It had been a sizzling minute since I final put collectively a weblog, and I used to be eager about what is perhaps an attention-grabbing subject. Nicely, as is typical, I considered what I’d lately run throughout, or labored on, in my “day job” as a part of the engineering workforce that builds and helps the lab environments for all of the Studying at Cisco coaching supplies.
On this explicit day, I used to be reviewing the present configurations of the core community routers (layer 3 switches actually) in our information facilities. I’m pretty new to this a part of the workforce, and I used to be to find that we had been leveraging Coverage Based mostly Routing to govern the forwarding habits for several types of visitors. I’m certain lots of you studying this weblog are accustomed to the truth that there are at all times 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 typically 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 referred to as “Technically Talking… with Hank Preston” on the Cisco U. by Studying and Certifications YouTube channel. Particularly, we’re going to take a look at how to configure policy-based routing to affect forwarding habits. The why I’ll depart for one more put up. 🙂
Additionally, for anybody learning for the CCNP Enterprise certification, policy-based routing is on the ENARSI – Implementing Cisco Enterprise Superior Routing and Companies 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 an ideal subject for the Cisco Studying weblog. Let’s dive proper in!
Setting the Stage
Earlier than we take a look at altering the standard routing and forwarding habits, let’s begin with the essential forwarding habits. For this exploration, I put the under community collectively in a Cisco Modeling Labs simulation. (You’ll find the topology file right here.)
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 most effective path from R1 to R5 by R3. We are able to see that in a pair methods.
First, let’s take a look at the interface prices on R1.
R1#present ip ospf interface temporary Interface PID Space IP Tackle/Masks Price State Nbrs F/C Gi0/1.200 1 0 192.168.200.1/24 1 DR 0/0 Gi0/1.100 1 0 192.168.100.1/24 1 DR 0/0 Gi0/4 1 0 10.14.14.1/24 110 DR 1/1 Gi0/3 1 0 10.13.13.1/24 1 DR 1/1 Gi0/2 1 0 10.12.12.1/24 100 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 just one.
And now, we’ll confirm the routing desk entry for host H3 on R1.
R1#present ip route 172.16.10.11
Routing entry for 172.16.10.0/24
Identified by way of "ospf 1", distance 110, metric 3, kind intra space
Final replace from 10.13.13.3 on GigabitEthernet0/3, 00:23:02 in the past
Routing Descriptor Blocks:
* 10.13.13.3, from 5.5.5.5, 00:23:02 in the past, by way of GigabitEthernet0/3
Route metric is 3, visitors share rely 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 probably be a hint route from host H1.
H1:~$ traceroute -n 172.16.10.11
traceroute to 172.16.10.11 (172.16.10.11), 30 hops max, 46 byte packets
1 192.168.100.1 5.534 ms 5.004 ms 3.038 ms
2 10.13.13.3 5.528 ms 5.531 ms 4.137 ms <- R3's G0/1 interface
3 10.35.35.5 5.533 ms 5.656 ms 6.339 ms
4 172.16.10.11 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 bit of bit.
Suppose there was some motive that you just needed to direct visitors obtained at router R1 from host H1 destined for H3 to go by R2 . Perhaps there was some visitors evaluation that occurred on that router. Or maybe that hyperlink is extra dependable, even when slower. There are any variety of causes this would possibly come up in a community design. The important thing half is that you just don’t wish to change ALL forwarding habits, simply a few of it. You might have a “coverage,” so to talk, that identifies some visitors you wish to modify. That is the place coverage primarily based routing, also known as PBR, is available in.
Coverage primarily based routing can appear sophisticated. To be honest, if overused, it might probably make networks very sophisticated and laborious to keep up. Nonetheless, the technical fundamentals of PBR are fairly simple.
First, you want a strategy to establish the visitors that you just wish to apply the coverage to. Like many “matching” use circumstances in networking, that is typically completed with an access-list. So, right here’s the entry record that I’ll use to match the visitors I’m all in favour of.
ip access-list prolonged H1-to-H3 10 allow ip host 192.168.100.11 host 172.16.10.11
This single line prolonged ACL is all that’s wanted. I’m matching all IP visitors from H1 to H3, however I might be extra particular, to a particular port as properly. Perhaps simply internet 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” circumstances to establish the visitors and “set” circumstances to make the “coverage primarily based adjustments” to the 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 by R2 match ip deal with H1-to-H3 set ip next-hop 10.12.12.2
I’ve used the access-list I created in my “match ip deal with” command. And, I’ve indicated that when visitors “matches” this coverage, I wish to “set” the next-hop to be 10.12.12.2.
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 explicit coverage entry holds. A route-map could be made up of many coverage units – every with a “match” and “set” assertion. On this approach, community engineers can have very granular management over how visitors is forwarded within the community. Fairly helpful proper!
Earlier than I am 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 be used as a part of high quality of service (QoS) configurations, routing protocol filtering, and BGP path manipulations. So when you discover the configuration choices accessible for match and set one can find a number of different choices. Most of those are used to be used circumstances apart 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 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.
Testing the Outcomes
We’ll begin by rerunning the identical hint route command as earlier than and evaluating the outcomes.
1:~$ traceroute -n 172.16.10.11
traceroute to 172.16.10.11 (172.16.10.11), 30 hops max, 46 byte packets
1 192.168.100.1 7.306 ms 3.017 ms 3.337 ms
2 10.12.12.2 3.844 ms 4.335 ms 3.688 ms <- R2's G0/1 interface
3 10.25.25.5 7.906 ms 5.125 ms 5.962 ms
4 172.16.10.11 8.951 ms 8.912 ms 7.348 ms
Have a look at that, visitors is certainly going by R2 now. However let’s confirm that it’s only for visitors to H3 by hint routing the visitors to H4.
H1:~$ traceroute -n 172.16.10.21
traceroute to 172.16.10.21 (172.16.10.21), 30 hops max, 46 byte packets
1 192.168.100.1 3.681 ms 3.153 ms 2.563 ms
2 10.13.13.3 3.613 ms 3.185 ms 3.747 ms <- R3's G0/1 interface
3 10.35.35.5 5.957 ms 7.555 ms 5.040 ms
4 172.16.10.21 14.915 ms 7.157 ms 7.853 ms
Yep, visitors from H1 to H4 is certainly nonetheless following the “customary path” by R3. However what about visitors from H2 -> H3? Will it’s redirected by R2?
H2:~$ traceroute -n 172.16.10.11
traceroute to 172.16.10.11 (172.16.10.11), 30 hops max, 46 byte packets
1 192.168.200.1 7.284 ms 2.840 ms 3.173 ms
2 10.13.13.3 3.526 ms 4.514 ms 3.498 ms <- R3's G0/1 interface
3 10.35.35.5 6.375 ms 7.212 ms 4.900 ms
4 172.16.10.11 6.642 ms 6.270 ms 5.884 ms
Nope, solely visitors from H1 -> H3 is being redirected.
If we take a look at the routing desk on R1, we’ll see nothing has modified.
R1#present ip route 172.16.10.11
Routing entry for 172.16.10.0/24
Identified by way of "ospf 1", distance 110, metric 3, kind intra space
Final replace from 10.13.13.3 on GigabitEthernet0/3, 00:23:02 in the past
Routing Descriptor Blocks:
* 10.13.13.3, from 5.5.5.5, 00:23:02 in the past, by way of GigabitEthernet0/3
Route metric is 3, visitors share rely 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 10.12.12.2
Coverage routing matches: 12 packets, 756 bytes
This command supplies “coverage match” statistics. We are able to see that after I ran this command there have been 12 matches up to now.
One other command that’s helpful is the “debug ip coverage” command. It supplies helpful particulars in regards to the processing of the coverage as visitors flows by the router. However as with all “debug” command, watch out utilizing it on a manufacturing system as it might probably put a heavy load on community units if there’s a variety of visitors flowing by.
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 R1# *Apr 26 00:29:58.282: IP: s=192.168.100.11 (GigabitEthernet0/1.100), d=172.16.10.11, len 84, FIB coverage match *Apr 26 00:29:58.282: IP: s=192.168.100.11 (GigabitEthernet0/1.100), d=172.16.10.11, len 84, PBR Counted *Apr 26 00:29:58.282: IP: s=192.168.100.11 (GigabitEthernet0/1.100), d=172.16.10.11, g=10.12.12.2, len 84, FIB coverage routed
Evaluate the above output to the debug output after I ping H1 -> H4.
*Apr 26 00:31:00.294: IP: s=192.168.100.11 (GigabitEthernet0/1.100), d=172.16.10.21, 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 reveals that the 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.
Closing down
And with that, we’ve come to the top of this put up. I hope this quick take a look at coverage primarily based routing helped break it down and introduce you to a brand new expertise software that you may put into your toolkit. Perhaps it’ll assist you remedy a enterprise problem sometime. Or perhaps it’ll assist you in your preparation for the ENARSI examination or different research. Both approach, thanks for hanging out with me right this moment.
Obtained a subject you’d like me to breakdown? Let me know within the feedback.
Sources
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