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Influencing Forwarding Habits with Coverage Based mostly Routing

It had been a scorching minute since I final put collectively a weblog, and I used to be interested by what is likely to be an fascinating matter. Properly, as is typical, I thought of what I’d not too long ago run throughout, or labored on, in my “day job” as a part of the engineering staff that builds and helps the lab environments for all of the Studying at Cisco coaching supplies.

On this specific day, I used to be reviewing the present configurations of the core community routers (layer 3 switches actually) in our knowledge facilities. I’m pretty new to this a part of the staff, and I used to be to find that we had been leveraging Coverage Based mostly Routing to control the forwarding conduct for various kinds of site visitors. I’m positive a lot of you studying this weblog are accustomed to the truth that there are all the time a number of methods to perform a job in networking (life actually, however undoubtedly in networking). As such, policy-based routing is a software within the community engineer’s toolkit that may usually be leveraged to deal with “odd enterprise necessities.”

And with that, I had a subject to make use of for this weblog and an accompanying video to kick off a brief video sequence known as “Technically Talking… with Hank Preston” on the Cisco U. by Studying and Certifications YouTube channel. Particularly, we’re going to have a look at how to configure policy-based routing to affect forwarding conduct. The why I’ll go away for one more submit. 🙂

Additionally, for anybody learning for the CCNP Enterprise certification, policy-based routing is on the ENARSI – Implementing Cisco Enterprise Superior Routing and Providers blueprint – “1.6 Configure and confirm policy-based routing.” 300-410 ENARSI is a focus examination that earns you the Cisco Licensed Specialist – Enterprise Superior Infrastructure Implementation certification.  So, it’s undoubtedly a terrific matter for the Cisco Studying weblog. Let’s dive proper in!

Setting the Stage

Earlier than we have a look at altering the standard routing and forwarding conduct, let’s begin with the fundamental forwarding conduct. For this exploration, I put the under community collectively in a Cisco Modeling Labs simulation. (You will discover the topology file right here.)

Network Toplogy
The community topology used on this exploration of coverage primarily based routing and forwarding conduct.

This community has two small LANs separated by a fundamental, single space OSPF community within the center. The prices within the OSPF community have been configured to make the very best path from R1 to R5 via R3. We will see that in a pair methods.

First, let’s have a look at the interface prices on R1.

R1#present ip ospf interface transient 

Interface    PID   Space            IP Tackle/Masks    Price  State Nbrs F/C
Gi0/1.200    1     0        1     DR    0/0
Gi0/1.100    1     0        1     DR    0/0
Gi0/4        1     0           110   DR    1/1
Gi0/3        1     0           1     DR    1/1
Gi0/2        1     0           100   DR    1/1

Discover the prices for interface G0/2 and G0/4 (in direction of R2 and R4) have a value of 100 and 110 respectively, whereas the price of G0/3 (in direction of R3) is only one.

And now, we’ll confirm the routing desk entry for host H3 on R1.

R1#present ip route   

Routing entry for
  Identified through "ospf 1", distance 110, metric 3, sort intra space
  Final replace from on GigabitEthernet0/3, 00:23:02 in the past
  Routing Descriptor Blocks:
  *, from, 00:23:02 in the past, through GigabitEthernet0/3
      Route metric is 3, site visitors share depend is 1

The routing desk lists the route as in direction of R3 out interface G0/3 — precisely as we’d count on.

The ultimate verify will likely be a hint route from host H1.

H1:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
 1   5.534 ms  5.004 ms  3.038 ms
 2      5.528 ms  5.531 ms  4.137 ms       <- R3's G0/1 interface
 3      5.533 ms  5.656 ms  6.339 ms
 4   14.180 ms  9.787 ms  7.908 ms

And no huge shocker right here, the second hop within the hint is certainly R3.

Let’s shake issues up a little bit bit.

Suppose there was some cause that you simply needed to direct site visitors acquired at router R1 from host H1 destined for H3 to move via R2 . Possibly there was some site visitors evaluation that occurred on that router. Or maybe that hyperlink is extra dependable, even when slower. There are any variety of causes this may come up in a community design. The important thing half is that you simply don’t wish to change ALL forwarding conduct, simply a few of it. You may have a “coverage,” so to talk, that identifies some site visitors you wish to alter. That is the place coverage primarily based routing, also known as PBR, is available in.

Coverage primarily based routing can appear sophisticated. To be truthful, if overused, it will possibly make networks very sophisticated and exhausting to keep up. Nonetheless, the technical fundamentals of PBR are fairly easy.

First, you want a technique to establish the site visitors that you simply wish to apply the coverage to. Like many “matching” use instances in networking, that is usually finished with an access-list. So, right here’s the entry checklist that I’ll use to match the site visitors I’m eager about.

ip access-list prolonged H1-to-H3
  10 allow ip host host

This single line prolonged ACL is all that’s wanted. I’m matching all IP site visitors from H1 to H3, however I may very well be extra particular, to a particular port as nicely. Possibly simply internet site visitors (tcp/80 & tcp/443) for instance.

Subsequent, a route-map is used to describe the coverage that we wish to configure. The “coverage” is made up of “match” situations to establish the site visitors and “set” situations to make the “coverage primarily based modifications” to the site visitors that was matched.

Right here is the route-map for my coverage instance.

route-map POLICY-BASED-ROUTING allow 10
  description Site visitors from H1 -> H3 route via R2
  match ip deal with H1-to-H3
  set ip next-hop

I’ve used the access-list I created in my “match ip deal with” command. And, I’ve indicated that when site visitors “matches” this coverage, I wish to “set” the next-hop to be

And see the primary line within the configuration instance. It ends with the quantity “10.” This quantity identifies the place within the route map that this specific coverage entry holds.  A route-map could be made up of many coverage units – every with a “match” and “set” assertion.  On this manner, community engineers can have very granular management over how site visitors is forwarded within the community.  Fairly helpful proper!

Earlier than I’m going a lot farther it’s undoubtedly necessary to notice that route-maps are used for extra than simply coverage primarily based routing.  The route-map assemble can also be used as a part of high quality of service (QoS) configurations, routing protocol filtering, and BGP path manipulations.  So if you happen to discover the configuration choices obtainable for match and set you will see that a number of different choices.  Most of those are used to be used instances aside from coverage primarily based routing.

The final step to finish the configuration of my coverage is to use it to the router interface. Since this coverage is about controlling site visitors from the LAN related to interface Gig0/1 on R1, that’s the place I’ll apply it.

interface Gig0/1.100
  ip coverage route-map POLICY-BASED-ROUTING

That’s it, we’ve configured coverage primarily based routing. Let’s take a look at to see if it’s working.

We’ll begin by rerunning the identical hint route command as earlier than and evaluating the outcomes.

1:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
 1  7.306 ms  3.017 ms  3.337 ms
 2     3.844 ms  4.335 ms  3.688 ms      <- R2's G0/1 interface
 3     7.906 ms  5.125 ms  5.962 ms
 4   8.951 ms  8.912 ms  7.348 ms

Have a look at that, site visitors is certainly going via R2 now. However let’s confirm that it’s only for site visitors to H3 by hint routing the site visitors to H4.

H1:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
 1  3.681 ms  3.153 ms  2.563 ms
 2     3.613 ms  3.185 ms  3.747 ms     <- R3's G0/1 interface
 3     5.957 ms  7.555 ms  5.040 ms
 4  14.915 ms  7.157 ms  7.853 ms

Yep, site visitors from H1 to H4 is certainly nonetheless following the “commonplace path” via R3. However what about site visitors from H2 -> H3?  Will or not it’s redirected via R2?

H2:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
 1  7.284 ms  2.840 ms  3.173 ms
 2     3.526 ms  4.514 ms  3.498 ms    <- R3's G0/1 interface
 3     6.375 ms  7.212 ms  4.900 ms
 4   6.642 ms  6.270 ms  5.884 ms

Nope, solely site visitors from H1 -> H3 is being redirected.

If we have a look at the routing desk on R1, we’ll see nothing has modified.

R1#present ip route   

Routing entry for
  Identified through "ospf 1", distance 110, metric 3, sort intra space
  Final replace from on GigabitEthernet0/3, 00:23:02 in the past
  Routing Descriptor Blocks:
  *, from, 00:23:02 in the past, through GigabitEthernet0/3
      Route metric is 3, site visitors share depend is 1

There are a couple of helpful instructions on the router to verify the standing of coverage primarily based routing.

First up, a fundamental “present” command price noting.

R1#present route-map 

route-map POLICY-BASED-ROUTING, allow, sequence 10
  Match clauses:
    ip deal with (access-lists): H1-to-H3 
  Set clauses:
    ip next-hop
  Coverage routing matches: 12 packets, 756 bytes

This command offers “coverage match” statistics. We will see that once I ran this command there have been 12 matches thus far.

One other command that’s helpful is the “debug ip coverage” command. It offers helpful particulars in regards to the processing of the coverage as site visitors flows via the router. However as with every “debug” command, watch out utilizing it on a manufacturing gadget as it will possibly put a heavy load on community units if there’s a number of site visitors flowing via.

I’ll activate the debugging after which ship a single ICMP (ping) packet from H1 -> H3.

R1#debug ip coverage
Coverage routing debugging is on

*Apr 26 00:29:58.282: IP: s= (GigabitEthernet0/1.100), d=, len 84, FIB coverage match
*Apr 26 00:29:58.282: IP: s= (GigabitEthernet0/1.100), d=, len 84, PBR Counted
*Apr 26 00:29:58.282: IP: s= (GigabitEthernet0/1.100), d=, g=, len 84, FIB coverage routed

Evaluate the above output to the debug output once I ping H1 -> H4.

*Apr 26 00:31:00.294: IP: s= (GigabitEthernet0/1.100), d=, len 84, FIB coverage rejected(no match) - regular forwarding

Within the first instance, “FIB coverage match” signifies that the PRB coverage was triggered. And a following debug line exhibits that the site visitors was “FIB coverage routed.” That’s the PBR in motion. Evaluate that to the output from the second ping that’s “FIB coverage rejected (no match) – regular forwarding.” That output is fairly descriptive.

And with that, we’ve come to the top of this submit. I hope this brief have a look at coverage primarily based routing helped break it down and introduce you to a brand new know-how software which you could put into your toolkit. Possibly it’ll aid you remedy a enterprise problem sometime. Or perhaps it’ll aid you in your preparation for the ENARSI examination or different research. Both manner, thanks for hanging out with me immediately.

 Acquired a subject you’d like me to breakdown? Let me know within the feedback.



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