An Extra Edge for your Network

 Edge routers are nothing new. They've been around for a

long time. It's just that their importance in the enterprise has started

becoming



increasingly important due to several factors. One is the increasing reliance of

enterprises on their WAN links for different business applications, which

include voice, video, as well as data. So for effective business communication

to happen, an  enterprise must choose the right edge router, which not only

meets all the requirements, but also provides the necessary routing performance,

and security.

Here we'll explore some of the new



capabilities that have been introduced in edge routers, and also explore the

various types.

Key features



Edge routers are devices that connect an external network to the core

network of an enterprise, using some access mechanism. They're called edge

routers because they sit at the edge of your network. The other type of routers

are called core routers, which we'll talk about later in this article.

The external network could be a public



network like the Internet, or just a connection link between the branch office

and the head office.

The fact is that edge routers can also aggregate

traditional forms of traffic such as VPN, frame relay, and at the same time they

also offer support for newer, timelier methodologies of connectivity, such as

MPLS and VPLS.

Edge routers give flexibility to deliver the right VPN

service for each customer. Layer 2 VPN services are simple to provide and

manage, and are mostly appealing to enterprises with a small number of sites.

However, large organizations prefer a Layer 3 VPN, where each site interfaces to

the service provider over a single connection and not over hundreds or thousands

as would have been required to connect many sites in a Layer 2 model. Layer 3

VPN services leverage the MPLS core to



interconnect numerous customer sites in a hierarchical and scalable fashion,

using BGP (Border Gateway Protocol) to propagate routing information. A service

edge router can be the cornerstone for deploying this service.

Edge routers can also work with IPv6. They accept and

process an IPv6 packet, encapsulate it into an IPv4 packet, and forward it to

another edge router. The other one removes the IPv4 header, reinserts the IPv6

packet and its associated data back onto the network headed for the IPv6

destination. This helps service providers in deploying IPv6-based video-on-



demand, IP television, videoconferencing, voice and video telephony services.

Two other key features of edge routers are their support

for IP/MPLS and VPLS services. MPLS (Multiprotocol Label Switching) is defined

as a label-swapping framework with Layer 3 (Network Layer) routing. It

integrates Layer 2 (Data Link Layer) information into Layer 3 routing. It is

called multiprotocol because it works with the IP (Internet Protocol), ATM

(Asynchronous Transport Mode), and FR (Frame Relay) network protocol. MPLS

allows most packets to be forwarded at the Layer 2, ie switching level rather

than at the Layer 3 (routing level).

In addition to moving traffic faster overall,  MPLS

makes it easy to manage a network for QoS (Quality of Service). For these

reasons,  the technique is readily adopted as networks begin to carry more

and different mixtures of traffic, including Video Services and VoIP features.

MPLS provides any-to-any connectivity (ie meshed or partially-meshed network).

It gives routers the ability to handle different applications, such as voice,

video, and data. It also lets youprioritize different applications. With MPLS,

different performance levels can be given to different application types.

VPLS is a class of VPNs that supports the connection of

multiple sites in a single bridged domain over a managed IP/MPLS (Multi-protocol

Label Switching) network. VPLS uses edge routers that can learn, bridge and

replicate on a per-VPLS basis.

These routers are connected by a full mesh of MPLS LSP

(Label Switched Path) tunnels, enabling any-to-any connectivity. Multiple

services can be carried within each LSP tunnel.

All services in a VPLS are identified by a unique virtual

channel label, which is exchanged between each pair of edge routers. Edge

routers use these virtual channel labels to demultiplex traffic arriving from

different VPLS nodes over the same LSP tunnel. The VPLS technology



allows service providers to easily configure multipoint-to-multipoint Layer 2

VPN tunnels between the sites. VPLS, in conjunction with MPLS Layer 2 VC, allows

multiple customer sites connected across an MPLS network to share a single

bridged domain. So, all customer sites appear to be on the same LAN, regardless

of their geographical location.

Types of edge routers



While there are many types of edge routers like wireless edge router,

multi-services edge router, label edge router,



edge aggregation router, etc-we detail the two most common of these in use.

Label edge router It is a device that sits at the

edge of an MPLS domain and uses routing information to assign labels to

datagrams and then forwards them to the MPLS domain.

MPLS establishes an end-to-end connection path before

transferring information, and the paths may be selected based on application

requirements such as bandwidth and maximum latency. An end-



to-end MPLS connection is called a Label Switch Path (LSP). Information about

the connection is summarized into an MPLS label, which is inserted between the

Layer 2 and Layer 3 headers of each packet. A



label edge router adds the first label to an incoming packet.

Labels are a simple indexing mechanism that replaces

traditional Layer 2 (Ethernet/ATM) or Layer 3 (IP) packet forwarding mechanism

with fast and simple switching. At each hop in the network, a router examines

the incoming label to



figure out the next forwarding hop for the packet. This eliminates resource

intensive address lookups that reduce overall packet throughput and limit

scalability.

Each Label Switch Router (LSR) makes forwarding decisions

based solely on the contents of the label. At each hop, the LSR strips off the

existing label and applies a new label, which tells the next hop LSR how to

forward the packet. All MPLS routers within the network regularly exchange label

and reachability information to build a complete picture of the network, which

is then used to determine paths and specify the new label to place onto the

packet.

Aggregation router These routers terminate large

number of dedicated access connections such



as leased line services, or ATM and frame relay virtual circuits, and aggregate

the traffic for hand off to a core backbone router.

Edge aggregation routers are typically single-purpose

devices optimized for Internet access and pure IP forwarding. They typically

lack the QoS, connection management, carrier class availability and service

management required to deliver switched data services. Aggregation routers are

essential for scaling IP networks and for providing a clean separation between

access and core functions.

Edge vs core



Edge routers accept any traffic from any source outside the network and

route it into the internal network while a core router provides

packet-forwarding service between other core and edge routers. Another thing

edge routers do is to classify the packets received from the



outside network and mark each packet with the class of service that they should

receive. Edge routers characterize, police, and mark traffic between other edge

or core routers. They provide security to the core and function as admission

controllers to external traffic.

On the other hand, core routers only manage traffic to

avoid and cope with transient congestion within the core. The core router

differentiates the incoming packets based on their code points and the entries

in PHB (Per-HopBehavior) table. Edge router on the entrance of the network

analyzes the Layer 2 address of the Ethernet frame and attaches a 32-bit tag to

the packet. Core network routers use the tag to determine the treatment that

should be afforded to that packet. Each of the



intervening core router analyzes the tag to select the most appropriate path,

over which to forward the packet. Through a



series of such link-by-link decisions,



the end-to-end path is established and maintained for each traffic flow, which

brings connection-oriented characteristics to a connectionless IP network.

Since the edge router usually represents a single point of

failure, it's what most service providers consider as the most vulnerable

point of their network after the core is protected. So instead of using

additional routers and links as in the core, redundancy within the edge router

is provided via redundant control processor cards, redundant line cards, and

redundant links (such as SONET/SDH Automatic Protection Switching ) for

fault tolerance.

Swapnil Arora