EIGRP IP FRR & OSPF LFA

In today's modern networks, fast convergence has become a mandatory requirement.
If we want to achieve fast convergence, each of the steps below need to be optimized

1. Failure Detection
2. Failure Propagation
3. Processing of new information
4. Updating RIB/FIB

1. Failure Detection:- "How long does it take me to detect a failure?"
This normally depends on the Hello/Hold down/Dead timers of the routing protocol. We can either tune these timers or use a mechanism such as "BFD" which we have seen in our earlier post.

2. Failure Propagation:- "How long does it take me to tell everyone else?"
In EIGRP, this is done through Query/Reply packets. We can reduce the Query domain by configuring the routers as "stub".

In OSPF, this depends on the area size and the LSA flooding procedure. We can tune the LSA timers to change this.

Carrier Supporting Carrier (CSC)

In this post we will look at the Carrier Supporting Carrier design where smaller service providers use large service providers as backbone in order to connect parts of their network which eliminates the need to build and maintain their own MPLS network.

From customer's point of view, there is no difference in terms of connectivity and it will still appear as they have a normal Layer 3 MPLS connection from the provider.

Let's look at the below topology to understand how it works.

We have a Tier 2 SP who is providing services to customer sites in two different geographical locations. The service provider have their own network within specific regions but not connected with each other directly hence they are using Tier 1 SP's backbone to connect both of their networks and provide end-to-end connectivity to the customer.

Inter-AS MPLS VPN - Option C (BGP+Label)

In this post, we will look into the Inter-AS MPLS VPN - Option C which is also known as "BGP + Label". 

Option C uses eBGP IPv4 session between ASBRs to exchange reachability to PE loopbacks. There will be a VPNv4 neighbourship between service providers VPNv4 route-reflectors (RRs).

Option C takes away the heavy reliance on ASBRs. In this case, ASBRs are only used to exchange the loopback prefixes using eBGP IPv4 sessions.

To understand how it works, let's look at our topology below.

We have two service providers. Both are connected through ASBR routers. In this specific instance, we have local VPNv4 route-reflector for each SP. The customer site CE1 is connected to SP1 and CE2 is connected to SP2.

Inter-AS MPLS VPN - Option B (VPNv4 EBGP between ASBRs)

So far we have seen the Inter-AS MPLS VPN using "back to back VRF" method. Even though it's a relatively easy method to implement, it has few drawbacks.

This procedure doesn't require MPLS enabled on the link between ASBRs however it does not scale very well. 

There is another method in which ASBR's establish VPNv4 neighbourship, exchange MPLS labels and can maintain end-to-end LSP. It is know as "Option B".

Let's see how it works. We are going to use the same topology as we used in last post.

The IP addressing and the core routing remains the same. The only difference is the configuration on ASBR where I have removed the EBGP neighbourship statement and removed the local VRF configuration.

Inter-AS MPLS VPN - Option A (Back to Back VRF)

We have seen how standard L3 MPLS works in the previous post http://ciskonetwork.blogspot.co.uk/2015/03/layer-3-mpls-vpn.html. 

In normal implementation, customer sites are served by a single service provider. The PE routers maintain an IBGP session and the routing information gets exchanged through VPNv4 neighbourship.

What if two customer sites are connected through different service providers? The PE routers of each service provider will not be able to establish IBGP neighbourship with each other hence won't be able to exchange VPNv4 routes.

There are mainly three ways to handle this type of a situation. One of three methods is called "Option A - Back to Back VRF".

IPv6 NAT-PT

In the recent posts, we discussed how we can transport IPv6 traffic over IPv4 backbone using the tunneling techniques. 

These techniques will not work if we have one part of network which can only run IPv4 and the other part can only run IPv6. Since IPv4 and IPv6 are not compatible with each other, we have to use some sort of translation mechanism. The way we can achieve this is through NAT-PT (NAT protocol translation).

Let's look at the below topology and see how NAT-PT can be implemented.

We have three routers R1, R2 and R3. We are running RIPng and RIPv2 between R1-R2 and R2-R3 respectively. R1 is only aware about IPv6 addresses, similarly R3 is only aware about IPv4 addresses.

Our goal is to establish reachability between the IPv6 loopback (2001:1111::1/128) of R1 to IPv4 loopback (3.3.3.3/32) of R3.

IPv6 tunneling over IPv4 - 6to4 Autotunnel

In the previous post, we looked at the IPv6 tunneling technique using IPv4 manual tunnels. We will now see how dynamic multipoint IPv6 tunnel works.

6to4 tunnels allow for the dynamic creation of IPv6 within IPv4 tunnels. Manual tunnel is normally used as point-to-point however 6to4 tunnels can be point-to-multipoint.

The 2002::/16 range has been reserved to use for tunneling. We will have to add the IPv4 address into this range to generate /48 prefix. i.e. if the IPv4 address of the border router is 192.168.1.1 then the 6to4 site address prefix becomes 2002:C0A8:101::/48 (Convert 192.168.1.1 to Hex which results in C0A8:0101)

We will work on the same topology we used in the previous post.

IPv6 tunneling over IPv4 - Manual Tunnel

We know that IPv4 and IPv6 addresses are not compatible with each other. Let's say the existing infrastructure is running on IPv4. If we need to implement IPv6 on few of the hosts, we need to use some sort of a technique for IPv6 hosts to communicate with each other using existing IPv4 network. One of the ways we can achieve this is by using a tunneling method.

In this post we will see how we can implement "Manual Tunnel". 

As shown in the diagram below, we have R1, R2 and R3 connected with each other. 

All three routers are running EIGRP and there is an IPv4 reachability from R1 to R3.

BFD (Bidirectional Forwarding Detection)

In this post, we will look into a feature called BFD (bidirectional forwarding detection). In normal routing protocol operation, the link failure is detected by using the hello/holddown/dead timers. 

Depending on the routing protocol, you can lower the timers to achieve fast failure detection. e.g. in the case of OSPF the lowest dead time can be one second and one can set the hello interval as low as 50ms.

However lowering these timers can result in higher CPU utilization. Also it will unnecessarily waste the link bandwidth. 

There is a better way of achieving fast failure detection through BFD. BFD is a UDP-based protocol that provides fast (in milliseconds) routing protocol independent detection of layer-3 next hop failures.

Let's see how it works. As shown in the diagram below, routers R1 and R2 are connected via Switch 1 and Switch 2. Both the routers are running OSPF and we can see the OSPF neighbourship is up.

Multicast Notes

Well Known Multicast Addresses

224.0.0.1 - All Hosts
224.0.0.2 - All Multicast Routers
224.0.0.9 - RIPv2 Routers
224.0.0.10 - EIGRP Routers
224.0.0.12 - HSRPv2
224.0.0.13 - PIM Routers
224.0.0.22 - IGMPv3
224.0.1.1 - NTP
224.0.1.39 - Cisco-RendezvousPoint-Announce
224.0.1.40 - Cisco-RendezvousPoint-Discovery
232.0.0.0/8 - Reserved for SSM
233.0.0.0/8 - GLOP address range

PIM (Protocol Independent Multicast)

• Protocol independent means that it can work with any unicast routing protocol 
• Pim can be implemented in three modes: sparse mode, dense mode and sparse-dense mode.