can you explain this point
“The destination MAC address is a broadcast address of the subnet that the router interface is connected to.” are you referring to the next hop arp entry?
Thanks
Hello Sulaiman
Not quite. Essentially, Rene here is stating the circumstances under which R1 would accept the frame for further decapsulation and processing. The criteria of accepting the frame have to do with the contents of the destination MAC address field. It will accept the frame only if the MAC address either matches that of the Gi0/1 interface of R1, is a broadcast MAC address, or is a multicast MAC address that the router has been configured to listen to. Otherwise, the frame will be dropped simply because the router determines that it itself is not the intended destination.
I believe that the statement can be confusing. It says:
The destination MAC address is a broadcast address of the subnet that the router interface is connected to.
where I believe it should say:
The destination MAC address is a broadcast address.
Unlike IP addresses which have a broadcast address for each subnet, MAC addresses only have one broadcast address, which is FF:FF:FF:FF:FF:FF. A frame with this destination MAC address will reach all hosts within a broadcast domain.
I will ask Rene to clarify this statement.
I hope this has been helpful!
Laz
Hello all,
We are using EIGRP protocol but there is below default gateway configuration.
For example
S* 0.0.0.0[1/0] via 1.1.1.1
D EX 1.222.111.222 via 1.1.1.1
D EX 2.222.222.222 via 1.1.1.1
.
.
.
Inside network: HSRP is configured but no track interface option configured
there are L2 and L3 etherchannel connection between routers.
Is there any reason why default gate has been set up?
It is normal configuration?
Many thanks!!
DY
Hello DY HAN
According to the routing table entry of “S” this is a statically configured default route. This means that there is a command in the config in the form of
ip route 0.0.0.0 0.0.0.0 1.1.1.1
SInce the “S” is indicated at the beginning of the route, it means that no dynamic routing protocol is involved, nor is HSRP. It is manually configured on the device. as shown above.
Now the “*” indicates that it is a candidate default route. This just indicates that, if there is more than one default route, this one, in particular, is the currently used one. It looks like there is only one configured on your device, so by definition, this is the one being used.
I hope this has been helpful!
Laz
Hello Laz.
Above all , many thanks for your reply.
I need to time to think about this more and ask you again later.
Many thanks!!
DY
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Conclusion
You have now learned how an IP packet is forwarded from one router to another, also known as IP routing.Let’s summarize this process.
The host has a simple decision to make:
Is the destination on the local subnet?
Check ARP table for **destination IP address
**, if empty, send an ARP request.
Is the destination on a remote subnet?
Check ARP table for default gateway IP address, if empty, send an ARP request.
The host Check ARP table for MAC address or the IP addres?
Hello Tariq
The local ARP table of the host maintains a list of IP address to MAC address correspondences. When a host is preparing to send data, it knows the IP address of the destination, but it must find the MAC address that corresponds to this so it can place it in the destination MAC address field of the frame.
When Rene says “Check ARP table for destination IP address” he means we look up the destination IP address in the local ARP table to see if we have a corresponding MAC address entry. If we don’t, then we send out an ARP request. The ARP request is used to find the MAC address that corresponds to the IP address we’re looking for.
As stated in the lesson, the IP address may be of the destination host, if it is on the same subnet, or of the default gateway, if the destination host is on a different subnet.
I do agree that the wording may be a bit confusing here, so I will let Rene know to take a look and see if he can make some modifications to make the text clearer.
I hope this has been helpful!
Laz
“Is the destination on the local subnet?
Check ARP table for destination IP”
This not correct information , Check ARP table for distinction MAC address not IP
Hello Tariq
What Rene is saying here is the following:
- Is the destination on the local subnet? If yes,
- Check the ARP table to see if the destination IP address is in the table, and if it is, use the corresponding MAC address in the destination field of the Ethernet Frame.
- Is the destination on a remote subnet? If yes,
- Check the ARP table to see if the IP address of the default gateway is in the table, and if it is, use the corresponding MAC address in the destination field of the Ethernet Frame.
Since we’re talking about the ARP table, this meaning is assumed to be understood. However, if this is confusing, I will let Rene know to take a look and consider rewording it to make it more straightforward.
I hope this has been helpful!
Laz
Please do you have any thoughts on this? when i enter the ip routing command on my cisco 9300 switch my SSH connection times out, i would have to use console cable to disable the command and the SSH session will start to work.
My configuration:
line vty 0 5
login local
transport input ssh
Hello Temitope
There could be several reasons for this. Some default security parameters may be enabled when you issue this command that causes all SSH and other connections to fail. It may be that by issuing this command, the management IP address changes on the device, and you may be trying to SSH to the wrong IP address. Another possible reason is that the switch’s SSH configuration may have been reset or altered during the process of enabling “ip routing.”
I suggest you issue the ip routing command only via the console and then perform some additional configuration and troubleshooting tasks locally to ensure that you have access via SSH remotely.
Once IP routing has been enabled, you can check to see what IP addresses are configured on the device, and you can attempt to SSH into all of them to see which one may be configured as the management IP. Also, you can try configuring an access list to permit SSH traffic and apply it to the switch’s interface or to the VTY to ensure that SSH traffic is not being blocked by the switch’s routing configuration.
By being physically present during the configuration and troubleshooting will allow you to resolve any connectivity issues that cannot be resolved remotely.
As you do your troubleshooting, let us know how you get along.
I hope this has been helpful!
Laz
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Hi Laz
in Cisco book you referred to previously they talked about how the router lookup for the best route met the destination address, but they didn’t mention that if I configured Default route 0.0.0.0/0 ---- it will take it as best match immediately.
Can you plz explain this for me
Hello Abdullah
A router will look through all of its routing entries to find the one that best matches the intended destination. If no match is found, and if the “gateway of last resort” (or the default route) is set, only then will it send the packet to the default route.
For example, take a look at this routing table:
R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 192.168.24.254 to network 0.0.0.0
C 192.168.12.0/24 is directly connected, FastEthernet0/0
S* 0.0.0.0/0 [1/0] via 192.168.24.254, FastEthernet0/1
D 192.168.23.0/24 [90/307200] via 192.168.12.2, 00:00:07, FastEthernet0/0
A destination address of 192.168.12.5 will match the directly connected network on FastEthernet 0/0. However, a destination address of 10.10.10.5 will not match any entry, and will thus be sent via the gateway of last resort, which has a next hop of 192.168.24.254, and an exit interface of FastEthernet 0/1.
So the gateway of last resort, as its name suggests, will only be used if all other entries do not match.
I hope this has been helpful!
Laz
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Do dynamic routing protocols propagate static routes or are static routes only relevant to the router they are configured on?
Hello CJ
By default, static routes are relevant only to the local router. Even if you have a dynamic routing protocol enabled on the device, it will not automatically advertise such routes. However, you can redistribute a static route into a dynamic routing protocol such that it advertises them.
This can be done using the redistribute static command within the configuration of the routing protocol. For example,
Router(config)#router eigrp 1
Router(config-router)#redistribute static
This command will take any configured static routes on the local router and will redistribute them into the EIGRP routing protocol, such that they will be advertised throughout the EIGRP domain. For more information on redistribution, take a look at this lesson:
I hope this has been helpful!
Laz
Thanks for confirming! I had suspected this was the case but it was hard to find any info out there to confirm it.
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