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DSL Access 2009-01-16 08:44:47 Historically, DSL has been an asymmetric service (ADSL), evolving into a symmetric one (G.SHDSL) designed to replace E1 TDM circuits and provide voice, ATM, raw IP, and ISDN transport.DSL copper cables are terminated at a central office (CO) DSLAM port (digital subscriber access line multiplexer). The DSLAM serves two purposes:One is to physically terminate the subscriber line and separate the voi Read more:Access
Route Cloning 2009-01-16 08:34:51 Cloned routes are a concept unique to BSD networks stacks. The concept refers to on-demand generation (cloning) of host routes (/32). In other words (quoted from the FreeBSD arp(4) manual page), "The ARP cache is stored in the system routing table as dynamically created host routes. The route to a directly attached Ethernet network is installed as a 'cloning' route (one with the RTF_CLONING flag s Read more:Cloning
Floating Static Routes 2009-01-16 08:31:42 Floating static routes are a useful and simple measure to provide backup routes via another hop or link. However, a floating static route just "lurks" there and does not provide load balancing! This can be as simple as two default routes that just differ in terms of metric or cost. As long as the preferred route with the better metric is available, the floating static route with the less attractiv Read more:Floating
Classification of Dynamic Routing Protocols 2009-01-16 08:27:21 Dynamic routing protocols are based on an algorithm, such as Bellman-Ford-Fulkerson, Dijkstra SPF (Shortest Path First), or the Enhanced Interior Gateway Routing
Protocol (EIGRP) DUAL (Diffuse Update Algorithm). Based on these algorithms, dynamic IGPs can be classified in link-state and distance-vector protocols.NOTEThe Border Gateway Protocol (BGP) discussed in the next chapter represents a path- Read more:Classification
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Introduction to Link-State Routing Protocols 2009-01-16 08:15:56 Link-state routing protocols are based on Edsger W. Dijkstra's Shortest Path First (SPF) algorithm, a result of applied graph theory. Link-state protocols establish and maintain adjacencies via hello packets (connection-oriented) with their neighbors (peers), speaking the same routing protocol. The name link-state stems from the underlying concept that every participant distributes (floods) all th Read more:Routing
, Protocols
, Introduction
, State
Introduction to Link-State Routing Protocols 2009-01-16 08:12:10 Link-state routing protocols are based on Edsger W. Dijkstra's Shortest Path First (SPF) algorithm, a result of applied graph theory. Link-state protocols establish and maintain adjacencies via hello packets (connection-oriented) with their neighbors (peers), speaking the same routing protocol. The name link-state stems from the underlying concept that every participant distributes (floods) all th Read more:Routing
, Protocols
, Introduction
, State
OSPFv2 2009-01-16 08:05:40 OSPF (Open Shortest Path First) is the most popular among the link-state routing protocols. The current IPv4 version, OSPFv2, is widely deployed throughout carrier, ISP, and enterprise networks. OSPFv3 essentially is an IPv6-enabled OSPF. It is a well documented protocol in terms of standards, books, guides, and white paper density.The knowledge level is high among those who deploy and operate OSP
OSPF Authentication 2009-01-16 07:57:43 Configuring authentication for OSPF or RIP is pretty straightforward under Zebra. You have the choice between clear-text passwords and MD5 hashes (Example 9-25). However, consider that this contributes to CPU load.Example 9-25. Configuring MD5 Authentication
for Zebra OSPFcastor-ospfd# show running-configCurrent configuration:!hostname castor-ospfdpassword 8 4DwwIFdKLWvU.enable password 8 dV8x4Mhx
IS-IS (Intermediate System-to-Intermediate System) 2009-01-16 07:55:22 I have included this section to raise more appreciation for the IS-IS routing protocol. In Open System
Interconnection (OSI) CLNS environments, CLNP provides a network layer service to peer CLNS entities. CLNP can be seen as the ISO equivalent of (connectionless) IP datagram delivery.The following dynamic routing approaches can be used to route CLNP:IS-IS (Intermediate
System-to-Intermediate Syste
User-Space Tunneling 2008-12-15 12:59:39 User-space tunnels are not an integral part of the operating system. They carry out their duty on top of TCP and UDP. This section discusses several representative examples, but a much larger variety exists. Several approaches are designed to circumvent corporate security by creating transparent tunnels (tcp80/tcp443) over HTTP(S) proxies or SOCKS5 relays.Note that I do not discuss these approache Read more:Space
Road-Warrior Scenarios (Road Warrior-to-OpenBSD/FreeBSD Gateway with IKE) 2008-12-15 12:45:18 Road warriors (multiuser configurations) are roaming user clients with dynamically assigned IP addresses unknown to the home IPSec gateway or VPN concentrator. Hence the configuration has to rely on other means of authentication such as deployment of signatures or certificates. This requires a PKI.Deployment of preshared secrets does not scale and often compromises entire architectures that rely o Read more:Scenarios
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Dynamic Routing Protocols 2008-12-15 12:39:33 Dynamic RoutingProtocols
over Point-to-Point Tunnels—Transparent Infrastructure VPNIn general, IPSec tunnel setups cannot transfer routing protocols such as OSPF. IPSec does not always support the notion of an interface on which a routing engine (such as ospfd) can rely. (Remember, IPSec deals with SAs.) This can be accomplished by deploying OSPF over IP-IP/GRE tunnels over IPSec or out-of-band Read more:Dynamic
Designing for High Availability 2008-12-15 12:34:25 High-availability architectures represent a wide-ranging subject of interlocked complexity stretching over all layers of the OSI (Open System Interconnection) stack.Keep in mind that the end-user's perception of service availability is the ultimate and most relevant criterion; perception will be favorable if you did your job right. Toward that end, high-availability architectures satisfy the follo Read more:Designing
, Availability
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Withstanding a (D)DoS Attack 2008-12-15 12:32:23 In light of recent Internet attacks, whether a sound HA architecture should withstand a massive (distributed) denial-of-service ([D]DoS) attack or be able to mitigate its effects has become a legitimate question. From my point of view, a state-of-the-art HA architecture should have some inherent self-healing capabilities; HA architects should also add another line of defense to assist in at least
Simple but Effective Approaches to Server HA 2008-12-15 12:30:12 Let us consider the network vicinity of a server in the context of its connected network interface cards (NICs), its LAN switch environment, VLAN membership, and exit gateways. Note that two or more NICs attached to redundant switch access ports provide sufficient redundancy, and channel bonding or interface teaming provides another useful combination of link aggregation (with an added redundancy Read more:Simple
Dynamic Routing Protocols 2008-12-15 12:29:05 Dynamic routing is the most flexible and effective approach to provide redundancy for alternative paths and the only way to detect network node, port, or link failures reliably. Routing
and standby protocols rely on the simple principle that if a speaker hasn't heard from a neighbor in a certain time, something must be wrong. Load balancing over multiple links can be accomplished in several ways: Read more:Dynamic
, Protocols
Clustering and Distributed Architectures 2008-12-15 12:19:01 The basic idea behind redundant systems and services is to provide at least a second resource that can either take over the duty in a hot-standby fashion or, even better, is a member of a server/service farm that constantly contributes via load-balancing schemes. One step further would distribute such architecture geographically, and that would pretty much define the boundaries (not limits) of suc Read more:Distributed
, Architectures
A Few Words About Content Caches and Proxies 2008-12-15 12:14:18 Content-caching architectures and engines such as in the Cisco product palette deal with the challenge to deliver content reliably, efficiently, and effectively to the network edge and access layer where customers subscribe to certain content. Vice versa, they are necessary to provide sufficiently clustered server farms to feed these requests.NOTEHistorically, caching was the initial purpose of pr Read more:Words
, Content
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Cisco HA and Load-Balancing Approaches 2008-12-15 12:12:24 Cisco offers several architectural approaches to high availability, ranging from lower-layer concepts such as resilient packet ring and Multiprotocol Label Switching (MPLS) node protection up to protocol-intrinsic or application layer approaches.The lower-layer concepts (Layers 1 through 3) are summarized under the Cisco Global Resilient IP Framework (GRIP). This framework consists of the followin Read more:Cisco
, Balancing
No title 2008-12-15 11:47:25 VRRPGateway redundancy protocols solve the problem of eliminating single points of failure in a LAN environment for clients that require a reliable HA default path and next hop. Provisioning of a default route/multiple routes can, in general, be achieved in various ways:Dynamic Host Configuration Protocol (DHCP) with default gateway provisioningManual default route entry/entries plus load balancin
No title 2008-12-15 11:44:21 The Internet Router Discovery Protocol (IRDP, RFC 1256), sometimes also referred to as the ICMP Router Discovery Protocol, is the ancestor of redundancy concepts such as VRRP or HSRP. It is a timelessly elegant concept that recently has attracted renewed attention because of its essential role in mobile IP deployments (RFC 3344). Extensions to the original protocol were necessary to provide for th
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