Content-Type: text/x-zim-wikiWiki-Format: zim 0.4Creation-Date: 2015-07-22T13:50:41+03:00====== 1. Multi Protocol Label Switched ======Created Wednesday 22 July 2015**- MPLS Header where :**- 20 bits are reserved for Label Space - 3 bits used for QoS- 1 bit for Bottom Stack - 8 bits used for TTL mechanism{{./pasted_image.png}}- MPLS supports Label Stacking , here the BoS dictatest the header position , if "0" zero it indicates that it is not the last Label , if it is "1" one it indicatest that it is the last label of the stack- Once a Label is pushed the frame on the wire change meaning that in the L2 header we will have a new Protocol ID value - Labe Switch Router is a router that supports MPLS functions and forward MPLS traffic :- Ingress LSR : receives a unlabeled packet inserts a label or label stack in front of the packet and forward it on the data link- Performs the PUSH operation- If the PUSH operations is performed on an unlabeld packet the LSR is also called an __imposiong LSR__ - Intermediate LSR / Tranzit LSR : receives an incoming labeled packet performs operations on it , switch the packet on the correct data link- Performs the SWAP operation- Egress LSR : receives a labeled packet , removes the label or label stack and send them on a data link - Performs the POP operation- if after the POP results a packet without labeled the LSR is also called a __disposing LSR__ - Ingress LSR and Egress LSR are EDGE LSR**- Label Switched Path:**- Is a sequence of LSR that switch a labeled packet through an MPLS network :- LSP are unidirectional - **- Forwarding Equivalence Class :**- a group or flow of packets that are forwarded along the same path and ar treated the same with regard to the forwarding treatment- all packets belonging to the same FEC have the same label- not all packets that have the same label belong to the same FEC , as example their EXP values might differ , so the fowarding threatment could be different - the router that decide which packet belong to which FEC is the ingress LSR - Examples of FEC:- Packets with Layer 3 destination IP address matching a certain prefix- Multicast packets belonging to a certain group- Packets with the same forwarding treatment , based on the precedence or IP DiffServ Code Point- Layer 2 frames carred acros an MPLS network received on one VC or subinterface ingress LSR and transmitted on onve VC or subinterface on the egress LSR - Packets with Layer 3 destination IP addresses that belong to a set of BGP prefixes all with the same BGP next hop**- Label Operations :**- Pop : the top label is removed , the packet is forwarded with the remaining label stack or as an unlabeled packet- Swap : The top label is removed and replaced with a new label- Push : the top label is replaced with a new label swapped , and one or more labels are added ( pushed ) on top of the swapped label- Untagged/No label : The stack is removed , and the packet is fowarded unlabled - Aggregate : the label stack is removed and an IP lookup is done on the IP packet**- Reserved Labels (0-15)** :- unknown Labeled packets are dropped - Implicit NULL Label is 3 , it is used by the egress PE to signal to its nearest P router to perform PHP on the packet destined to it- Explicit NULL it is used to preserve the Qos of the packet , it has the label 0- Router Alert Label has value 1 , present anyware in the label stack but not at bottom , when present the packet will strip the label and will process the packet in software , not hardware over the LFIB- OAM Alert Label 14 , used to signal different OAM funcitonality over the MPLS Network**- TTL Operations :**- Push : once the router decides how a packet will be forward , it will decrement the TTL by 1, it will update the TTL in the IP packet , it will copy the new TTL in the new label attached and will forward the packet- Swap : the old label is stripped , from the stripped label the TTL decresed by 1 and copied in the new imposed label , and the packet is forwarded- Pop : the old label is stripped , from the stripped label the TTL is decresed by 1 and overrides the TTL in the next heade ( IP or MPLS label)- When TTL expires in transit , an message of ICMP time exceeded is sent along the LSP of the original packet , one reaching the destination it will bounce back to the source where the packet was originated**- MPLS Fragmentation:**- if a LSR detects that the outgoing Data link has its MTU smaller than the fowarded packet it will fragment the packet - the fragmentation process:1. The label stack is stripped off2. The packet is fragmented3. Each fragment will pe imposed with the label previous stripped label stack4. Each fragment will be fowarded along the Data Link- - **MPLS Control Plane Elements:**__- Label Distribution Protocol:__- used to discover neighor LSR - it uses the Routing Information Base to form a local binding database ( Label Information Base )- it advertise and received Label bindings populating and manteining the LIB__- Label Information Base :__- buid and stores the prefix to label binding __- MAC Table (ARP) :__- it stores the MAC to port bindings - it is used in conjunction with some tables (RIB/LIB) to generate fast forwarding structures (FIB/LFIB) , by providing the L2 encapsulation details- **An LSR can use different modes when distributiing labels to other LSR .:**- **Label distribution mode:****- Donwstream-on-Demand label distribution:**- each LSR request its next-hop ( in downstream) LSR a label binding for each FEC - each LSR receive one binding per FEC from its downstream LSR on that FEC - the downstream LSR is the next-hop router indicated by the IP routing table- in this mode the LIB shows only one remote binding - used on LC-ATM interface**- Unsolicited Downstream label distribution**- each LSR distributes a binding to its adjacent LSR without those LSRs requesting a label- each LSR receive remote label binding from each LSR - each LSR distributes a binding to its local FEC- in this model we can see more than one remote binding in LIB- All non LC-ATM interface uses UD mode - **Label retention mode** :**- Liberal label retention mode :**- an LSR keeps all received remote bindings in the LIB - not all received FEB bindings received from its neigbhors will be placed in LFIB , - in this mode a LDP peers doesnt need to request new bindings in case of a incident , it has store all its neighbor mappings- LLR gives quicker adaptation to routing changes - All non LC-ATM interfaces uses LLR mode- **Conservative label retention m**ode - does not stores the all remote bindings in the LIB , but it stores only the remote bindings that is associated with the next hop LSR for a particular FEC- gives fewer labels to store and a better usage of the available memory- CLR is the default mode for LC-ATM interfaces **- LSP Control Modes:****- Independent LSP Control mode:**- each LSR creates a local binding for a particular FEC as soon as it recognizes the FEC - Usuali this means that the prefix for that FEC is in the RIB- one disadvantage is that some LSR begin to label switch packets before the complet LSP is set up end to end - It assigns a label mapping for each IGP prefixe in the RIB - Independent is used Ethernet Switches**- Ordered LSP Control mode:**- an LSR only creates a local binding for a FEC if it recognize that it is the egress LSR for a FEC or if the LSR has received a label binding from next hop for this FEC- It assings a label mapping for each IGP prefixes marked as connected in RIB , or for prefixes for which it received the binding- Used by ATM switches