We have an edge router (Cisco), and has a marking of packets based on DSCP. The edge router is connected to an ISP (not Cisco) which has a QoS based on IP Precedence. I understand that there is a conversion table between DSCP and IP Precedence, I wondered if this conversion is done automatically between routers, or must I have to configure a package marked on the edge router with IP Precedence to the exit to the ISP.
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Short version, you probably don't need to do anything. Long version . . .
As Vasileios correctly describes, IP Prec and DSCP both use the ToS byte. DSCP usage, which uses the next 3 bits (as also noted by Vasileios), was designed for backward compatibility as DSCP Class groups map into IP Prec, e.g. AF2x maps to IP Prec 2.
As long as your provider only examines IP Prec, you seldom have a problem beyond agreeing that a IP Prec will treat the DSCP class as you desire. An example of possible treatment difference, under DSCP recommendations, CS1 and AF1x are used for scavenger and bulk traffic and may receive priority lower than best effort traffic, but under IP Prec, precedence 1 might receive higher priority than IP Prec 0 (best effort).
Under really, really old RFCs, the extra bits in the ToS byte have meanings too, but very unlikely your provider uses these rules, although wouldn't hurt to ask - they might answer huh?
If your provider wants to remark ToS, that can cause some issue as just resetting IP Prec would leave the extra bits as they were. For example, you have a packet marked as AF43, your provider remarks it to IP Prec 3, you'll then see it as AF33 not CS3. Most providers, though, usually don't remark.
On other than really old IOS images, ToS marking are copied from an original packet's ToS to the tunnel's ToS. QoS pre classify is only need when, on the tunneling router, you want to look at other header fields after the packet has been encapsulated.
I think you do not need to do remarking if your isp ip precedence match you dscp.You will need rematking if the isp does not match you dscp marking. let's say you made you defaulft class to af1 instead of zero. but if you isp has the narkping as you (which rarely happens) you do nit need remarking. so my example above using dscp ef and ip precedence 5 may not need remarking.
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