multilayer switch(cef) connected to another switch.

Answered Question
Dec 15th, 2008
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Hi every body!

Scenario #1

Let say two switches(MLS),sw1 and sw2 are connected to each other by f0/1 ports.

. 199.199.199.0/24

sw1 f0/1-------------- f0/1 sw2

where:

199.199.199.1 ---sw1'f0/1

199.199.199.2----- sw2'f0/1

Then

sw1# show ip route

199.199.199.0/24 directly connected via f0/1.


sw1# show ip cef.


prefix next hop interface

199.199.199.0/24 attached f0/1


199.199.199.0/32 receive

199.199.199.1/32 receive.

199.199.199.255/32 receive.

Would there be another entry for directly connected sw2 i.e 199.199.199.2?

My undestanding is If sw2 has not sent any ip packets and cdp is disabled, there should not be any such entry. However my book shows the entry for directly connected sw2 in output:

Book out put:

prefix next hop interface

199.199.199.0/24 attached f0/1

199.199.199.0/32 receive

199.199.199.1/32 receive

199.199.199.2/32 199.199.199.2/32 f0/1

199.199.199.255/32 receive.

Am I correct in saying the entry 199.199.199.2 is present because ip packet was received from sw2 by sw1?


If i am correct, when sw1 has not recived any ip packet,cdp packets, then fib should be

sw1# show ip cef

prefix next hop interface

199.199.199.0/24 attached f0/1

199.199.199.0/32 receive

199.199.199.1/32 receive

199.199.199.255/32 receive.

Since there is no entry for 199.199.199.2, any packet destined to 199.199.199.2 will be process switched(by router).

AS the packet is forwarded to 199.199.199.2, Adjacency for 199.199.199.2 is build and an entry is also added to cef in sw1.Now fib in sw1 should have entry for 199.199.199.2/32.

Any subsequent packets to 199.199.199.2 will be cef-switched.

Am i correct?

thanks a lot!

Correct Answer by Edison Ortiz about 8 years 4 months ago

The fib had an entry already for that subnet with the following entry;


199.199.199.0/24 attached FastEthernet0/13

In a nutshell, that's the information from the RIB.


a more specific entry was entered into the fib once the directly connected device responded to an ARP request, in this case a ICMP reply.


HTH,


__


Edison.

Correct Answer by Edison Ortiz about 8 years 4 months ago

Sarah,


Correct


Rack1SW1#sh ip cef | i 199

199.199.199.0/24 attached FastEthernet0/13

199.199.199.0/32 receive

199.199.199.1/32 receive

199.199.199.255/32 receive

Rack1SW1#

Rack9Pod1>2

[Resuming connection 2 to sw2 ... ]


Rack1SW2(config-if)#


Rack1SW2(config-if)#do ping 199.199.199.1


Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 199.199.199.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/9 ms

Rack1SW2(config-if)#

Rack9Pod1>1

[Resuming connection 1 to sw1 ... ]


Rack1SW1#sh ip cef | i 199

199.199.199.0/24 attached FastEthernet0/13

199.199.199.0/32 receive

199.199.199.1/32 receive

199.199.199.2/32 attached FastEthernet0/13

199.199.199.255/32 receive

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Jon Marshall Mon, 12/15/2008 - 12:21
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Sarah


I will add this to tests but i would say that following on from our recent discussion you are correct in that if no IP traffic has been sent between the switches then i would not expect to see an entry for sw2 on sw1.


Furthermore i would expect in the CEF adjacency table on both sw1 and sw2 to see a glean adjacency for the 199.199.199.0/24 subnet.


Jon

sarahr202 Mon, 12/15/2008 - 13:02
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Thanks Jon!

How about punt adjacency?

My reasoning is since no mac -address is present,therefore packet is punted to layer 3 engine.The reason for punt adjacency in our case could be:

no-adj,no-encap.

However I am not sure which one i.e no-adj or no-encap.

Jon Marshall Mon, 12/15/2008 - 13:13
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Sarah


As i said i think it will be punted to the CPU but i think the adjacency would show as a glean adjacency in the adjacency table.


I would expect it to be no-adj.


Jon

Correct Answer
Edison Ortiz Mon, 12/15/2008 - 12:23
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Sarah,


Correct


Rack1SW1#sh ip cef | i 199

199.199.199.0/24 attached FastEthernet0/13

199.199.199.0/32 receive

199.199.199.1/32 receive

199.199.199.255/32 receive

Rack1SW1#

Rack9Pod1>2

[Resuming connection 2 to sw2 ... ]


Rack1SW2(config-if)#


Rack1SW2(config-if)#do ping 199.199.199.1


Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 199.199.199.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/9 ms

Rack1SW2(config-if)#

Rack9Pod1>1

[Resuming connection 1 to sw1 ... ]


Rack1SW1#sh ip cef | i 199

199.199.199.0/24 attached FastEthernet0/13

199.199.199.0/32 receive

199.199.199.1/32 receive

199.199.199.2/32 attached FastEthernet0/13

199.199.199.255/32 receive

sarahr202 Mon, 12/15/2008 - 13:08
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Thanks Edison !

It means entries in Fib are not only sourced from routing table,but also from packets received by a switch. For example entry for 199.199.199.199.2/32 was built in fib after the sw1 received ping from sw2.


thanks a lot Edison and Jon !

Correct Answer
Edison Ortiz Mon, 12/15/2008 - 13:15
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The fib had an entry already for that subnet with the following entry;


199.199.199.0/24 attached FastEthernet0/13

In a nutshell, that's the information from the RIB.


a more specific entry was entered into the fib once the directly connected device responded to an ARP request, in this case a ICMP reply.


HTH,


__


Edison.

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