Lecture 5.0. Virtual LANs

Transcription

1 Lecture 5.0 Virtual LANs Standard 802.1Q, 802.1v, 802.1s Broadcast issues Switches: - did partition collision domains - bud DID not partition broadcast domain 1

2 The obvious obvious solution: : IP subnets Partition network into several subnets Critical approach (especially in the past): routers were slow Need to replace switches with routers No more a problem of efficiency, today layer 3 switches = hardware-based routers, very fast! However Cons of physical IP subnets Floor 2 LAB 1 (telecom) LAB 2 (nanotech) OFFICES One switch per lab! Even if all switches in a same floor box, manual connection necessary Different LAB rooms = different subnets! Broadcast domain cannot extend through routers more complex management needed Floor 1 LAB 2 (telecom) 2

3 Physical Network Design vs Logical Network Design Standard design for physical network Well before network partitioning needs emerge from customers of the building! Armadio di piano Armadio di piano Cablaggio orizzontale in rame Prese RJ45 Stanza Stanza Stanza Canalina metallica forata Canalina in PVC Prese RJ45 Stanza Stanza Stanza Tubo in PVC Cablaggio verticale in Fibra Ottica Canalina metallica - Cablaggio verticale di backup in rame Solution: Virtual LAN (VLAN) VLAN = area which limits the broadcast domain Benefits Broadcast confinement solves scalability issues of large flat networks Isolation of failures and network impairments Security (more later) Multiple VLANs may coexist over a same Switched LAN 3

4 VLAN Membership Per Port THE typical VLAN approach The IEEE 802.1Q approach Per User Via MAC address Via VLAN tag Results: anarchic VLAN but too easy to break into Per Protocol New feature in IEEE 802.1V Combination (cross-layer) Supported as proprietary extensions Via IP subnet address. Classification hierarchy may be defined E.g. per IP subnet; if not IP per protocol; if not in the set of classified protocols per MAC; if not in MAC list per port. Per-Port Port + Per-Protocol Protocol Control (example) Default = tag with PVID (Port VLAN ID) 4

5 Physical vs logical view (i.e. why VLANS instead of IP network) Layer 3 subnets ought to be physically separated BUT many VLANs may overlap on the same, unique physical network structure! Robust, failureproof, single managed VLANs and IP subnets /1 1 VLAN = 1 IP subnet Routers are needed to move frames from different VLANs Even if STAs are in the same physical network Inter-VLAN connectivity through router: improves security May apply packet filtering mechanisms such as ACL, etc 5

6 VLANs and IP subnets / / /24 Routers for VLAN interconnection may have as little as just one physical interface Also called, in jargon, one-armed routers Multiple IP addresses on the single interface VLAN tagging 6

7 Port types TRUNK port: transmits and receives tagged frames i.e. with explicit VLAN membership indication ACCESS port: transmits and receives untagged frames i.e. with no VLAN membership indication HYBRID ports: may handle both tagged and untagged frames Access links A link connected to an access port Typically the PC-to-switch link or small-hub-to-switch link Access port Connected STAs belong to only 1 VLAN Connected STAs DO NOT NEED TO KNOW they are on a VLAN They just assume to be on a dedicated IP subnet TX/RX frames: standard Ethernet (no QTAG prefix) S1 S2 S3 HUB 7

8 Access links (legacy regions) May be switched LANs themselves Made up by VLAN-unaware switches VLAN-aware switch Access port VLAN-unaware switch VLAN-unaware switch S3 S1 S2 Trunk links A link connected to a trunk port Typically switch-to-switch or switch-to-router links frequently server-to-switch link If PC-to-switch link: Anarchic VLANs considered Trunk port Support tagged Ethernet frames Explicit tagging mechanism to differentiate them Does not belong to a VLAN but transport VLAN frames Either from all VLANs Or just from selected VLANs However, may belong to a VLAN Case of hybrid link Untagged frames assumed to belong to a VLAN 8

9 Hybrid links Support both tagged and untagged Ethernet frames Untagged frames belong to the same VLAN (in the example, VLAN C) Modern understanding and implementations: all links are of hybrid type Ethernet Frame format for VLAN (802.3ac, 1998) QTag type = 0x8100 QTag prefix = 4 bytes Maximum frame: 1522 (!!) > 1528 = baby giant processed correctly but might be recorded as error 9

10 User Priority (802.1p) BE BK --- EE CL VI VO NC Best Effort (default) Background Unspecified Excellent Effort Controlled Load Video < 100ms latency/jitter Voice < 10 ms latecny/jitter Network Control Managed via separated output queues - typically with priority queueing - but more complex scheduling mechanisms can be used Proprietary solutions (e.g. CISCO ISL) Cisco Inter Switch Link Protocol ISL Frame encapsulated in ISL (26 bytes) External tagging (encapsulation) frame 10 bits VLAN tag Other space for proprietary usage FCS (4 bytes) 10

11 May a station belong to more than 1 VLAN? Access links Trunk link Access links Yes! (typical case: servers) Switch operation with VLANs 11

12 VLAN and forwarding Green Blue, Green Trunk ports may forward only selected VLAN tags Manual (static) configuration Red, Green Automatic (dynamic) configuration via specially devised protocols (GVRP: GARP VLAN Registration Protocol) GARP = Generic Attribute Registr. Prot. See clause 10, 802.1D 1998 version No spanning tree considerations at the moment VLAN switch: relay functions Ingress function Classification of each received frame as belonging to one and only one VLAN Based on tag Based on port (e.g.) for untagged frames Discard frame based on normal bridging rules PLUS VLAN classification E.g. unallowed VLAN tag from port Ingress function = Access control using switches rather than routers! Forward function Only on specific enabled ports for given VLAN Egress function Add tag (or leave previous tag) if trunk link; Remove tag if access link 12

13 Learning Learning process affected by VLAN MAC address is no more the only information to consider! VLAN Identifier is also necessary Shared VLAN Learning (SVL) 1 single filtering DB if individual MAC Address learned in one VLAN, learned information used in forwarding decisions relative to all other VLANs Independent VLAN Learning (IVL) 1 filtering DB per each VLAN ID if individual MAC Address learned in one VLAN, learned information NOT used in forwarding decisions relative to all other VLANs General case (SVL/IVL) Many filtering DBs (each with a Filtering ID FID) Each FID may include more than 1 VLAN Filtering DB - SVL Dest MAC Address Ports Age vlan aa-01 1/ b0-8d-13-1a-f1 1/ a b-b4 2/ a7-64 2/ ff /

14 Filtering DB - IVL FID=12 Dest MAC Address Ports Age aa-01 1/1 1 a b-b4 2/ ff /6 5 FID=43 Dest MAC Address Ports Age b0-8d-13-1a-f1 1/7 4 FID=1 Dest MAC Address Ports Age a7-64 2/4 1 Distinct Filtering DBs (each assigned a Filtering ID) SVL vs IVL In most cases, no matter wthere IVL or SVL is used However, in some particolar cases, IVL or SVL are necessary Notation used in what follows: Member set Set of ports through which members of the VLAN can be reached Untagged set Set of ports through which, if frames are to be transmitted, they shall be transmitted without tag» Untagged set for a port may include multi VLANs (see SVL example next) PVID (Port VLAN ID) VLAN associated to the port See 802.1Q-2003, Annex B for detailed explanation of following examples 14

15 Why IVL? /1 Note: is a bridge device! Were it a router, no problems! SVL would not work!! (A learned from both port 1 and 4) (no STP in the example ) Why IVL? /2 SVL would not work!! (A learned from both port 1 and 3) (STP enabled, VLAN-aware connector) 15

16 Why SVL? VLAN unaware server to be shared among VLANs Must use untagged access link Asymmetric VLANs! Spanning Tree and VLANs (just motivations MSTP details in 802.1Q, clause 13+14) 16

17 VLANs and Spanning Tree Original 802.1Q specification: Common Spanning Tree (CTS) One for all VLANs Easy to maintain No load balancing possible Bridge priorities (or VLAN trunking) must be carefully selected To guarantee connectivity for ALL VLANs Multiple Spanning Tree Based on an early proprietary idea: Per VLAN Spanning Tree Problem: several VLANs BPDU load! Idea: aggregate VLANs 17

18 Based on RSTP Hierarchical approach One single spanning tree connects regions Common Spanning Tree (CTS) across regions Each region has at least an Internal Spanning Tree (IST) Called Common IST (CIST) One region acts as a virtual single bridge in terms of spanning tree! Multiple spanning tree instances (MSTI) are possible inside each region MSTP (802.1s, 2002) Details and new BPDU format quite complex - Refer to standard (and RFC 2014 for VLAN to MSTI crypted (HMAC-MD5) mapping) CIST + MSTI 18