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Unicast Reverse Path Forwarding

July 17th, 2017 in ROUTE 300-101 Go to comments

Question 1

Explanation

The Unicast Reverse Path Forwarding feature (Unicast RPF) helps the network guard against malformed or “spoofed” IP packets passing through a router. A spoofed IP address is one that is manipulated to have a forged IP source address. Unicast RPF enables the administrator to drop packets that lack a verifiable source IP address at the router.

Unicast RPF is enabled on a router interface. When this feature is enabled, the router checks packets that arrive inbound on the interface to see whether the source address matches the receiving interface. Cisco Express Forwarding (CEF) is required on the router because the Forwarding Information Base (FIB) is the mechanism checked for the interface match.

Unicast RPF works in one of three different modes:
+ Strict mode: router will perform two checks for all incoming packets on a certain interface. First check is if the router has a matching entry for the source in the routing table. Second check is if the router uses the same interface to reach this source as where it received this packet on.
+ Loose mode: only check if the router has a matching entry for the source in the routing table
+ VRF mode: leverage either loose or strict mode in a given VRF and will evaluate an incoming packet’s source IP address against the VRF table configured for an eBGP neighbor.

Reference: CCIE Routing and Switching v4.0 Quick Reference, 2nd Edition

Question 2

Explanation

When Unicast Reverse Path Forwarding is enabled, the router checks packets that arrive inbound on the interface to see whether the source address matches the receiving interface.

Question 3

Explanation

First we need to understand the “allow-default” keyword here:

Normally, uRPF will not allow traffic that only matches the default route. The “allow-default” keyword will override this behavior and uRPF will allow traffic matched the default route to pass through.

In answer A, The “ip verify unicast source reachable-via rx allow-default” command under interface Fa0/0 enables uRPF strict mode on Fa0/0. Therefore traffic from the 172.16.1.0/24 network (and any traffic) can go through this interface except the 10.0.0.0/8 network because this network is matched on Fa0/1 interface only. The network 10.0.0.0/8 can only enter TUT router from Fa0/1, thus “limiting spoofed 10.0.0.0/8 hosts that could enter router”.

Question 4

Explanation

Unicast Reverse Path Forwarding (uRPF) examines the source IP address of incoming packets. If it matches with the interface used to reach this source IP then the packets are allowed to enter (strict mode).

uRPF.jpg

The syntax of configuring uRPF in interface mode is:

ip verify unicast source reachable-via {rx | any} [allow-default] [allow-self-ping] [access-list-number]
The any option enables a Loose Mode uRPF on the router. This mode allows the router to reach the source address via any interface.
The rx option enables a Strict Mode uRPF on the router. This mode ensures that the router reaches the source address only via the interface on which the packet was received.
You can also use the allow-default option, so that the default route can match when checking source address -> Answer “allow default route” is a valid option
The allow-self-ping option allows the router to ping itself -> Answer “allow self ping to router” is a valid option.
Another feature of uRPF is we can use an access-list to specify the traffic we want or don’t want to check -> Answer “allow based on ACL match” is a valid option. An example is shown below:
Router(config)#access-list 110 permit ip 192.168.1.0 0.0.0.255 any
Router(config)#interface fa0/1
Router(config-if)#ip verify unicast source reachable-via any 110
Note: Access-list “permit” statements allow traffic to be forwarded even if they fail the Unicast RPF check, access list deny statements will drop traffic matched that fail the Unicast RPF check. In above example, 192.168.1.0/24 network is allowed even if it failed uRPF check.
The last option is “source reachable via both” is not clear and it is the best answer in this case. Although it may mention about the uRPF loose mode.

Question 5

Explanation

Unicast Reverse Path Forwarding (uRPF) examines the source IP address of incoming packets. If it matches with the interface used to reach this source IP then the packets are allowed to enter (strict mode).

uRPF.jpg

Unicast RPF is enabled on a router interface. When this feature is enabled, the router checks packets that arrive inbound on the interface to see whether the source address matches the receiving interface. Cisco Express Forwarding (CEF) is required on the router because the Forwarding Information Base (FIB) is the mechanism checked for the interface match.

Unicast RPF works in one of three different modes:
+ Strict mode: router will perform two checks for all incoming packets on a certain interface. First check is if the router has a matching entry for the source in the routing table. Second check is if the router uses the same interface to reach this source as where it received this packet on.
+ Loose mode: only check if the router has a matching entry for the source in the routing table
+ VRF mode: leverage either loose or strict mode in a given VRF and will evaluate an incoming packet’s source IP address against the VRF table configured for an eBGP neighbor.

Reference: CCIE Routing and Switching v4.0 Quick Reference, 2nd Edition

This question only mentioned about “the network to which the packet’s source IP address belongs is found in the router’s FIB” so surely loose mode will accept this packet.

Question 6

Explanation

When a packet is received at the interface where Unicast RPF and ACLs have been configured, the following actions occur:
Step 1: Input ACLs configured on the inbound interface are checked.
Step 2: Unicast RPF checks to see if the packet has arrived on the best return path to the source, which it does by doing a reverse lookup in the FIB table.
Step 3: CEF table (FIB) lookup is carried out for packet forwarding.
Step 4: Output ACLs are checked on the outbound interface.
Step 5: The packet is forwarded.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/security/configuration/guide/fsecur_c/scfrpf.html

Question 7

Question 8

Explanation

The command “ip verify unicast source reachable-via any” enables uRFP in loose mode, which only checks if the router has a matching entry for the source in the routing table.

Question 9

Question 10

Comments
  1. Anonymous
    February 18th, 2017
  2. Messi
    May 14th, 2017

    What’s uRPF checking first when the packet enters the interface? or when unicast reverse path forwarding is configured on an interface.The answer should be, it is checking for a route in the table for the source.
    What if both ACL and a URPF command are present on the interface, then what would it go by first, would it drop the packet if a deny ACL is matched even if the URPF check is successfull? Is this even possible? I just ran test commands on a router, the router accepted both ingress ACL (access-group) as well as the URPF command. Any body can help ?

  3. zzz
    May 16th, 2017

    @Messi
    I have been looking into this also and this I the best info I have found.

    http://www.cisco.com/c/en/us/td/docs/ios/12_2/security/configuration/guide/fsecur_c/scfrpf.html

    When a packet is received at the interface where Unicast RPF and ACLs have been configured, the following actions occur:

    Step 1 Input ACLs configured on the inbound interface are checked.
    Step 2 Unicast RPF checks to see if the packet has arrived on the best return path to the source, which it does by doing a reverse lookup in the FIB table.
    Step 3 CEF table (FIB) lookup is carried out for packet forwarding.
    Step 4 Output ACLs are checked on the outbound interface.
    Step 5 The packet is forwarded.

  4. zzz
    May 16th, 2017

    Access Control Lists and Logging
    If an ACL is specified in the command, then when (and only when) a packet fails the Unicast RPF check, the ACL is checked to see if the packet should be dropped (using a deny statement in the ACL) or forwarded (using a permit statement in the ACL). Whether a packet is dropped or forwarded, the packet is counted in the global IP traffic statistics for Unicast RPF drops and in the interface statistics for Unicast RPF.
    If no ACL is specified in the Unicast RPF command, the router drops the forged or malformed packet immediately and no ACL logging occurs. The router and interface Unicast RPF counters are updated.
    Unicast RPF events can be logged by specifying the logging option for the ACL entries used by the Unicast RPF command. Using the log information, administrators can see what source addresses are being used in the attack, the time the packets arrived at the interface, and so on.

  5. AnyMaster
    July 9th, 2017

    What is not supported by Unicast Reverse Path Forwarding interface?
    Right Answer: Searchable both.

    Expl.: the answer ‘searchable both’, maybe, is trying to say ‘rx’ and ‘any’ that are the two possible ways uRPF works (strict and loose). You can only choose one! The other answers are supported (ping-to self , default-route, ACL).

    Let’s see the command:
    interface FastEthernet 0/0
    ip verify unicast source reachable-via {rx | any} [allow-default] [allow-self-ping] [list]

    *the only thing you MUST choose just one is {rx OR any}!

  6. AnyMaster
    July 9th, 2017

    completing my previous post… dump is saying ACL because just router 12000 cannot use that! it’s completely insane that Cisco will get to this specifics in all questions, this way it would be impossible to pass. Dump tries to find googling the easy way to find for an answer.

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    July 18th, 2017

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    July 26th, 2017

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    July 28th, 2017

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    July 28th, 2017

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  11. Guacca
    August 11th, 2017

    Q6 and Q10-

    Same questions with differing answers??

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    August 28th, 2017

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  13. Urumiko
    August 29th, 2017

    Question 6 and 10 contradict each other.

  14. Urumiko
    August 29th, 2017

    Cisco link confirms ACLs are checked first

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    November 11th, 2017

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    November 14th, 2017

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    November 21st, 2017

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  18. Corner
    November 21st, 2017

    Q6. Correct answer should be C.
    Read the question thoroughly: it’s asking what uRPF will check first.
    When a packet enters an interface, it will be tested against an inbound acl first, but that is NOT handeld by uRPF

  19. Raj7437
    November 22nd, 2017

    why people trust only with the dumps ? dumps is just for revision. Please refer books and lab videos. If everyone completes CCNP then no point of conducting the exam.

  20. Raj7437
    November 22nd, 2017

    http://www.digitaltut.com/final_flash/Unicast_Reverse_Path_Forwarding/quiz.html

    Please advise if Q3 answer is correct for A? or D?

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    December 13th, 2017

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  22. Oyama
    January 4th, 2018

    Question 6
    What’s uRPF checking first when the packet enters the interface?

    A. It checks the ingress access list
    B. It checks the egress access list
    C. Route available in FIB or it verifies a reverse patch via the FIB to the source
    D. It verifies that the source has a valid CEF adjacency

    it says right answer is A, but what if there is no ingress access list??? and by default there is not. then Option C is correct, i guess.

  23. Question 6
    January 17th, 2018

    I believe the correct answer for question 6 is C.

    The question does not specified that the packet is received on an interface that has an Access-list. If that was the case, then answer would be A.

    From How Unicast RPF works section of the below link:
    https://www.cisco.com/c/en/us/td/docs/ios/12_2/security/configuration/guide/fsecur_c/scfrpf.html#wp1000928

    “Unicast RPF checks to see if any packet received at a router interface arrives on the best return path (return route) to the source of the packet. Unicast RPF does this by doing a reverse lookup in the CEF table”

    AND…

    When a packet is received at the interface where Unicast RPF and ACLs have been configured, the following actions occur:

    Step 1 Input ACLs configured on the inbound interface are checked.

    Step 2 Unicast RPF checks to see if the packet has arrived on the best return path to the source, which it does by doing a reverse lookup in the FIB table.

    Step 3 CEF table (FIB) lookup is carried out for packet forwarding.

    Step 4 Output ACLs are checked on the outbound interface.

    Step 5 The packet is forwarded.

  24. Anonymous
    January 19th, 2018

    For Q10

    No explaination. Why answer is A ?

    I think …….B is correct. If CEF is not enabled uRPF fails as it uses FIB tables to verify source.

  25. Maja
    January 21st, 2018

    Q10
    uRPF doesn’t fail it is just having issues :)
    CEF is enabled and uRPF is working but asymmetric routing is making problems
    this is typical cisco question!

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    February 1st, 2018

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    March 22nd, 2018

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  28. EAGLE_EYE
    March 23rd, 2018

    @Digital Q7 answer in the (quiz) is (ALL) but her its (any).
    Could you please correct the quiz asap so people don’t get confused.

    CORRECT: C. interface GigabitEthernet0/0
    ip verify unicast source reachable-via any

  29. Q6 Corect
    August 6th, 2018

    The access list check it is refering to is not an interface access list, it is the access list applied to the rpf check command. uRPF allows an acceess list override filter to specify addresses that will be forwarded without checking the uRPF validity.

    If an ACL is specified in the command, then when (and only when) a packet fails the Unicast RPF check, the ACL is checked to see if the packet should be dropped (using a deny statement in the ACL) or forwarded (using a permit statement in the ACL). Whether a packet is dropped or forwarded, the packet is counted in the global IP traffic statistics for Unicast RPF drops and in the interface statistics for Unicast RPF.

  30. Dmitry
    October 23rd, 2018

    About Question 6, right answer is A:
    Step 1 Input ACLs configured on the inbound interface are checked.
    Step 2 Unicast RPF checks to see if the packet has arrived on the best return path to the source, which it does by doing a reverse lookup in the FIB table.
    Step 3 CEF table (FIB) lookup is carried out for packet forwarding.
    Step 4 Output ACLs are checked on the outbound interface.
    Step 5 The packet is forwarded.
    You can read this:
    https://www.cisco.com/c/en/us/td/docs/ios/12_2/security/configuration/guide/fsecur_c/scfrpf.html

  31. Kuiwal
    November 19th, 2018

    New urpf question on exam, I can’t remember whole question sorry but it’s something along the lines of
    “What is valid options for configuring uRPF”
    1) Enabled globally
    2) Enabled on interface
    3) both strict and loose can use the keyword any
    4) strict mode may drop legitimate traffic
    5) Can’t remember last option

  32. Kuiwal
    November 19th, 2018

    Just noticed the question i posted above is on New ROUTE Questions – Part 4 question 5 :)

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