How does a router know the MTU of a network?
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
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
asked Jan 7 at 21:44
Josemi Josemi
402
add a comment |
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
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
asked Jan 7 at 21:44
Josemi Josemi
402
add a comment |
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
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
asked Jan 7 at 21:44
Josemi Josemi
402
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
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
asked Jan 7 at 21:44
Josemi Josemi
402
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
asked Jan 7 at 21:44
Josemi Josemi
402
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
edited Jan 7 at 22:26
Ron Maupin♦
67.9k1369126
67.9k1369126
asked Jan 7 at 21:44
Josemi Josemi
402
asked Jan 7 at 21:44
Josemi Josemi
402
asked Jan 7 at 21:44
Josemi Josemi
402
402
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
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.
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
add a comment |
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.
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
– Ron Maupin♦
Jan 7 at 21:52
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 ofinterface MTU.
– Josemi
Jan 7 at 22:06
@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♦
Jan 7 at 23:35
add a comment |
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2 Answers
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2 Answers
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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.
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
add a comment |
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.
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
add a comment |
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.
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
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.
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
answered Jan 7 at 21:51
Ron Maupin♦Ron Maupin
67.9k1369126
67.9k1369126
add a comment |
add a comment |
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.
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
– Ron Maupin♦
Jan 7 at 21:52
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 ofinterface MTU.
– Josemi
Jan 7 at 22:06
@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♦
Jan 7 at 23:35
add a comment |
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.
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
– Ron Maupin♦
Jan 7 at 21:52
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 ofinterface MTU.
– Josemi
Jan 7 at 22:06
@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♦
Jan 7 at 23:35
add a comment |
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.
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
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.
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
answered Jan 7 at 21:51
Ron TrunkRon Trunk
39.4k33780
39.4k33780
I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
– Ron Maupin♦
Jan 7 at 21:52
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 ofinterface MTU.
– Josemi
Jan 7 at 22:06
@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♦
Jan 7 at 23:35
add a comment |
I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
– Ron Maupin♦
Jan 7 at 21:52
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 ofinterface MTU.
– Josemi
Jan 7 at 22:06
@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♦
Jan 7 at 23:35
I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
– Ron Maupin♦
Jan 7 at 21:52
I think the OSPF confusion is that the MTU must match for OSPF to form a neighbor.
– Ron Maupin♦
Jan 7 at 21:52
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
Jan 7 at 22:06
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
Jan 7 at 22:06
@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♦
Jan 7 at 23:35
@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♦
Jan 7 at 23:35
add a comment |
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