Dragon0x40
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Static route pointing to an interface
If you use an interface instead of a next hop ip address how does that work?
example: ip route 0.0.0.0 0.0.0.0 s1/0
If a packet for 10.10.10.9 came into the router and the best route was the above default route would the router try to arp for 10.10.10.9 out the s1/0 interface?
I have read some articles that say that packet would just get sent out s1/0 but others which say that the router would try to resolve the L3 address to a L2 mac address.
If the router must resolve the L3 address then the 10.10.10.9 must be in the same broadcast domain as s1/0 or use proxy arp on the downstream router/switch?
One article said this configuration is better for interfaces connected via point to point networks but multi-access networks need more configuration and are not scalable.
I have not come across a config where the interface is used instead of the next hop but I am curious as to when or how it might be used.
example: ip route 0.0.0.0 0.0.0.0 s1/0
If a packet for 10.10.10.9 came into the router and the best route was the above default route would the router try to arp for 10.10.10.9 out the s1/0 interface?
I have read some articles that say that packet would just get sent out s1/0 but others which say that the router would try to resolve the L3 address to a L2 mac address.
If the router must resolve the L3 address then the 10.10.10.9 must be in the same broadcast domain as s1/0 or use proxy arp on the downstream router/switch?
One article said this configuration is better for interfaces connected via point to point networks but multi-access networks need more configuration and are not scalable.
I have not come across a config where the interface is used instead of the next hop but I am curious as to when or how it might be used.
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Can a serial interfacde use ethernet encapsulation?
If the interface used as the next hop is not mulit-access then the frames are broadcast out s1/0 and that is no problem because HDLC and PPP are layer 2 and broadcast anyway?
If the interface used as the next hop is multi-access layer 3 protocol (such as ethernet) then the router will try to resolve the L3 address to a L2 address out of the s1/0 interface which is not acceptable/recommended because the far end may not be able or be configured to answer the L3 to L2 resolution query.
If the interface used as the next hop is not mulit-access then the frames are broadcast out s1/0 and that is no problem because HDLC and PPP are layer 2 and broadcast anyway?
If the interface used as the next hop is multi-access layer 3 protocol (such as ethernet) then the router will try to resolve the L3 address to a L2 address out of the s1/0 interface which is not acceptable/recommended because the far end may not be able or be configured to answer the L3 to L2 resolution query.
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The contents of the previous post are alll questions not statements.
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I am still not getting the complete picture in my head.
Whether we have a serial interface or an ethernet interface we are still using IP at L3 and the same IP protocols are in effect wherever the packets are sent. (ppp or ethernet)
Would understanding how ppp and hdlc on a serial interface encapsulates the packets help me or am I missing something with how IP and arp works?
Does arp not work on a serial interface?
Does ppp have physicall addresses?
Whether we have a serial interface or an ethernet interface we are still using IP at L3 and the same IP protocols are in effect wherever the packets are sent. (ppp or ethernet)
Would understanding how ppp and hdlc on a serial interface encapsulates the packets help me or am I missing something with how IP and arp works?
Does arp not work on a serial interface?
Does ppp have physicall addresses?
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So my takeaway:
You can create static routes to point to an interface or next hop ip address if using serial interface or an interface connected to a point to point link but it more of the standard to use the next hop ip address? You may point to the interface for a technical reason which can not be easily summarized here. Using the ip address is self documenting and is easier for the next admin doing a show ip route?
It is not recommended to point a static route to an ethernet interface because of the increased cpu utilization of the L3 to L2 resolution workload (because the router thinks every ip address that matches the static route is directly connected) instead you should point to the next hop ip address? The next hop ip address is then resolved once and placed in the arp cache.
You can create static routes to point to an interface or next hop ip address if using serial interface or an interface connected to a point to point link but it more of the standard to use the next hop ip address? You may point to the interface for a technical reason which can not be easily summarized here. Using the ip address is self documenting and is easier for the next admin doing a show ip route?
It is not recommended to point a static route to an ethernet interface because of the increased cpu utilization of the L3 to L2 resolution workload (because the router thinks every ip address that matches the static route is directly connected) instead you should point to the next hop ip address? The next hop ip address is then resolved once and placed in the arp cache.
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Agreed, but on the final point frankly do not.
>It is not recommended to point a static route to an ethernet interface because of
>the increased cpu utilization of the L3 to L2 resolution workload...
This is certainly a possibility, Cisco has documented this. Also, if default setting are in effect on the remote router the packet will certainly make it.
See link ....
http://www.cisco.com/en/US/products/hw/routers/ps359/products_tech_note09186a00801c2af6.shtml#arp
As an example;
Host1(10.1.1.1/24) |--------|fa0/1-R1-fa0/0|--------- 10.1.3.0/24 ---------|fa0/1| R2 |fa0/0|---------|Servers(10.1.2.0/24)
Lets assume R1 is configured as follows;
ip route 0.0.0.0 0.0.0.0 fa0/0
And R2
ip route 10.1.1.0 255.255.255.0 fa0/1
And everything else is default.
When Host1 connects to Servers on 10.1.2.0/24, R1 will send ARP requests out fa0/0 for every Server that Host1 tries to communicate with. Because proxy-arp is enabled by default the packet will definitely make it to the servers and back.
Now consider if we replace the server segment with the Internet, this means an ARP request would be generated for every host on the Internet Host1 tries to communicate with. This is not ideal.
>It is not recommended to point a static route to an ethernet interface because of
>the increased cpu utilization of the L3 to L2 resolution workload...
This is certainly a possibility, Cisco has documented this. Also, if default setting are in effect on the remote router the packet will certainly make it.
See link ....
http://www.cisco.com/en/US/products/hw/routers/ps359/products_tech_note09186a00801c2af6.shtml#arp
As an example;
Host1(10.1.1.1/24) |--------|fa0/1-R1-fa0/0|--------- 10.1.3.0/24 ---------|fa0/1| R2 |fa0/0|---------|Servers(10.1.2.0/24)
Lets assume R1 is configured as follows;
ip route 0.0.0.0 0.0.0.0 fa0/0
And R2
ip route 10.1.1.0 255.255.255.0 fa0/1
And everything else is default.
When Host1 connects to Servers on 10.1.2.0/24, R1 will send ARP requests out fa0/0 for every Server that Host1 tries to communicate with. Because proxy-arp is enabled by default the packet will definitely make it to the servers and back.
Now consider if we replace the server segment with the Internet, this means an ARP request would be generated for every host on the Internet Host1 tries to communicate with. This is not ideal.
The problem with this theory is that it requires proxy ARP to be enabled. Which is why I stated " the packet will most likely never make it to the destination."
Best practices recommends disabling of proxy ARP.
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080120f48.shtml
If proxy ARP is not enabled, the packets will not make to the destination.
Best practices recommends disabling of proxy ARP.
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080120f48.shtml
If proxy ARP is not enabled, the packets will not make to the destination.
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It sounds to me like you both took opposite sides of the question I am trying to clarify.
donjohnston: no need to know a mac just send frame out s1/0
nazsky: the router will try to resolve the L3 address to a mac adress out the s1/0 interface.
I assumed that all frames must have a source and destination mac address (ethernet)
When you send frames out s1/0 does the frame have the mac address of s1/0 or does the router just physically send it out?