Ethernet Switches & Routers

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Occasional Contributor
Posts: 11
Registered: ‎05-29-2013

are these commands used for finding outgoing interface for a incoming packet.

In order to find a outgoing interface for a incoming packet does the router refer below two commands ? First table to get the mapping of IP-MAC and then to find out from which interface the destination MAC address can be reached.


show arp

show mac-address-table



Frequent Contributor
Posts: 118
Registered: ‎06-15-2009

Re: are these commands used for finding outgoing interface for a incoming packet.

Jrshah2,

You can use "show ip route" command first. You need to know what’s the destination IP address of traffic. For example, destination IP is 100.x.x.y. Then you do:

FCX_STK_2#show ip route 100.x.x.y

Type Codes - B:BGP D:Connected OSmiley SurprisedSPF R:RIP SSmiley Frustratedtatic; Cost - Dist/Metric

BGP  Codes - i:iBGP e:eBGP

OSPF Codes - i:Inter Area 1:External Type 1 2:External Type 2

Destination Gateway Port Cost          Type Uptime

1 100.x.x.0/24 150.x.x.x       ve 150 x/x           S 18h41m

You can see the outgoing interface is VE  150 and gateway is 150.x.x.x. Then you can do show arp to find out where is 150.x.x.x learned.

FCX_STK_2#show arp 150.x.x.x

  1. No.   IP Address       MAC Address Type     Age Port           Status

1 150.1.1.1        00e0.5200.0100 Dynamic  0 2/1/23        Valid

So the outgoing int is 2/1/23.


Hope this helps,

Deepti



Occasional Contributor
Posts: 11
Registered: ‎05-29-2013

Re: are these commands used for finding outgoing interface for a incoming packet.

Thanks Deepti that was pretty helpful but now I am trying to look for a command to view the forwarding table of the router.

I tried "show cam ip 1/1 " . This command was per interface. Does not there exist a system wide forwarding table that I can view using a command ?

Frequent Contributor
Posts: 118
Registered: ‎06-15-2009

Re: are these commands used for finding outgoing interface for a incoming packet.

Jrshah2, sure glad I could help. Could you tell me which switch you are talking about here?

Occasional Contributor
Posts: 11
Registered: ‎05-29-2013

Re: are these commands used for finding outgoing interface for a incoming packet.

Hi Deepti,

               Unfortunately I could not get all the routers on which I am looking for the forwarding tables. I had asked for show version but it does not have what type of router it is. My Bad.

But I remember MLX and ADX. I am looking for info like : Destination , its next hop and outgoing interface in the forwarding table.

thanks.

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