Saturday , November 18 2017
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IP Routing

RIP loop protection

I am going to show you how RIP loop protection works. Let’s take the topology from the previous post to demonstrate the features.

RIP topology

There are 2 methods of loop protection: Split Horizon and Poison Reverse.

Split Horizon

It prohibits a router to advertise a route back to neighbors, through the interface that receives the routes. Split Horizon is enabled by default on Huawei routers:

[R1]display rip 1 interface Serial 0/0/0 verbose
Serial0/0/0(192.168.20.1)
State : UP MTU : 500
Metricin : 0
Metricout : 1
Input : Enabled Output : Enabled
Protocol : RIPv2 Multicast
Send version : RIPv2 Multicast Packets
Receive version : RIPv2 Multicast and Broadcast Packets
Poison-reverse : Disabled

Authentication type : None
Replay Protection : Disabled

Split Horizon is disabled

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basic concept of RIP

Routing Information Protocol RIP is an interior gateway protocol IGP, implemented based on a distance-vector algorithm.

RIPv1 characteristics
  1. A classful routing protocol
  2. Uses broadcast address to update routes
  3. Uses UDP port 520 to send and receive packets

Let’s look at the captured request and response packets of RIPv1:

RIPv1_request

RIPv1_response

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OSPF stub area on Huawei router

Instead of transmitting learned AS external routes, area border router, in a stub area, generates a default route and advertises the route to non-ABRs in the stub area. In short, stub area reduces entries in the routing table of ABR and the amount of routing information to be transmitted.

We have to remember that:

  • The backbone area cannot be a stub area
  • All routers in a stub area need to be configured using stub attributes
  • The ASBR cannot exist in a stub area
  • Virtual links cannot be configured in stub area.

Let’s try to configure a simple lab. We would like to see what happens if AREA1 becomes a stub area.

  1. Based on the topology, configure IP address of each interface.
  2. Enable OSPF on each router and configure basic OSPF functions
  3. Configure AREA1 as stub and and check routing information on router Gdansk.
  4. Check routing information on router Gdansk, previously stopping advertising type 3 LSA into the stub area.

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routing policy configuration

Some time ago I wrote about local PBR and interface PBR.

It’s time to talk about routing policy, that is a different mechanism. Routing policy is applied to routing information and it is combined with routing protocols to form policies. PBR mechanism is applied to data flows and and packets are forwarded according to the configured policy.

Routing policy is a tool which can be used to filter routes and set route attributes, when importing routing information into OSPF, RIP, ISIS or BGP protocols. BGP can use routing policy to filter advertising routes as well. Routing policy defines which of the routes from the specific routing protocol are allowed to be imported into the target routing protocol. It can be also used to match routes or certain route attributes and to change these attributes when the matching rules are met.

Routing policy command syntax:
route-policy route-policy-name { permit | deny } node node

A route-policy may consists of multiple nodes, for example:

route-policy LABNARIO-POLICY permit node 10
route-policy LABNARIO-POLICY deny node 20

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IP FRR on Huawei routers

What do we have in traditional IP networks?

Let’s assume that there is a fault at the physical or data link layers. Router sees that a physical interface becomes DOWN. After the router detects this fault, it informs upper layer routing system to update routing information. The convergence time is several seconds, what is critical for sensitive services.

That’s why IP FRR has been developed. After we configure IP FRR, a router doesn’t wait for network convergence but a backup link is immediately used to forward packets.

We have 2 scenarios of using IP FRR:

  1. To protect routers in public networks.
  2. To protect CE routers in private networks.

Let’s focus on the first one.

IP FRR topology

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