Ethernet Urpf stuff to note

ETHERNET:

·       IEEE documentation lists Ethernet addresses with the most significant byte on the left. However, inside each byte, the leftmost bit is the least significant bit, and the rightmost bit is the most Significant bit. Many documents refer to the bit order as canonical; other documents refer to it as little-endian.

·       Duplex mismatch:  intermittent or poor connectivity, collisions both late and excessive.

·       Collisions, Runt, CRC errors are normal for 100Mbs half-duplex connections.

·       802.3z fiber Gige 1000BASE-X use flow control, pause packet 01:80:C2:00:00:01 to overcome buffer overflow.

·       802.3ab copper Gige 1000Base-T

·       802.3ah added two more Gigabit fiber standards, 1000BASE-LX10 and 1000BASE-BX10 (Ethernet in the first mile)

·       Auto on switch, hard coded on the station results in duplex mismatch; auto defaults to half duplex.

·       Runt packets that are smaller than allowed size, 64 bytes in Ethernet, usually caused by duplex mismatch.

·       Ether-channels; on, off do not send any PAgP packets, auto and desirable do.

·       Auto and auto will NOT form ether-channel, neither port initiates negotiation.

·       On and on mode will form ether-channel although they don’t exchange PAgP packets.

·       EtherTypes.

o    0x0800              IP, Internet Protocol.

o    0x0806              ARP, Address Resolution Protocol.

o    0x0808              Frame Relay ARP.

o    0x8035              RARP, Dynamic RARP.

o    0x814C              SNMP, Simple Network Management Protocol.

o    0x86DD              IPv6, Internet Protocol version 6.

o    0x8808              MPCP, Multi-Point Control Protocol.

o    0x880B              PPP, Point-to-Point Protocol.

o    0x8847              MPLS, Multi-Protocol Label Switching (unicast).

o    0x8848              MPLS, Multi-Protocol Label Switching (multicast).

o    0x8863              PPPoE, PPP Over Ethernet (Discovery Stage).

o    0x8864              PPPoE, PPP Over Ethernet (PPP Session Stage).

o    0xAAAA              Cisco Proprietary; STP, PAgP, VTP, PVST+, CDP, DTP, UDLD

<arch>-<feature set>-<type>.<major minor rev>-<micro rev>

c7200-js-mz.121-3a.T1

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uRPF:

·       URPF is an input function and applies only to the input interface of a router.

·       If the packet was received from one of the best reverse path routes, the packet is forwarded as normal.

·       URPF considers all equal-cost best return paths to be considered valid as long as cost and FIB match.

·       Dependent on ip cef since FIB is generated by CEF.

·       If there is no reverse path route on the same interface from which the packet was received, it might mean that the source address was modified.  If Unicast RPF does not find a reverse path for the packet, the packet is dropped or forwarded, depending on whether an access control list (ACL) is specified in the ip verify unicast reverse-path interface configuration command.

·       When administrators use Unicast RPF in strict mode, the packet must be received on the interface that the router would use to forward the return packet.

·       Unicast RPF configured in strict mode may drop legitimate traffic that is received on an interface that was not the router's choice for sending return traffic. Dropping this legitimate traffic could occur when asymmetric routing paths are present in the network.

·       When administrators use Unicast RPF in loose mode, the source address must appear in the routing table.

·       Administrators can change this behavior using the allow-default option, which allows the use of the default route in the source verification process.

·       Additionally, a packet that contains a source address for which the return route points to the Null0 interface will be dropped.

·       An access list may also be specified that permits or denies certain source addresses in Unicast RPF loose mode.

·       uRPF does not inspect packets encapsulated in tunnels such as GRE, L2TP, PPTP. uRPF needs to be configure at the gateways after the encapsulation and encryption layers have been stripped off the packets.

#ip cef                                                # required for urpf, needs fib.

#interface Serial 0/0/0

 #ip verify unicast reverse-path [acl]                 # configures urpf, legacy.

 #ip verify unicast source reachable-via (any [allow-default] | rx)  # rx strict any loose mode.

R2#sh cef int gi0/0

R2#sh ip int gi0/0