network protocols handbook.pdf

274Protocols GuideCisco ProtocolsProtocol NameISL & DISL: Cisco Inter-SwitchLink Protocol and DynamicISL ProtocolProtocol DescriptionInter-Switch Link. Protocol (ISL), a Cisco-proprietary protocol,maintains VLAN information as traffic flows between switchesand routers.Inter-Switch Link (ISL) tagging accomplishes the same task as802.1Q trunking but uses a different frame format. ISL trunksare Cisco proprietary and define only a point-to-point connectionbetween two devices, typically switches. The name Inter-SwitchLink hints at this design. ISL frame tagging uses a low-latencymechanism for multiplexing traffic from multiple VLANs on asingle physical path. ISL has been implemented for connectionsamong switches, routers, and network interface cards (NICs)used on nodes such as servers. To support the ISL feature, eachconnecting device must be ISL-configured. A router that is ISLconfiguredcan allow inter-VLAN communications. A non-ISL devicethat receives ISL-encapsulated Ethernet frames will mostlikely consider them protocol errors because of the format andsize of the frames.Like 802.1Q, ISL functions at Layer 2 of the OSI model, but itdiffers by encapsulating the entire Layer 2 Ethernet frame insidean ISL header and trailer. Because ISL encapsulates theentire frame, it is protocol-independent and can carry any typeof Layer 2 frame or upper-layer protocol between the switches.The encapsulated frames may be token-ring or Fast Ethernet,and are carried unchanged from transmitter to receiver. ISL hasthe following characteristics:• Performed with application-specific integrated circuits(ASIC)• Not intrusive to client stations; client does not see theISL header• Effective between switches, routers and switches, andswitches and servers with ISL NICsDynamic Inter-Switch Link Protocol (DISL), also a Cisco protocol,simplifies the creation of an ISL trunk from two interconnectedFast Ethernet devices. Fast EtherChannel technologyenables aggregation of two full-duplex Fast Ethernet links forhigh-capacity backbone connections. DISL minimizes VLANtrunk configuration procedures because only one end of a linkneeds to be configured as a trunk.Protocol StructureISL header structure:40 4 4 48 16 8 24 15 1 16 16bitsDA Type User SA Len AAA03 HSA VLAN BPDUIndex• DA - 40-bit multicast destination address.• Type - 4-bit descriptor of the encapsulated frametypes—Ethernet (0000), Token Ring (0001), FDDI(0010), and ATM (0011).• User - 4-bit descriptor used as the type field extensionor to define Ethernet priorities. This is a binaryvalue from 0, the lowest priority, to 3, the highest priority.• SA - 48-bit source MAC address of the transmittingCatalyst switch.• LEN - 16-bit frame-length descriptor minus DA type,user, SA, LEN, and CRC.• AAAA03 - Standard SNAP 802.2 LLC header.• HSA - First 3 bytes of SA (manufacturer’s ID or organizationalunique ID).• VLAN - 15-bit VLAN ID. Only the lower 10 bits areused for 1024 VLANs.• BPDU - 1-bit descriptor identifying whether the frameis a Spanning Tree bridge protocol data unit (BPDU).Also set if the encapsulated frame is a Cisco DiscoveryProtocol (CDP) frame.• INDEX - 16-bit descriptor that identifies the transmittingport ID. Used for diagnostics.• RES - 16-bit reserved field used for additional information,such as Token Ring and Fiber DistributedData Interface (FDDI) frame Frame Check (FC)field.Related protocolsIEEE 802.1Q,VTP, DTP, Ethernet, Token RingSponsor SourceISP and DISL are Cisco protocols.Referencehttp://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008019f048.html#1017196Understanding and Configuring VLAN Trunking Protocolhttp://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/sw_5_2/cofigide/e_trunk.htmConfiguring VLAN TrunksResv