                    • I don't think so, if you consult the rfc of OSPF, version 2: rfc-editor.org/pdfrfc/rfc2328.txt.pdf , in page 195 where the information database packet is described, there is a field of interface MTU.
                      – Josemi
                      5 hours ago










                    • @Josemi, from Why Are OSPF Neighbors Stuck in Exstart/Exchange State?: "OSPF states for adjacency formation are Down, Init, Attempt, 2-way, Exstart, Exchange, Loading and Full. There can be number of reasons why the Open Shortest Path First (OSPF) neighbors are stuck in exstart/exchange state. This document focuses on an MTU mismatch between OSPF neighbors resulting in exstart/exchange state. For more details on troubleshooting OSPF refer to Troubleshooting OSPF."
                      – Ron Maupin♦
                      3 hours ago
















                    I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
                    – Ron Maupin♦
                    5 hours ago




                    I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
                    – Ron Maupin♦
                    5 hours ago












                    I don't think so, if you consult the rfc of OSPF, version 2: rfc-editor.org/pdfrfc/rfc2328.txt.pdf , in page 195 where the information database packet is described, there is a field of interface MTU.
                    – Josemi
                    5 hours ago




                    I don't think so, if you consult the rfc of OSPF, version 2: rfc-editor.org/pdfrfc/rfc2328.txt.pdf , in page 195 where the information database packet is described, there is a field of interface MTU.
                    – Josemi
                    5 hours ago












                    @Josemi, from Why Are OSPF Neighbors Stuck in Exstart/Exchange State?: "OSPF states for adjacency formation are Down, Init, Attempt, 2-way, Exstart, Exchange, Loading and Full. There can be number of reasons why the Open Shortest Path First (OSPF) neighbors are stuck in exstart/exchange state. This document focuses on an MTU mismatch between OSPF neighbors resulting in exstart/exchange state. For more details on troubleshooting OSPF refer to Troubleshooting OSPF."
                    – Ron Maupin♦
                    3 hours ago




                    @Josemi, from Why Are OSPF Neighbors Stuck in Exstart/Exchange State?: "OSPF states for adjacency formation are Down, Init, Attempt, 2-way, Exstart, Exchange, Loading and Full. There can be number of reasons why the Open Shortest Path First (OSPF) neighbors are stuck in exstart/exchange state. This document focuses on an MTU mismatch between OSPF neighbors resulting in exstart/exchange state. For more details on troubleshooting OSPF refer to Troubleshooting OSPF."
                    – Ron Maupin♦
                    3 hours ago


















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