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Do routers change MAC address when forwarding
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Lets say host A wants to send a packet to host B through R1(interface 1 then 2) and R2 (interface 3), so my questions is:
When the packet is forwarded inside R1 (from interface 1 to interface 2), ready to be sent to R2, does the pack's source MAC(A's MAC address) and destination MAC(interface 1's MAC address) get changed to interface 2's MAC address as new source address and interface 3(of R2) as new destination MAC address? If they do get changed, why router need to change source and destination MAC address?
router mac-address
edited 4 hours ago
Zac67
19.7k21047
asked 4 hours ago
amjad
634
add a comment |Â
up vote
1
down vote
favorite
Lets say host A wants to send a packet to host B through R1(interface 1 then 2) and R2 (interface 3), so my questions is:
When the packet is forwarded inside R1 (from interface 1 to interface 2), ready to be sent to R2, does the pack's source MAC(A's MAC address) and destination MAC(interface 1's MAC address) get changed to interface 2's MAC address as new source address and interface 3(of R2) as new destination MAC address? If they do get changed, why router need to change source and destination MAC address?
router mac-address
edited 4 hours ago
Zac67
19.7k21047
asked 4 hours ago
amjad
634
add a comment |Â
up vote
1
down vote
favorite
up vote
1
down vote
favorite
Lets say host A wants to send a packet to host B through R1(interface 1 then 2) and R2 (interface 3), so my questions is:
When the packet is forwarded inside R1 (from interface 1 to interface 2), ready to be sent to R2, does the pack's source MAC(A's MAC address) and destination MAC(interface 1's MAC address) get changed to interface 2's MAC address as new source address and interface 3(of R2) as new destination MAC address? If they do get changed, why router need to change source and destination MAC address?
router mac-address
edited 4 hours ago
Zac67
19.7k21047
asked 4 hours ago
amjad
634
Lets say host A wants to send a packet to host B through R1(interface 1 then 2) and R2 (interface 3), so my questions is:
When the packet is forwarded inside R1 (from interface 1 to interface 2), ready to be sent to R2, does the pack's source MAC(A's MAC address) and destination MAC(interface 1's MAC address) get changed to interface 2's MAC address as new source address and interface 3(of R2) as new destination MAC address? If they do get changed, why router need to change source and destination MAC address?
router mac-address
router mac-address
edited 4 hours ago
Zac67
19.7k21047
asked 4 hours ago
amjad
634
edited 4 hours ago
Zac67
19.7k21047
asked 4 hours ago
amjad
634
edited 4 hours ago
Zac67
19.7k21047
edited 4 hours ago
Zac67
19.7k21047
edited 4 hours ago
Zac67
19.7k21047
19.7k21047
asked 4 hours ago
amjad
634
asked 4 hours ago
amjad
634
asked 4 hours ago
amjad
634
634
add a comment |Â
add a comment |Â
3 Answers
3
active
oldest
votes
up vote
3
down vote
When a frame comes into a router, the router strips off and discards the frame, losing any layer-2 addressing, including MAC addresses. The router will build a new frame for the next interface.
Not all layer-2 protocols use MAC addresses, and of those that do, some are 48-bit MAC addresses, and some are 64-bit MAC addresses. It is the IEEE LAN protocols that use MAC addressing, but, for example, frame relay uses DLCI number, or ATM uses VPI/VCI. Point-to-point protocols may not use any addressing because there is only one other device on the link.
If the next router interface also uses MAC addresses, the router will build a new frame with the MAC address of its interface as the source address, and the MAC address of the destination on the link as the destination MAC address. If the next interface doesn't use MAC addressing, the router builds a frame for the protocol on the interface.
answered 4 hours ago
Ron Maupin♦
56.4k94795
add a comment |Â
up vote
2
down vote
isn't a router only deal with IP address
A router with Ethernet (or Ethernet-like) interfaces needs to deal with MAC addresses because it needs to send and receive IP packets over those Ethernet interfaces and MAC addresses are a core part of Ethernet.
MAC addresses only have meaning within an Ethernet network.
Logically what happens at R1 is.
- The destination MAC address is checked. For a Unicast MAC address* if it doesn't match the frame is dropped by the network card, if it does match the Ethernet frame is stripped and the packet is passed up to the IP stack. This check is important to prevent packet duplication.
- The IP address is examined and determined to not be one of the local interfaces of the router.
- The IP address is looked up in the routing table and an interface and next-hop IP address is chosen.
- What happens next depends on the nature of the connection between R1 and R2. If the connection is Ethernet then the IP packet will be encapsulated in an Ethernet frame with the router's interface as source MAC and a MAC address looked up using the next-hop IP address as destination MAC address. On the other hand if the link is a serial point to point interface then there may be no MAC addresses involved at all and other protocols may have their own addressing scheme.
* Broadcast and multicast packets need more special handling, but that is outside the scope of this question.
answered 4 hours ago
Peter Green
6,6302922
do those MAC address do get changed?
– amjad
4 hours ago
Will expand my answer, it's not so much a case of "changed" as "re-generated"
– Peter Green
4 hours ago
add a comment |Â
up vote
2
down vote
It's only rewording what others have already said, but the way I usually explain this is with a thought experiment:
- Suppose there's no ethernet, and all of the hosts and routers have a direct point-to-point links (with the same connectivity, ie A to R1)
- Many point to point links have no link addressing: just send the packet in some kind of framing and the other end receives it.
- Then, exactly as you imagine, packets just have fixed IP source and destination address from host A to host B.
- If it's convenient, a given set of links might be replaced by ethernet
- To pass a packet in ethernet, it must have ethernet source and destination addresses in order to get to just the right node on the ethernet
- On ethernet the frame says "I have this for you", where "I" and "you" are defined by the source and destination ethernet addresses
- As the IP packet goes from hop to hop, different routers are saying "I have this for you", so of course the frame would have different source and destination addresses
- If it's not ethernet, you get the appropriate kind of MAC addressing for that medium; 48-bit, 64-bit, none.
answered 4 hours ago
jonathanjo
5,725323
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
3
down vote
When a frame comes into a router, the router strips off and discards the frame, losing any layer-2 addressing, including MAC addresses. The router will build a new frame for the next interface.
Not all layer-2 protocols use MAC addresses, and of those that do, some are 48-bit MAC addresses, and some are 64-bit MAC addresses. It is the IEEE LAN protocols that use MAC addressing, but, for example, frame relay uses DLCI number, or ATM uses VPI/VCI. Point-to-point protocols may not use any addressing because there is only one other device on the link.
If the next router interface also uses MAC addresses, the router will build a new frame with the MAC address of its interface as the source address, and the MAC address of the destination on the link as the destination MAC address. If the next interface doesn't use MAC addressing, the router builds a frame for the protocol on the interface.
answered 4 hours ago
Ron Maupin♦
56.4k94795
add a comment |Â
up vote
3
down vote
When a frame comes into a router, the router strips off and discards the frame, losing any layer-2 addressing, including MAC addresses. The router will build a new frame for the next interface.
Not all layer-2 protocols use MAC addresses, and of those that do, some are 48-bit MAC addresses, and some are 64-bit MAC addresses. It is the IEEE LAN protocols that use MAC addressing, but, for example, frame relay uses DLCI number, or ATM uses VPI/VCI. Point-to-point protocols may not use any addressing because there is only one other device on the link.
If the next router interface also uses MAC addresses, the router will build a new frame with the MAC address of its interface as the source address, and the MAC address of the destination on the link as the destination MAC address. If the next interface doesn't use MAC addressing, the router builds a frame for the protocol on the interface.
answered 4 hours ago
Ron Maupin♦
56.4k94795
add a comment |Â
up vote
3
down vote
up vote
3
down vote
When a frame comes into a router, the router strips off and discards the frame, losing any layer-2 addressing, including MAC addresses. The router will build a new frame for the next interface.
Not all layer-2 protocols use MAC addresses, and of those that do, some are 48-bit MAC addresses, and some are 64-bit MAC addresses. It is the IEEE LAN protocols that use MAC addressing, but, for example, frame relay uses DLCI number, or ATM uses VPI/VCI. Point-to-point protocols may not use any addressing because there is only one other device on the link.
If the next router interface also uses MAC addresses, the router will build a new frame with the MAC address of its interface as the source address, and the MAC address of the destination on the link as the destination MAC address. If the next interface doesn't use MAC addressing, the router builds a frame for the protocol on the interface.
answered 4 hours ago
Ron Maupin♦
56.4k94795
When a frame comes into a router, the router strips off and discards the frame, losing any layer-2 addressing, including MAC addresses. The router will build a new frame for the next interface.
Not all layer-2 protocols use MAC addresses, and of those that do, some are 48-bit MAC addresses, and some are 64-bit MAC addresses. It is the IEEE LAN protocols that use MAC addressing, but, for example, frame relay uses DLCI number, or ATM uses VPI/VCI. Point-to-point protocols may not use any addressing because there is only one other device on the link.
If the next router interface also uses MAC addresses, the router will build a new frame with the MAC address of its interface as the source address, and the MAC address of the destination on the link as the destination MAC address. If the next interface doesn't use MAC addressing, the router builds a frame for the protocol on the interface.
answered 4 hours ago
Ron Maupin♦
56.4k94795
answered 4 hours ago
Ron Maupin♦
56.4k94795
answered 4 hours ago
Ron Maupin♦
56.4k94795
answered 4 hours ago
Ron Maupin♦
56.4k94795
56.4k94795
add a comment |Â
add a comment |Â
up vote
2
down vote
isn't a router only deal with IP address
A router with Ethernet (or Ethernet-like) interfaces needs to deal with MAC addresses because it needs to send and receive IP packets over those Ethernet interfaces and MAC addresses are a core part of Ethernet.
MAC addresses only have meaning within an Ethernet network.
Logically what happens at R1 is.
- The destination MAC address is checked. For a Unicast MAC address* if it doesn't match the frame is dropped by the network card, if it does match the Ethernet frame is stripped and the packet is passed up to the IP stack. This check is important to prevent packet duplication.
- The IP address is examined and determined to not be one of the local interfaces of the router.
- The IP address is looked up in the routing table and an interface and next-hop IP address is chosen.
- What happens next depends on the nature of the connection between R1 and R2. If the connection is Ethernet then the IP packet will be encapsulated in an Ethernet frame with the router's interface as source MAC and a MAC address looked up using the next-hop IP address as destination MAC address. On the other hand if the link is a serial point to point interface then there may be no MAC addresses involved at all and other protocols may have their own addressing scheme.
* Broadcast and multicast packets need more special handling, but that is outside the scope of this question.
answered 4 hours ago
Peter Green
6,6302922
do those MAC address do get changed?
– amjad
4 hours ago
Will expand my answer, it's not so much a case of "changed" as "re-generated"
– Peter Green
4 hours ago
add a comment |Â
up vote
2
down vote
isn't a router only deal with IP address
A router with Ethernet (or Ethernet-like) interfaces needs to deal with MAC addresses because it needs to send and receive IP packets over those Ethernet interfaces and MAC addresses are a core part of Ethernet.
MAC addresses only have meaning within an Ethernet network.
Logically what happens at R1 is.
- The destination MAC address is checked. For a Unicast MAC address* if it doesn't match the frame is dropped by the network card, if it does match the Ethernet frame is stripped and the packet is passed up to the IP stack. This check is important to prevent packet duplication.
- The IP address is examined and determined to not be one of the local interfaces of the router.
- The IP address is looked up in the routing table and an interface and next-hop IP address is chosen.
- What happens next depends on the nature of the connection between R1 and R2. If the connection is Ethernet then the IP packet will be encapsulated in an Ethernet frame with the router's interface as source MAC and a MAC address looked up using the next-hop IP address as destination MAC address. On the other hand if the link is a serial point to point interface then there may be no MAC addresses involved at all and other protocols may have their own addressing scheme.
* Broadcast and multicast packets need more special handling, but that is outside the scope of this question.
answered 4 hours ago
Peter Green
6,6302922
do those MAC address do get changed?
– amjad
4 hours ago
Will expand my answer, it's not so much a case of "changed" as "re-generated"
– Peter Green
4 hours ago
add a comment |Â
up vote
2
down vote
up vote
2
down vote
isn't a router only deal with IP address
A router with Ethernet (or Ethernet-like) interfaces needs to deal with MAC addresses because it needs to send and receive IP packets over those Ethernet interfaces and MAC addresses are a core part of Ethernet.
MAC addresses only have meaning within an Ethernet network.
Logically what happens at R1 is.
- The destination MAC address is checked. For a Unicast MAC address* if it doesn't match the frame is dropped by the network card, if it does match the Ethernet frame is stripped and the packet is passed up to the IP stack. This check is important to prevent packet duplication.
- The IP address is examined and determined to not be one of the local interfaces of the router.
- The IP address is looked up in the routing table and an interface and next-hop IP address is chosen.
- What happens next depends on the nature of the connection between R1 and R2. If the connection is Ethernet then the IP packet will be encapsulated in an Ethernet frame with the router's interface as source MAC and a MAC address looked up using the next-hop IP address as destination MAC address. On the other hand if the link is a serial point to point interface then there may be no MAC addresses involved at all and other protocols may have their own addressing scheme.
* Broadcast and multicast packets need more special handling, but that is outside the scope of this question.
answered 4 hours ago
Peter Green
6,6302922
isn't a router only deal with IP address
A router with Ethernet (or Ethernet-like) interfaces needs to deal with MAC addresses because it needs to send and receive IP packets over those Ethernet interfaces and MAC addresses are a core part of Ethernet.
MAC addresses only have meaning within an Ethernet network.
Logically what happens at R1 is.
- The destination MAC address is checked. For a Unicast MAC address* if it doesn't match the frame is dropped by the network card, if it does match the Ethernet frame is stripped and the packet is passed up to the IP stack. This check is important to prevent packet duplication.
- The IP address is examined and determined to not be one of the local interfaces of the router.
- The IP address is looked up in the routing table and an interface and next-hop IP address is chosen.
- What happens next depends on the nature of the connection between R1 and R2. If the connection is Ethernet then the IP packet will be encapsulated in an Ethernet frame with the router's interface as source MAC and a MAC address looked up using the next-hop IP address as destination MAC address. On the other hand if the link is a serial point to point interface then there may be no MAC addresses involved at all and other protocols may have their own addressing scheme.
* Broadcast and multicast packets need more special handling, but that is outside the scope of this question.
answered 4 hours ago
Peter Green
6,6302922
edited 4 hours ago
answered 4 hours ago
Peter Green
6,6302922
answered 4 hours ago
Peter Green
6,6302922
answered 4 hours ago
Peter Green
6,6302922
6,6302922
do those MAC address do get changed?
– amjad
4 hours ago
Will expand my answer, it's not so much a case of "changed" as "re-generated"
– Peter Green
4 hours ago
add a comment |Â
do those MAC address do get changed?
– amjad
4 hours ago
Will expand my answer, it's not so much a case of "changed" as "re-generated"
– Peter Green
4 hours ago
do those MAC address do get changed?
– amjad
4 hours ago
do those MAC address do get changed?
– amjad
4 hours ago
Will expand my answer, it's not so much a case of "changed" as "re-generated"
– Peter Green
4 hours ago
Will expand my answer, it's not so much a case of "changed" as "re-generated"
– Peter Green
4 hours ago
add a comment |Â
up vote
2
down vote
It's only rewording what others have already said, but the way I usually explain this is with a thought experiment:
- Suppose there's no ethernet, and all of the hosts and routers have a direct point-to-point links (with the same connectivity, ie A to R1)
- Many point to point links have no link addressing: just send the packet in some kind of framing and the other end receives it.
- Then, exactly as you imagine, packets just have fixed IP source and destination address from host A to host B.
- If it's convenient, a given set of links might be replaced by ethernet
- To pass a packet in ethernet, it must have ethernet source and destination addresses in order to get to just the right node on the ethernet
- On ethernet the frame says "I have this for you", where "I" and "you" are defined by the source and destination ethernet addresses
- As the IP packet goes from hop to hop, different routers are saying "I have this for you", so of course the frame would have different source and destination addresses
- If it's not ethernet, you get the appropriate kind of MAC addressing for that medium; 48-bit, 64-bit, none.
answered 4 hours ago
jonathanjo
5,725323
add a comment |Â
up vote
2
down vote
It's only rewording what others have already said, but the way I usually explain this is with a thought experiment:
- Suppose there's no ethernet, and all of the hosts and routers have a direct point-to-point links (with the same connectivity, ie A to R1)
- Many point to point links have no link addressing: just send the packet in some kind of framing and the other end receives it.
- Then, exactly as you imagine, packets just have fixed IP source and destination address from host A to host B.
- If it's convenient, a given set of links might be replaced by ethernet
- To pass a packet in ethernet, it must have ethernet source and destination addresses in order to get to just the right node on the ethernet
- On ethernet the frame says "I have this for you", where "I" and "you" are defined by the source and destination ethernet addresses
- As the IP packet goes from hop to hop, different routers are saying "I have this for you", so of course the frame would have different source and destination addresses
- If it's not ethernet, you get the appropriate kind of MAC addressing for that medium; 48-bit, 64-bit, none.
answered 4 hours ago
jonathanjo
5,725323
add a comment |Â
up vote
2
down vote
up vote
2
down vote
It's only rewording what others have already said, but the way I usually explain this is with a thought experiment:
- Suppose there's no ethernet, and all of the hosts and routers have a direct point-to-point links (with the same connectivity, ie A to R1)
- Many point to point links have no link addressing: just send the packet in some kind of framing and the other end receives it.
- Then, exactly as you imagine, packets just have fixed IP source and destination address from host A to host B.
- If it's convenient, a given set of links might be replaced by ethernet
- To pass a packet in ethernet, it must have ethernet source and destination addresses in order to get to just the right node on the ethernet
- On ethernet the frame says "I have this for you", where "I" and "you" are defined by the source and destination ethernet addresses
- As the IP packet goes from hop to hop, different routers are saying "I have this for you", so of course the frame would have different source and destination addresses
- If it's not ethernet, you get the appropriate kind of MAC addressing for that medium; 48-bit, 64-bit, none.
answered 4 hours ago
jonathanjo
5,725323
It's only rewording what others have already said, but the way I usually explain this is with a thought experiment:
- Suppose there's no ethernet, and all of the hosts and routers have a direct point-to-point links (with the same connectivity, ie A to R1)
- Many point to point links have no link addressing: just send the packet in some kind of framing and the other end receives it.
- Then, exactly as you imagine, packets just have fixed IP source and destination address from host A to host B.
- If it's convenient, a given set of links might be replaced by ethernet
- To pass a packet in ethernet, it must have ethernet source and destination addresses in order to get to just the right node on the ethernet
- On ethernet the frame says "I have this for you", where "I" and "you" are defined by the source and destination ethernet addresses
- As the IP packet goes from hop to hop, different routers are saying "I have this for you", so of course the frame would have different source and destination addresses
- If it's not ethernet, you get the appropriate kind of MAC addressing for that medium; 48-bit, 64-bit, none.
answered 4 hours ago
jonathanjo
5,725323
answered 4 hours ago
jonathanjo
5,725323
answered 4 hours ago
jonathanjo
5,725323
answered 4 hours ago
jonathanjo
5,725323
5,725323
add a comment |Â
add a comment |Â
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