brian dell wrote:
>
> say if cdp is enabled for an interface, then since cdp uses sap
> encapsulation (as Priscilla mentioned), then i don't think one
> configures encapsulation for that interface as sap ?
>
> the question is that why is this statement ("encap sap")in the
> configuration not needed if an interface has cdp enabled ?
> (i guess "encap arpa" is by default understood for an ethernet
> interface ? is that correct ? )
No. There's no default for an interface. There's only defaults for
particular protocols. CDP uses snap (not sap). IP uses ARPA. Spanning Tree
uses sap. Novell users novell-ether.
With the exception of Novell, VLAN tagging, and ARP, you can't change the
encapsulation that will be used for Ethernet frames for the various protocols.
Try it on a real router. It's nothing like changing encapsulation on a WAN,
which causes all traffic across the WAN link to use that encapsulation. Here
are some hints:
Albany#config t
Enter configuration commands, one per line. End with CNTL/Z.
Albany(config)#ipx routing
Albany(config)#int e0
Albany(config-if)#ipx network 400 encapsulation ?
arpa Novell Ethernet_II
hdlc HDLC on serial links
novell-ether Novell Ethernet_802.3
sap IEEE 802.2 on Ethernet, FDDI, Token Ring
snap IEEE 802.2 SNAP on Ethernet, Token Ring, and FDDI
Albany(config-if)#ipx network 400 encapsulation snap
Albany(config-if)#ipx network 100 encapsulation arpa secondary
Albany(config-if)#ipx network 200 encapsulation sap secondary
Albany(config-if)#ipx network 300 encapsulation novell-ether secondary
Albany#config t
Enter configuration commands, one per line. End with CNTL/Z.
Albany(config)#int e0
Albany(config-if)#arp ?
arpa Standard arp protocol
frame-relay Enable ARP for a frame relay interface
probe HP style arp protocol
snap IEEE 802.3 style arp
timeout Set ARP cache timeout
Albany(config-if)#arp snap
If you try just the encapsulation command in interface configuration mode,
you get a choice of VLAN tagging methods. I can't show you that because my
routers don't support it. VLAN tagging is a topic for another disertation,
not really related to the question you are asking.
Priscilla
>
>
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