Solved: Question about ToS

teru-lei · ‎03-20-2003

Hi All,

When we config qos in router and suppose we use ToS, so can define precedence of the traffic, so the ToS bit change and forward the traffic to next hop. But when the next hop router receive the traffic and forward to its next hop, will the router reset the ToS bits? I know that in switch, the CoS bit will be forwarded if you trust the CoS bits in the switch. But how is in router? Can anybody give me some idea? Thank You!

Best Regards

Teru Lei

bbranch · ‎03-21-2003

This depends if you are refering to TOS (IP Header) or COS (802.1p). TOS bit settings in the IP header (TOS, IP Precedence, DSCP) are maintained by routers unless a policy is implemented to change them. COS settings in Ethernet frames are not copied by routers as they are viewed as link local, they can however be matched and reset via a service policy (as can the IP Precedence, DSCP settings) using the match cos and set cos commands (http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt1/qcfcbmrk.htm#1010904).

The exception to this is Layer-3 switches which support the Trust commands, these will honour COS, TOS (IP Precedence) and DSCP (DiffServ) if configured.

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bbranch · ‎03-21-2003

This depends if you are refering to TOS (IP Header) or COS (802.1p). TOS bit settings in the IP header (TOS, IP Precedence, DSCP) are maintained by routers unless a policy is implemented to change them. COS settings in Ethernet frames are not copied by routers as they are viewed as link local, they can however be matched and reset via a service policy (as can the IP Precedence, DSCP settings) using the match cos and set cos commands (http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos_c/fqcprt1/qcfcbmrk.htm#1010904).

The exception to this is Layer-3 switches which support the Trust commands, these will honour COS, TOS (IP Precedence) and DSCP (DiffServ) if configured.