MPLS FAQs for BeginnersJune 8, 2015 16:04 reading 186
Q: What is Multiprotocol Label switching (MPLS)?
A: MPLS is a packet-forwarding technology that uses tags to make data-forwarding decisions. With MPLS Technology, a 3rd-level header analysis is done only once (when the packet enters the MPLS domain). The label check can facilitate subsequent packet forwarding. MPLS provides benefits for the following applications:
· Virtual Private Network (VPN)
· Flow Engineering (TE)
· Quality of service (QoS)
· Any MPLS-based transport (AToM)
In addition, it reduces the forwarding overhead on the core routers. MPLS technology is applicable to any network layer protocol.
Q: What is a label? What is the structure of the label?
A. The label is a fixed-length short identifier consisting of four bytes and only locally meaningful, used to identify the forwarding equivalent class (FEC). The label placed on a particular packet represents the FEC that the packet will be assigned to.
· Label-label value (no structure), 20-bit
· EXP-For testing purposes, 3 bits; currently used as a service level (CoS) field.
· S-bottom of the stack, 1 bits
· TTL-Time to live, 8-bit
Q: Where does the label attach to the packet?
A. The label will be appended to the Data Link layer (layer 2nd) header and the Network layer (layer 3rd) header. The top of the tag stack appears first in the packet, and the bottom appears at the end. The network layer packet immediately follows the last label in the tag stack.
Q: What is the forwarding equivalent class (FEC)?
A: FEC is a set of IP packets that are forwarded in the same way, through the same path, and through the same forwarding process. A FEC may correspond to a destination IP subnet, but may also correspond to any traffic category that the edge LSR considers meaningful. For example, all traffic that has an IP priority of a specific value can constitute a FEC.
Q: What is an upstream label switching router (LSR)? What is a downstream LSR?
A: Upstream and downstream in the MPLS field is a relative two words. They always refer to a prefix (to be exact, FEC). This is further explained in the following example.
For FEC 10.1.1.0/24,R1 is the "downstream" LSR of R2.
For FEC 10.1.1.0/24,R2 is the "upstream" LSR for R1.
For FEC 10.1.1.0/24,R1 is the "downstream" LSR of R2. And R2 is the "downstream" LSR of R3.
For FEC 10.1.1.0/24,R1 is the "downstream" LSR of R2. For FEC10.2.2.0/24,R2 is the "downstream" LSR for R1.
Data flows from upstream to downstream to reach the appropriate network (prefix).
The R4 routing table uses R1 and R2 as the "next hop" to reach 10.1.1.0/24.
Q: Is the "downstream" LSR for 10.1.1.0/24,R3 R4?
A. No, data flows downstream from upstream.
Q: What are the meanings of the term incoming, outgoing, local, and remote when referring to tags?
A. Observe the R2 and R3 in the following topologies. For FEC F,r2 assigns a label L to R3. R3 uses the label L when forwarding data to fec-f (because the FEC-F,R2 is its downstream LSR). In this case:
· For F,l is the incoming label on the R2.
· For Fec-f,l is the outgoing label on the R3.
· For FEC F,l is a local binding on the R2.
· For fec-f,l is a remote binding on the R3.
Q: Can LSR transmit/receive local IP packets (non-MPLS) on the MPLS interface?
A. Can. If the IP protocol is enabled on the interface, local packets can be received/transmitted as usual. IP is just a different protocol. MPLS packets have a different 2nd-layer encoding. The receiving LSR can determine the MPLS packet according to the 2nd layer encoding.
Q: Can LSR receive/transmit tagged packets on a non-MPLS interface?
Answer: No. You must not transmit packets on an interface that does not have the appropriate protocol enabled. MPLS has a specific type of ether code associated with it (just like IP, IPX, and AppleTalk each have a unique type of ether). If the Cisco router receives a packet whose ether type is not enabled on the appropriate interface, it discards the packet. For example, if a router receives an AppleTalk packet on an interface that does not have AppleTalk enabled, it discards the packet. Similarly, if an MPLS packet is received on an interface that does not have MPLS enabled, the packet is discarded.
Q: Which platforms and Cisco IOS support MPLS?
A. Cisco series 2691, 3640, 3660, 3725, 3745, 6400-nrp-1, 6400-NRP-2SV, 6400-NSP, Catalyst 5000, 7200, 7301, 7400, with Routing switching module (RSM), 7500. Catalyst 6500/cisco 7600 Series with ws-sup720-3b and WS-SUP720-3BXL, Gigabit switch Router (GSR), routing processor Module (RPM), Universal Broadband Router (UBR) 7200, AS5350 and Igx8400-urm both support MPLS.
These platforms support the use of the Cisco Tag Allocation Protocol (TDP) as a label allocation protocol.
You can use tools to find information about the Tag allocation Protocol (LDP), Resource Reservation Protocol (RSVP), and Border Gateway Protocol (BGP). The software Advisor provides a complete list of different Cisco IOS versions and feature sets supported by different platforms.
Q: The cost of a generic Routing encapsulation (GRE) tunnel is 24 bytes. So, what is the overhead of an MPLS LSP tunnel?
A. The overhead of an MPLS LSP tunnel is a label (four bytes) or two tags (for example, when using link protection to quickly reroute). Unlike GRE tunnels, MPLS does not change the IP header. Instead, the label stack is attached to the packet that takes the tunnel path.
Q: How does LSR determine which is the top label, Bottom label, and Middle label of the tag stack?
A. The label immediately following the 2nd level header is the top label, while the label with the S bit set to 1 is the bottom label. No application will require the LSR to read/identify the intermediate label. However, if the label is not on the top of the label stack and the S bit is set to 0, the label is an intermediate label.
Q: What is the range of tag values? What label values are retained? What are the meanings of these reserved values?
A. These values can also be found in the RFC3032-MPLS tag stack encoding.
Theoretically, the range passed is 0 (2-120). The label values 0 to 15 are reserved values, and the values 4 to 15 remain for future use. Values 0 to 3 are defined as:
· A value of 0 means "IPv4 an explicit NULL label". This label indicates that a popup must be performed on the label stack, and packet forwarding must be based on the IPV4 header. This helps ensure that the EXP bit is secure before reaching the egress router. It is used in MPLS-based QoS.
· A value of 1 indicates "router alert label". If the received packet contains this tag value at the top of the label stack, the packet is routed to a local software module for processing. The actual packet forwarding is determined by the label in the tag stack that is located under the packet. However, if the packet is further forwarded, the Router alert label should be pushed back to the label stack before forwarding. The usage of this label is similar to the use of the Router Alert option in IP packets (for example, ping with the record route option)
· A value of 2 means "IPv6 an explicit NULL label". It indicates that a popup must be performed on the label stack, and packet forwarding must be based on the IPV6 header.
· A value of 3 indicates an "implicit NULL label". This is the label that the LSR can assign and distribute. However, it never really appears in the package. It indicates that the LSR will eject the top label from the tag stack and then forward the remainder of the packet (either marked or non-tagged) through the outgoing interface (as per the entry in Lfib). Although this value may never come out of the package, you need to specify this value in the label assignment protocol, thus preserving a value.
Q: What protocols and port numbers are used by LDP and TDP to assign tags to LDP/TDP peers?
A: LDP uses TCP protocol and port 646;TDP to use the TCP protocol and Port 711. These ports are opened on the interface only if MPLSIP is configured on the router interface. The use of TCP as a transport protocol enables reliable LDP/TDP information delivery and powerful data flow control and congestion handling mechanisms.
Q: What are the limitations of MPLS support for Catalyst6500 and 7600 Fibre service routers (OSR)?
A. Interfaces connected to the MPLS domain must use some kind of fiber service module (OSM) (for example, any module that uses parallel fast forwarding (PXF) composite technology) or an interface in the Flexwan module. The same limitations exist for MPLS 3rd VPN. That is, the IP frame must be entered on a WAN interface, which can be either an OSM or an interface in the Flexwan module. Supervisor 720 does not exist for this limitation.
Q: Where can I find the MPLS configuration example?
A. You can find many MPLS configuration documents in the following locations: implementation and configuration: MPLS.
Q: What options are available for load-balanced MPLS packets?
A: MPLS packets can be load balanced using the MPLS tag information and/or the source and destination addresses of the important IP header.
Q: Can I configure 802.1Q trunk between two ciscocatalyst switches on different sites via MPLS connection?
A. When connecting to a remote site via MPLS, it is a layer 3rd connection, and the 802.1Q trunk is a 2nd layer protocol, so the 802.1Q trunk cannot be configured with an MPLS connection. You need a Metro Ethernet connection or a 802.1Q tunnel to extend the VLAN provided by your ISP. In the MPLS cloud, the ISP communicates through a VRF.
For more information, see Configuring IEEE 802.1Q Tunnels.
Q: Does the outgoing MPLS exp value inherit the DSCP value of the incoming IP packet by default or does the incoming DSCP delegate have no more configuration enabled interfaces in MPLS?
A: Yes, no more configuration is needed.
Q: Does the DHCP relay feature work on MPLS VPN networks?
A. Yes, the DHCP request is forwarded within the VRF of the MPLS VPN network and the egress operator Edge sends it to the DHCP server in the same VRF.
MPLS FAQs for Beginners