Table of Contents Layer 2 ridging and Switching Karst Koymans Informatics Institute University of Amsterdam (version 43, 2015/02/14 12:39:33) Monday, February 16, 2015 Refinement Devices and sublayers Frame formats DIX Extended Unique Identifiers Ethernet Types 8023 ridges and Switches asics VLAs Layered Extensions Traffic Engineering Layer 3 view Refinement H3 Host Router etwork R etwork Host H3
Refinement (with IP enabled bridges) Layer 1 and 2 devices H3 3 R 3 Layer 1 (physical) Repeaters, Hubs (multiport repeaters) Single collision domain Together they form a LA (or link) segment Layer 2 (data link) ridges and Switches Single broadcast domain Together they form a LA (a link, usually 1 a layer 3 subnet) H3 1 multi-link subnets and multi-subnet links also occur Layer 2 sublayer: MAC Layer 2 sublayer: LLC Media Access Control Lower data link sublayer Work from the 8023 committees Most important example is Ethernet CSMA/CD Carrier Sense, Multiple Access, with Collision Detection ackoff and Retransmission Logical Link Control Upper data link sublayer Work from the 8022 committees Introduces three new fields DSAP: Destination Service Access Point SSAP: Source Service Access Point Control: 1 or 2 control bytes
DIX Ethernet (Ethernet II) packets/frames MAC addresses (1) 7 1 6 6 2 46-1500 4 Preamble SFD DstAddr SrcAddr Type PDU (with PAD) FCS Preamble and SFD 2 are handled by the physical layer (network card) A frame starts with 6 byte destination and source MAC addresses A 2 byte type field follows (multiplexing) PAD used to get a minimum PDU of 46 bytes 6 bytes (48 bits) MAC-48 (physical, now obsolete) EUI-48 (virtual, now including physical) EUI-64 (64-bit Extended Unique Identifier) Organizationally Unique Identifier 3 (OUI) Comprises the first three bytes of the MAC address Leaving three bytes for local assignment by the OUI-owner Maximum size of PDU is 1500 bytes The Frame Check Sequence is a Cyclic Redundancy Check (CRC) 2 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011 (In lsb first order canonical format this is 0x55 0x55 0x55 0x55 0x55 0x55 0x55 0xD5) 3 http://standardsieeeorg/develop/regauth/tut/euipdf MAC addresses (2) EUI-48 EUI-64 I/G bit: Individual (0) / Group (1) bit First bit on the wire roadcast or multicast U/L bit: Universal (0) / Local (1) bit Second bit on the wire I/G and U/L are the Least Significant its (LSs) of the first byte Sic!: this is called canonical format (LS-format) Token Ring uses noncanonical format (MS-format) Ample opportunity for confusion 4 Embedding of MAC-48 addresses OUI:extension OUI:FF:FF:extension Embedding of EUI-48 addresses 5 OUI:extension OUI:FF:FE:extension 00:30:65:7E:9D:5E 00:30:65:FF:FE:7E:9D:5E IPv6 is based on EUI-48 embedding for autoconfiguration but with a quirk 4 See IEEE Std 802-2001, section 95 5 Since MAC-48 addresses are obsolete, both FF:FF and FF:FE may be used
Modified EUI-64 Ethernet Types (1) The 64 bit host part of an IPv6 address Modified EUI-64 address U/L bit flipped 0030:65FF:FE7E:9D5E 0230:65FF:FE7E:9D5E The reverse happens to locally assigned IPv6 addresses 0200:0000:0000:0001 0000:0000:0000:0001 which immediately explains the reason for the modification Used for multiplexing Common ethertypes 0x0800 0x0806 0x8035 0x8137 0x86DD IP ARP RARP IPX (ovell) IPv6 Ethernet Types (2) 8023 frames (with 8022 LLC) Reintroducing the Ethernet Version I length field Less common ethertypes 0x0AD anyan Vines 0x0805 X25 level 3 (PLP) 0x809,0x80F3 AppleTalk 6 0x8847,0x8848 MPLS 7 0x8863,0x8864 PPPoE 8 7 1 6 6 2 1 1 1 9 43-1497 4 Preamble SFD DstAddr SrcAddr Length DSAP SSAP Ctrl LLC data (with PAD) FCS Introduces the LLC layer 2 sublayer Replaces type field by length field Used in EtherTalk packets (Apple) Disjoint values from ether types Later old DIX format also officially allowed 6 Ethertalk, AARP 7 Unicast, Multicast Ethernet 8 Discovery, Session Stage bytes 9 In some cases Ctrl is 2 bytes and then LLC data is between 42 and 1496
LLC/SAP IPX frames Sub etwork Access Protocol DSAP = 0xAA SSAP = 0xAA Control = 0x03 (UI, connectionless) Adds 5 extra bytes Organization Identifier (not used, set to 0x000000) Protocol type (guess what: ether type :-)) etto effect is the waste of 8 bytes so that the PDU size (MTU) is reduced from 1500 to 1492 bytes There are four variants DIX Ethernet with ethertype 0x8137 8023 SAP with protocol type 0x8137 8023 with DSAP = SSAP = 0xE0 Raw 8023 IPX packet starts with an (unimplemented) checksum looking like broadcast SAP DstAddr SrcAddr Length 0xFFFF Transparent ridges Switches o-frills Copies frames between its two sides (or multiple ports) Uses Store and Forward Separates collision domains Learning and filtering Remembers on which side/port a station resides y looking (promiscuously) to all addresses uilds a filtering database Standardized in 8021D Switches are a synonym for bridges In most cases have multiple ports Often remember on which port a station resides Switches may support more protocol extensions VLAs Quality of Service Management (SMP)
VLAs 8023 modifications for VLAs Separating a physically bridged LA into multiple logical/virtual LAs Each VLA has its own ID Standardized as 8021Q-2011 Ether type for VLAs: 0x8100 Conflicted with Wellfleet allocation 10 Official registration by IEEE 11 Add 4 extra bytes (VLA tag) between MAC source address and type/length field 2 bytes 0x8100 (VLA type) 3 bits priority (used by 8021p) 1 bit CFI 12 (Ethernet/Token Ring, usually 0 (Ethernet)) 12 bits VLA ID (0-4095) 10 http://wwwianaorg/assignments/ieee-802-numbers/ 11 http://standardsieeeorg/develop/regauth/ethertype/ethtxt 12 Related to the MS-LS-format issue: Canonical Format Indicator Later replaced by DEI (Drop Eligible Indicator) VLA membership Provider ridges (Ps) ested VLAs Port based Easy; static MAC based Administrative burden; dynamic Protocol and IP address based Wrong layer; not transparent Standardized as 8021ad Several non-standard Q-in-Q schemes Add a Service-VLA-tag (S-tag; outer tag) in front of the Customer-VLA-tag (C-tag; inner tag) Used by a P (Provider ackbone etwork) Ether type for S-VLAs: 0x88a8 (not in IAA list) Separates backbone VLA assignments from customer VLA assignments Placed between source MAC address and customer VLA tag
Provider ackbone ridges (Ps) MAC-in-MAC or M-in-M P-TE Provider ackbone ridges Traffic Engineering Standardized as 8021ah Adds a complete second Ethernet header with extended tags Used by a P (Provider ackbone ridging etwork) Adds provider backbone MAC destination and source addresses Followed by a ackbone tag (-tag) Ether type for -tags is again 0x88a8 (same as S-tag) And a new ackbone Service Instance tag (I-Tag) Ether type for I-tags is 0x88e7 (not in IAA list) Contains a 24 bit Service Instance Identifier Formally includes the client MAC destination and source address Derived from PT (Provider ackbone Transport; ortel) Connection oriented Ethernet transport Standardized as 8021Qay IEEE alternative for MPLS-TP (MPLS Transport Profile; ITU) which is based on T-MPLS (Transport MPLS) 8021Q-2011 standard 13 includes 8021ad, 8021ah and 8021Qay 13 aming explained in http://etherealmindcom/what-the-letters-802-1-ethernet-standards-stand-for/ TRILL TRansparent Interconnect of Lots of Links SP Shortest Path ridging Defines Rridges Routing on layer 2 Smart bridges, interoperable with 8021D and 8021Q Independent, localized routing in a single LA Uses layer 3 like headers with hop count Problem and applicability statement in RFC 5556 (May 2009) Specification in RFC 6325 (July 2011), 7176, 7177 (May 2014) Standardized as 8021aq (approved in March 2012) It replaces old spanning tree mechanisms, like STP, RSTP, MSTP It supports two flavours SPV (forwarding 8021ad Q-in-Q frames) SPM (forwarding 8021ah M-in-M frames)