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How does routing via a SVI technically work?

vv0bbLeS
Level 1
Level 1

Hello all,

I'm trying to wrap my mind around what a SVI technically "is" and as such I'm stuck on the question of how routing via a SVI technically works, i.e. by what process does the SVI determine the actual physical port out which to send the data?

  • I understand that the SVI is a virtual routed interface, but at some point the bits have to be sent out a physical port - how is this actually/technically accomplished if a routing entry's outgoing interface is a SVI ?

 

0xD2A6762E
1 Accepted Solution

Accepted Solutions

Jon Marshall
Hall of Fame
Hall of Fame

 

It is essentially the same process as routing out of a physical interface with one additional step. 

 

So a L3 device using physical interfaces consults the routing table, finds the next hop IP address and the outgoing interface, gets the mac address of the next hop IP from the arp table, rewrites the L2 header and transmits the packet (I have simplified because of CEF etc. but the principle is valid). 

 

A L3 device using SVIs consults the routing tables, finds the next hop IP and the outgoing interface, get the mac address of the next hop IP from the arp table but then as you say it still does not know which physical interface to use to get to the next hop IP address. 

 

This is where the mac address table comes in and is the additional lookup needed by a device using SVIs ie. it knows the mac address of the next hop IP from the arp table but it now needs to know on which port that mac address is reachable. 

 

Jon

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4 Replies 4

Jon Marshall
Hall of Fame
Hall of Fame

 

It is essentially the same process as routing out of a physical interface with one additional step. 

 

So a L3 device using physical interfaces consults the routing table, finds the next hop IP address and the outgoing interface, gets the mac address of the next hop IP from the arp table, rewrites the L2 header and transmits the packet (I have simplified because of CEF etc. but the principle is valid). 

 

A L3 device using SVIs consults the routing tables, finds the next hop IP and the outgoing interface, get the mac address of the next hop IP from the arp table but then as you say it still does not know which physical interface to use to get to the next hop IP address. 

 

This is where the mac address table comes in and is the additional lookup needed by a device using SVIs ie. it knows the mac address of the next hop IP from the arp table but it now needs to know on which port that mac address is reachable. 

 

Jon

Jon,

Excellent! Would it be the Forwarding Engine that also performs this additional "step" in getting the MAC address (when sending out a SVI interface) ?

0xD2A6762E

 

It gets a bit complicated with hardware devices especially the more modular switches. 

 

Basically forwarding is usually done in hardware and the forwarding tables, both L2 and L3, are loaded into specialised hardware memory for efficient lookups. 

 

Jon

Great! Thanks again!

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