Internet Control Message Protocol (ICMP) assists the operation of the IP network by delivering messages about the network’s functionality—or lack thereof. ICMP includes functions for the following:
...Communicating network errors—Such as host or network unreachable.
...Announcing network congestion—An example is the ICMP Source Quench messages used to cause a sender to slow down transmission because of a router buffering too many packets.
...Provide troubleshooting tools—The Echo function is used by the ping utility to test connectivity between two systems.
...Communicate timeouts in the network—If a packet’s TTL reaches 0, an ICMP message can be sent announcing this fact.
ICMP protocol unreachable messages
If the Cisco device receives a nonbroadcast packet destined for itself that uses an unknown protocol, it sends an ICMP protocol unreachable message back to the source. Similarly, if the device receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP host unreachable message to the source. This feature is enabled by default. To enable it if it’s disabled, use the following command:
Router(config-if)# ip unreachables
ICMP redirects
If the router resends a packet through the same interface on which it was received, the Cisco IOS Software sends an ICMP redirect message to the originator of the packet, telling the originator that the router is on a subnet directly connected to the receiving device and that it must forward the packet to another system on the same subnet. To enable the sending of ICMP redirect messages if this feature was disabled, use the following command:
Router(config-if)# ip redirects
Address Resolution Protocol
Address Resolution Protocol (ARP) is used to resolve IP addresses to MAC addresses in an Ethernet network. A host wanting to obtain a physical address broadcasts an ARP request onto the TCP/IP network. The host on the network that has the IP address in the request then replies with its physical hardware address. When a MAC address is determined, the IP address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests and replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP). Reverse Address Resolution Protocol (RARP) works the same way as ARP, except that the RARP request packet requests an IP address rather than a MAC address. Use of RARP requires a RARP server on the same network segment as the router interface. RARP often is used by diskless nodes that do not know their IP addresses when they boot. The Cisco IOS Software attempts to use RARP if it does not know the IP address of an interface at startup. Also, Cisco routers can act as RARP servers by responding to RARP requests that they can answer.
History of Tag Switching to MPLS
Reference:- MPLS Fundamentals
http://www.ciscopress.com/bookstore/product.asp?isbn=1587051974
Cisco Systems started off with putting labels on top of IP packets in what was then called tag switching. The first implementation was released in Cisco IOS 11.1(17)CT in 1998. A tag was the name for what is now known as a label. This implementation could assign tags to networks from the routing table and put those tags on top of the packet that was destined for that network. Tag switching built a Tag Forwarding Information Base (TFIB), which is, in essence, a table that stores input-to-output label mappings. Each tag-switching router had to match the tag on the incoming packet, swap it with the outgoing tag, and forward the packet.
Later on, the IETF standardized tag switching into MPLS. The IETF released the first RFC on MPLS—RFC 2547, “BGP/MPLS VPNs”—in 1999. The result of this was that much of the terminology changed. Below table shows an overview of the old and new terminology.
Old Terminology --- New Terminology
Tag switching --- MPLS
Tag --- Label
TDP (Tag Distribution Protocol) --- LDP (Label Distribution Protocol)
TFIB (tag forwarding information base) --- LFIB (label forwarding information base)
TSR (tag switching router) --- LSR (label switching router)
TSC (tag switch controller) --- LSC (label switch controller)
TSP (tag switched path) --- LSP (label switched path)
http://www.ciscopress.com/bookstore/product.asp?isbn=1587051974
Cisco Systems started off with putting labels on top of IP packets in what was then called tag switching. The first implementation was released in Cisco IOS 11.1(17)CT in 1998. A tag was the name for what is now known as a label. This implementation could assign tags to networks from the routing table and put those tags on top of the packet that was destined for that network. Tag switching built a Tag Forwarding Information Base (TFIB), which is, in essence, a table that stores input-to-output label mappings. Each tag-switching router had to match the tag on the incoming packet, swap it with the outgoing tag, and forward the packet.
Later on, the IETF standardized tag switching into MPLS. The IETF released the first RFC on MPLS—RFC 2547, “BGP/MPLS VPNs”—in 1999. The result of this was that much of the terminology changed. Below table shows an overview of the old and new terminology.
Old Terminology --- New Terminology
Tag switching --- MPLS
Tag --- Label
TDP (Tag Distribution Protocol) --- LDP (Label Distribution Protocol)
TFIB (tag forwarding information base) --- LFIB (label forwarding information base)
TSR (tag switching router) --- LSR (label switching router)
TSC (tag switch controller) --- LSC (label switch controller)
TSP (tag switched path) --- LSP (label switched path)
Redistribution
Route redistribution might be required in an internetwork because multiple routing protocols must coexist in the first place. Multiple routing protocols might be a necessity because of an interim period during conversion from one to another, application-specific protocol requirements, political reasons, or a lack of multivendor interoperability.
A major issue with redistribution is the seed metric to be used when the routes enter the new routing protocol. Normally, the seed metric is generated from the originating interface. For example, EIGRP would use the bandwidth and delay of the originating interface to seed the metric. With redistributed routes, however, these routes are not connected to the router. Some routing protocols feature a default seed metric for redistribution, whereas others do not. Here is a list of the defaults for the various protocols. Note that Infinity indicates a seed metric must be configured; otherwise, the route will not be used by the receiving protocol.
Protocol ------ Default Seed Metric
OSPF ------ 20; except BGP, which is 1
IS-IS ------ 0
RIP ------ Infinity
IGRP/EIGRP ------ Infinity
A major issue with redistribution is the seed metric to be used when the routes enter the new routing protocol. Normally, the seed metric is generated from the originating interface. For example, EIGRP would use the bandwidth and delay of the originating interface to seed the metric. With redistributed routes, however, these routes are not connected to the router. Some routing protocols feature a default seed metric for redistribution, whereas others do not. Here is a list of the defaults for the various protocols. Note that Infinity indicates a seed metric must be configured; otherwise, the route will not be used by the receiving protocol.
Protocol ------ Default Seed Metric
OSPF ------ 20; except BGP, which is 1
IS-IS ------ 0
RIP ------ Infinity
IGRP/EIGRP ------ Infinity
Link-state and distance vector protocols
Distance vector
1. Examples: Routing Information Protocol Version 1 (RIPv1), RIPv2, Interior Gateway Routing Protocol (IGRP).
2. Features periodic transmission of entire routing tables to directly connected neighbors
3. Mathematically compares routes using some measurement of distance Features hop-count limitation
Link State
1. Examples: Open Shortest Path First (OSPF), Intermediate Systemto-Intermediate System (IS-IS).
2. Sends local connection information to all nodes in the internetwork.
3. Forms adjacencies with neighboring routers that speak the same protocol; sends local link information to these devices.
4. Note that although this is flooding of information to all nodes, the router is sending only the portion of information that deals with the state of its own links.
5. Each router constructs its own complete “picture” or “map” of the network from all of the information received.
Hybrid
1. Example: Enhanced Interior Gateway Routing Protocol (EIGRP)
2. Features properties of both distance vector and link-state routing protocols
Path vector protocol
1. Example: Border Gateway Protocol (BGP).
2. Path vector protocols are a subset of distance vector protocols; BGP uses “path vectors” or a list of all the autonomous systems a prefix has crossed to make metric decisions and to ensure a loopfree environment.
3. In addition to the autonomous system path list, an administrator can use many other factors to affect the forwarding or receipt of traffic using BGP
IPv4 addresses
IPv4 addresses consist of 32 bits. These 32 bits are divided into four sections of 8 bits, each called an octet. Addresses are typically represented in dotted-decimal notation. For example: 10.200.34.201
Subnet masks identify which portion of the address identifies a particular network and which portion identifies a host on the network.
The address classes defined for public and private networks consist of the following subnet masks:
Class A 255.0.0.0 (8 bits)
Class B 255.255.0.0 (16 bits)
Class C 255.255.255.0 (24 bits)
Class A addresses begin with 0 and have a first octet in decimal of 1 to 127.
Class B addresses begin with 10 and range from 128 to 191.
Class C addresses begin with 110 and range from 192 to 223.
Class D and Class E addresses also are defined. The Class D address space has the first 4 bits set to 1110 and has a first octet of 224 to 247.These addresses are used for IP multicast.
Class E addresses have the first 4 bits set to 1111 and have a first octet of 248 to 255. These addresses are reserved for experimental use.
Subnet masks identify which portion of the address identifies a particular network and which portion identifies a host on the network.
The address classes defined for public and private networks consist of the following subnet masks:
Class A 255.0.0.0 (8 bits)
Class B 255.255.0.0 (16 bits)
Class C 255.255.255.0 (24 bits)
Class A addresses begin with 0 and have a first octet in decimal of 1 to 127.
Class B addresses begin with 10 and range from 128 to 191.
Class C addresses begin with 110 and range from 192 to 223.
Class D and Class E addresses also are defined. The Class D address space has the first 4 bits set to 1110 and has a first octet of 224 to 247.These addresses are used for IP multicast.
Class E addresses have the first 4 bits set to 1111 and have a first octet of 248 to 255. These addresses are reserved for experimental use.
RIB & FIB
The routing and forwarding architecture in Cisco routers and multilayer switches used to be a centralized, cache-based system that combined what is called a control plane and a data plane. The control plane refers to the resources and technologies used to create and maintain the routing table. The data plane refers to those resources and technologies needed to actually move data from the ingress port to the egress port on the device. This centralized architecture has migrated so that the two planes can be separated to enhance scalability and availability in the routing environment.
The separation of routing and forwarding tasks has created the Routing Information Base (RIB) and the Forwarding Information Base (FIB). The RIB operates in software, and the control plane resources take the best routes from the RIB and place them in the FIB. The FIB resides in much faster hardware resources. The Cisco implementation of this enhanced routing and forwarding architecture is called Cisco Express Forwarding (CEF).
The separation of routing and forwarding tasks has created the Routing Information Base (RIB) and the Forwarding Information Base (FIB). The RIB operates in software, and the control plane resources take the best routes from the RIB and place them in the FIB. The FIB resides in much faster hardware resources. The Cisco implementation of this enhanced routing and forwarding architecture is called Cisco Express Forwarding (CEF).
