Router Switching Paths

[kfarrington]

All,

Please can someone confirm a small point for me.

Process switching

-Packet comes in from an interface and the interface places this packet in memory (SRAM).

-Packet then gets placed in DRAM, does a routing table lookup and gets forwarded based on the routing table.

Fast Switching

-Packet comes in from an interface and the interface places this packet in memory (SRAM).

-The packet still stored in SRAM, consults the fast cache table and if it gets a hit, gets forwarded based on the fast cache table

-NO ROUTING TABLE LOOKUP.

CEF Switching

-Packet comes in from an interface and the interface places this packet in memory (SRAM).

-The packet still stored in SRAM, consults the FIB table and if it gets a hit, consults the adjacency table and gets f/wd based on these two tables.

-NO ROUTING TABLE LOOKUP.

Is this correct? So not every packet a router receives, does a routing table lookup?

Many thx for ur help.

[DALE FRANCIS]

Ken,

Packet memory is not always SRAM, this depends on the platform.

Fast switching - If a cache entry exists it will be switched based on that cache entry, if an entry does not exist a interrupt will be issues to the Route Processor and subsequent packets will be switched based on the newly created route entry.

CEF - CEF FIB is built from the RIB, the adjacency is built from ARP cache. If there is no entry in the CEF FIB the packet will de dropped, if there is a PUNT associated with the entry the packet will switching decision will move to the next process level such as netflow, fast etc.

So yes if a CEF entry is found the Routing Table itself will not be consulted.

[sbrobak]

Another question to the same theme:

Is QoS related to the switching method?

If yes, what is PQ, CQ, CBWFQ using?

[DALE FRANCIS]

If there is in packet options or Layer3 re-writes then it will not involve CEF to the best of my knowledge. Hence the reason Cisco moved to PXF on some devices.

Queing methods affect the outgoing interface and as such they are POST CEF, so no switching methods are required.

Eg: On the Cisco 7500 Series, the ability to configure Frame Relay TS . With dCEF and Frame Relay TS, a CEF "punt" adjacency causes all packets to be fast-switched by the RSP

[agugger]

From experience with QoS is the Process switching is little bit more complex:

- Packet comes in from an interface and the interface places this packet in the IO memory

- (Whatever the I/O Memory is depends the platform SRAM, DRAM, Particle pool etc.).

- Form the input queue pointers and counters

- Do fast switch cache lookup if fail continue with process switching

- Than will the IP portion only of the packet moved to system buffer

- If there are an input ACL it will be processed

- Now follow the Routing table lookup, drop packet if fail

- Form the fast switching cache entry, encapsulation failure if not possible

- If defined perform accounting - If there are an output ACL it will be processed

- Perform an output queuing depend configuration eventually classify the packet per priority (QoS!)

- Tray an arp request and/or form the rewrite header for a serial interface or wan protocols

- Form the output queue pointers and counters

- Move the packet from system buffer to the output memory queue

- Send the packet thru the interface- If an error arrive do error process

Fast switching:

- Packet comes in from an interface and the interface places this packet in the I/O memory- Form the input queue pointers and counters

- Do fast switch cache lookup- Do Header rewrite at the IO Memory

- In this case the packet stay still at the same place

- Perform an output queuing depend configuration eventually classify the packet per priority (QoS!)(performance with FE or GE???)

- Just transfer the pointers to the output queue

- Form the output queue pointers and counters

- Send the packet thru the interface

- If an error arrive do error process

Until now I don’t have the same detail’s with CEF but it will be not more simple

Regards,

Andy