Route caching

[igoksadze]

Hi, 

I cant understand technology of route caching.

From CCNP switching guide:

Route caching: The first generation of MLS, requiring a route processor (RP) and a switch engine (SE). The RP must process a traffic flow’s first packet to determine the destination. The SE listens to the first packet and to the resulting destination, and then sets up a “shortcut” entry in its MLS cache. The SE forwards subsequent packets belonging to the same traffic flow based on shortcut entries in its cache....

Please explain this two options: RP and SE...

[Joseph W. Doherty]

This particular example is in the context of MLS (multi-layer switching).

A good example of how this could work was on an Catalyst 5500 that didn't have a L3 module, but only had a L2 SE (switch engine).  This Catalyst could be connected to an external router (the RP).  (The Catalyst 5500 also could have an internal RP rather than an external router, but an external router, I think, makes how earlier MLS work, more clear.)

The first packet of a new flow would be sent do the external router, and the router would inform the switch where the packet needed to go.  The L2 only switch, which does not normally move packets between VLANs, would then note where "like" packets should go, i.e. it "shortcut" the need to use the router of subsequent flow packets.  So, the packets would be "routed" at the Catalyst's L2 switching speed vs. the routing speed of the external router.

"Route caching" is/was also used on routers.  For routers, the router would route the first packet of a flow normally, but save what it did.  Subsequent packets would only require a cache table lookup, rather than a route table lookup.  The prior, on a router was generally 10x, or more, faster than the latter.

[igoksadze]

Is RP only  present in L3 Switches or it is in L2 switches too?

And is it right to say: L2 switches have only SE ?

[Joseph W. Doherty · Accepted solution]

Really depends on the architecture, and how vendor wants to "name" it.

The RP was an optional module in some early chassis switches, e.g. Catalyst 5500, so the SE and RP are really separate components.

Architecture wise, Catalyst 6500 is similar, although I believe the RP was always a "standard" component.

For a L3 switch like a 3650/3850, I don't know whether they have an SE and RP, per se, although they also support L2 and/or L3.

[igoksadze]

Thank you for your explanation

[igoksadze]

Is the fast switching  same thing as route caching process?

[Joseph W. Doherty]

Depends on context. Sometimes, I believe, it's commonly used to denote a packet being forwarded by a software based router's "interrupt" switching rather than it being subject to "process switching". I don't recall whether "interrupt" switching required route caching or one of the other routing speed up variants prior to CEF.