12-09-2019
02:56 PM
- last edited on
12-09-2019
03:12 PM
by
Hilda Arteaga
This topic is a chance to clarify your questions about the configuration, verification, troubleshooting and general best current practices of dynamic routing protocols. All questions regarding to general design, flooding, in-depth mechanics, and features of RIP, OSPF, IS-IS EIGRP, and BGP are welcome.
Dynamic routing protocols are a fundamental piece for internetworking, their main function is to provide the mechanisms to generate, process, and distribute topological and network layer reachability information (NLRI). Having an in-depth knowledge of routing protocols is key to understand modern overlay solutions such as Cisco DNA and a plethora of SDN designs, since these use some form of underlying routing form to perform connections under the hood.
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12-09-2019 05:02 PM
Hello Guru's,
How can I discover what vlans are traversing a particular VRF across my wan? I issued the sh ip route vrf vrfName command and got a ton of subnet ip addresses and physical egress interfaces, but I need to distinguish which vlans are flowing out of this edge router out into the WAN. Thanks for any help.
12-16-2019 07:21 AM
Hi telestrial2020,
Thank you for your question!
Regularly this information should be known via an updated network diagram that reflects the connections and indicates what VLANs exist at each point of the network. Network diagrams and flow diagrams will be the sources of information where this is reflected.
If this does not exist, a way is by checking the associated interfaces within the VRF [show vrf / show running-config vrf <VRF>]. You will need to verify the subinterface and observe it on the local router, the subinterface will indicate what is the corresponding encapsulation.
Of course, querying the router using some kind of network management system would be ideal, like Cisco Prime / Prime Infrastructure among others.
As an additional note on this subject, there is also another VRF-to-VLAN application named Easy Virtual Networking Network (EVN), where you can create trunks of VRFs, and via this application, is easier to map and visualise from the local router perspective what VRFs corresponds to each VLAN by the virtue of what is called vnet trunks.
A white paper on this technology can be found here, https://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/layer-3-vpns-l3vpn/whitepaper_c11-638769.html
HTH,
Elvin
12-22-2019 06:19 AM
Elvin,
Thank you so much for taking the time to answer this question. Yes, all of your first three suggestions are valid. Generally, if accurate documentation exists this question shouldn't, but in its absence, the cli command, or a polished NMS like the one you suggested may illuminate things. We are very grateful - me and the community, I'm sure - for you exposing us to the EVN technological principles in the attached white paper. Cheers!
12-12-2019 12:09 PM
WAN Details From ISP:
WAN IP: 136.xxx.xxx.206
GATEWAY IP: 136.xxx.xxx.205
SUB-NET MASK: 255.255.255.252
DNS:49.xx.xx.1
I want to use above mentioned IP to setup internet Connection.
Question: How do we enter Default gateway and Dns for interface "GigabitEthernet1"
LAN IP Pool provided by ISP.
IP1: 115.XXX.X.209 Need to use this IP for gateway for below mentioned LAN IPs.
IP2: 115.XXX.X.210 Need to assign to a Client PC on network behind router.
IP3: 115.XXX.X.211 Need to assign to a Client PC on network behind router.
IP4: 115.XXX.X.212 Need to assign to a Client PC on network behind router.
IP5: 115.XXX.X.213 Need to assign to a Client PC on network behind router.
IP6: 115.XXX.X.214 Need to assign to a Client PC on network behind router.
Subnet Mask: 255.255.255.248
Then i guess we'll need NAT between WAN IP and LAN IPs
I am sorry i am new to Cisco configuration so bear with me.
12-17-2019 07:00 AM
Hi Sanga62,
The steps of how to perform this type of task will depend of many variables, I will create an example that will translate traffic from internal LAN towards the Internet. Note that based on the IP addresses you have, you want to assign public space to internal client PCs behind the router, which is rather rare, but still possible, as this is usually perform with server devices.
Since this design involves having public space at the internal LAN, the public space does not need to be NATted in order to be Internet-routable.
1. Settings:
WAN Details From ISP:
WAN IP: 136.xxx.xxx.206
GATEWAY IP: 136.xxx.xxx.205
SUB-NET MASK: 255.255.255.252
DNS:49.xx.xx.1
IP1: 115.XXX.X.209 Need to use this IP for gateway for below mentioned LAN IPs.
IP2: 115.XXX.X.210 Need to assign to a Client PC on network behind router.
IP3: 115.XXX.X.211 Need to assign to a Client PC on network behind router.
IP4: 115.XXX.X.212 Need to assign to a Client PC on network behind router.
IP5: 115.XXX.X.213 Need to assign to a Client PC on network behind router.
IP6: 115.XXX.X.214 Need to assign to a Client PC on network behind router.
Subnet Mask: 255.255.255.248
2. Topology:
Three routers connected in a line.
LAN --- GW --- ISP
3. Configuration
#LAN hostname LAN ! interface Ethernet0/0.12 description LAN PC encapsulation dot1Q 12 ip address dhcp
/End.
#GW
hostname GW ! ip dhcp excluded-address 115.0.0.209 ! ip dhcp pool ADD network 115.0.0.208 255.255.255.248 default-router 115.0.0.209 dns-server 49.0.0.1 ! interface Ethernet0/0.12 description TO INTERNAL CLIENTS_PCS encapsulation dot1Q 12 ip address 115.0.0.209 255.255.255.248 ! interface Ethernet0/0.23 description INTERNET_IFACE encapsulation dot1Q 23 ip address 136.0.0.206 255.255.255.252 ! ip route 0.0.0.0 0.0.0.0 136.0.0.205 name TO_ISP
#ISP
(Of course, we cannot configure the ISP, but this is a functional example with three routers, I will post the configuration of the ISP).
hostname ISP ! interface Loopback0 ip address 49.0.0.1 255.255.255.255 ! interface Ethernet0/0.23 encapsulation dot1Q 23 ip address 136.0.0.205 255.255.255.0 ! ip route 115.0.0.0 255.0.0.0 136.0.0.206 ! end
/
Let us know any further questions.
Elvin
12-16-2019 10:59 AM
Hello,
i need a good way to do mutual redistribution between IGP with tags
Thanks
12-17-2019 08:46 AM
12-18-2019 01:10 AM
Hello @Elvin Arias ,
very nice post we were writing at the same time :)
+5 for you
>> Note that when doing routing to destinations, routers will always pick the longest match
The concept of routing most important is the longest = more specific prefix is chosen first you are right on this.
A lot of people think at AD as the first parameter, but CEF, FIB and TCAM works in such a way (Cisco has its own US patents on this) that they are able to match the destination address A.B.C.D of an Ipv4/IPv6 flow with the most specific prefix present in CEF / FIB /TCAM the search is performed in parallel way an entry in the TCAM table is found from the most specific route.
The TCAM entry will tell us what is the next-hop for destination A.B.C.D
>> Route preference is based on a local value known as Administrative Distance, if same prefix needs to be evaluated, the prefix with the lowest administrative distance is chosen. Values in IOS are set from 0 through 255, where some 255 is infinite (routes with this value will not be installed).
Perfect explanation Cisco choices of AD is explained.
A complete list of AD values is reported in Cisco documents
A good doc can be the following link
Connected : AD = 0 code C
L /32 means primary IP address on the connected subnet
Static to an IP next-hop (to be used if the exit interface is a LAN segment or a form of physical NBMA like ATM or Frame Relay or a form of logical NBMA like DMVPN) AD=1
Static to interface in the past here the AD was 0. To be noted if used in the following way
ip route 0.0.0.0.0 0.0.0.0 eth0
requires the next router to have IP proxy enabled and causes the branch router ARP table to grow over time defeating the CEF /TCAM optimazations. There were threads about this answered by Jon Marshall, Peter Paluch or Rick Burts or Paolo Bevilaqua.
>> When doing redistribution, routes are taken from protocol a to protocol b, and this is performed from a routing table perspective, the prefix must exist in the router's RIB before being injected into the particular IGP
Correct IGP A will send a prefix to IGP B only if this prefix is installed in the IP routing table (with the execption of connected routes in IPv4, special commands are needed in IPv6 to emulate this)
>> EIGRP uses EIGRP RID to prevent route feedback when redistributing external routes (also this has been recently included for internal routes).
This is true and explained in Peter Palùch's book CCIE R/S Vol I 5th edition.
when OSPF receives routes the subnets keyword is important to avoid classful summarization so I would add a subnets keyword in your example.
Have a nice day !
Hope to help
Giuseppe
12-18-2019 04:36 AM
Indeed, we were in sync on the posts. : )
Thank you for your comments, appreciate it.
Elvin
12-23-2019 01:19 AM
Hello Elvin,
Thank you very much
12-17-2019 09:07 AM
Hello @sobasamst ,
for route tags and IGPs there is not a single case.
Let me recap what an IGP route tag is :
Protocol Route tag Value
EIGRP single value unsigned integer 32 bit
OSPF single value unsigned integer 32 bit
IS-IS single value unsigned integer 32 bit
RIPv2 single value unsigned integer 16 bit (as far as I know from my personal study notes and labs)
So a lot of possible cases arise.
First of all, ,mutual redistribution is not always needed an IGP the modern one can be the core/backbone protocol the other protocol can be seen as a leaf and just needs to receive a default route 0.0.0.0/0 in its messages from one or two ASBR ( I use OSPF terms here).
In case of merges or migration you may need mutual redisribution on two network devices in a controlled way.
Here, come into play some specific properties of each IGP
OSPF to OSPF work as "ships in the night" each OSPFv2 process needs to have a different process-id and a different router-id (used in SP env)
OSPF route type plays no role here and both processes compete for installing their own best routes in the IP routing table .
The IP routing table daemon sees [110/x] [110/y] problems can arise. It can be solved with route tags using a negative logic and by using the OSPF command distance ospf inside each process to tune AD value for O , OIA and O E1, O E2, O N1 , O N2 routes.
(used successfully on the field in 2005 to merge two SP networks)
EIGRP has admin cost 150 for external routes so EIGRP to OSPF mutual for example is not an issue.
IS-IS is like OSPF (with OSI net in place of router-id, the capability to support up to 3 different areas)
However, EIGRP summary route has AD 5 by default and this can create problems when the 0.0.0.0/0 route is created towards the edge protocol with a per interface summarization 0.0.0.0/0.
I have seen this issue too in the field.
Also in EIGRP named mode you can increase the AD of the 0.0.0.0/0 injected route to 200 to avoid a routing blackhole.
We cannot copy a 32 bit value into RIPv2.
So when dealing with RIPv2 we need to take in account this.
IGRP and RIPv1 I think miss the concept totally they are legacy.
A typical config for two OSPF processes will be :
route-map OSPF1-to-OSPF6762 deny 10
match tag 999
route-map OSPF1-to-OSPF6762 permit 20
set tag 999
route-map OSPF6762-to-OSPF1 deny 10
match tag 50
route-map OSPF6762-to-OSPF1 permit 20
set tag 50
router ospf 1
router-id 1.1.1.1
redistribute subnets route-map OSPF6762-to-OSPF1 metric-type 1
distance-ospf 105 108 150
router ospf 6762
router-id 192.168.1.1
redistribute subnets route-map OSPF1-to-OSPF6762 metric-type 1
distance-ospf 110 120 110
Hope to help
Giuseppe
12-19-2019 02:51 AM
Hello,
I am trying to simulate a real scenario with GNS3. Attached you will find the network diagram for further details.
All routers are running OSPF (point-to-point), except the uplinks of routers R4 & R5 that connect to the Internet provider. In this case, HSRP is configured.
When issuing "show ip route" from R2 to another "outside" network (not shown on the diagram), the next hop is 192.168.10.2 (R4).
However, if I issue the same command from R2 but changing the destination by the IP of the internet provider (200.10.1.4), there are two paths (same metric), one to R4 and another to R5. The one to R5 is the "active".
I am having some problems to understand why it exist this difference. OK, the second case won´t happen because the dst IP will never be the internet provider but it would be interesting to receive some feedback.
To understand better the diagram, I put the different OSPF costs on each link. For example, on the link between R4-R5, the OSPF costs are 10 on the R4 side and 20 on the R5 side. If the cost matches, for example between R2-R4, just put the cost (in this case, 1).
Thanks!
12-19-2019 03:01 AM
I forgot to mention that the R4 is the active and the R5 the standby for HSRP
12-20-2019 05:04 PM
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