1. S7C5 – Spanning Tree Protocol And other topics

2. Switch Port Aggregation Bundling –Combining 2 to 8 links of FE (Fast Ethernet) or GE (Gigabit) Full duplex bandwidth of up to 1600 Mbps or 16 Gbps –Load balancing can be implemented –Several bundles can provide redundancy –Redundancy can be provided from within bundle »One link goes down; another link takes over

3. Bundling Usually, contiguous ports required Usually, two or four ports must be bundled Generally, all ports must belong to same vlan Ports should have same speed and duplex setting before bundling

4. Bundling Continued IOS-based switches distribute by low-order bits of source MAC address (config-if)#port group G# distribution source CLI-based switches may require no configuration: if they do Switch> (enable) set port channel all distribution IP|MAC Source|Destination|both –If MAC addresses are remaining constant, use IP addresses

5. Port Aggregation Protocol (PAgP) Provides automatic EtherChannel configuration and negotiation between switches –Ports with same neighbor device ID and port group capability will be bundled as bidirectional, point to point link Identify static VLANS or trunking –On off auto desirable modes

6. EtherChannel Port Guidlines All ports should be assigned to same VLAN or configured for trunking If a trunk link, all ports should have same trunk mode and carry same VLANs All ports should have same speed and duplex mode All ports should be static VLAN ports All ports should be enabled (disabled port is viewed as failed link)

7. EtherChannel Configuration CLI –(enable) set port channel 2/2-6 mode on IOS-based –(config-if)# port group 4 distribution source Show commands –Show port channel info –Show port group 5 –Show port channel statistics

8. Spanning Tree Protocol Provides network redundancy and load balancing to allow switch to recover from failures without intervention –Layer 2 switch mimics transparent bridge No initial knowledge of location of end devices Listens to frames coming into ports to id host Updates bridging table Sends broadcasts out all ports Unknown destination addresses treated as broadcast Frames forwarded across bridge can’t be modified

9. Bridging Loops Process of forwarding a single frame around and around between two switches Learned location of PC keeps changing as frames get looped –Unicast frame can cause a bridging loop Break by disconnecting switch ports or shutting a switch down Better to prevent loops –Use STP (prevents loops but maintains redundant paths) – redundant paths placed in blocking or standby

10. BPDU STP Multicast address is 01-80-c2-00-00-00 Configuration BPDU – used to elect reference points –Hello sent every 2 seconds Topology Change Notification – (TCN) – announces changes in network topology

11. Root Bridge Election Unique Bridge ID – 8 bytes –Bridge priority (2 bytes) 0-65535; default 32768 –MAC address (6 bytes) – can be supervisor module, backplane, or from pool – is hardcoded, unique, and can’t be changed Switch with lowest Bridge ID is elected root –Ongoing process

12. Root Ports Each non-root switch elects a root port –Evaluate root path cost Cumulative cost of all links leading to root bridge Each switch link has cost called pathcost –1 byte value based on link bandwidth »100 mbpsold was 10; new is 19 »1 gbpsold was 1; new is 4 –Catalyst 5000 uses older linear scale; 4000 and 6000 use non- linear scale Costs calculated as BPDU comes in to port NOT as they go out

13. STP Decisions Lowest Root Bridge ID Lowest Root path cost to root bridge Lowest sender bridge ID Lowest port ID

14. STP States Disabled – shut down by network administrator or by system Blocking – when switch initializes – received BPDUs Listening – thinks can be elected root or designated port Learning – learns MAC addresses Forwarding – sends and receives data frames and sends and receives BPDUs –Show spantree 3/4

15. STP Timers Hello Time2 seconds Forward Delay15 + 15 seconds Maximum Age20 seconds –Assumed diameter is 7 switches If necessary, modify on root bridge switch only

16. Types of SPT Common Spanning Tree (CST) –IEEE 802.1q Single instance of STP for all VLANs –All BPDUs transmitted over VLAN 2 Per-VLAN Spanning Tree (PVST) –Cisco proprietary STP Separate instance of STP for each individual VLAN –Allows load balancing over redundant links –Requires ISL trunking PVST+ –Translates between groups of CST and PVST switches

17. Redundant Link Convergence Portfast – enables fast connectivity to be established on access layer switch ports to workstations UplinkFast --- enables fast uplink failover on an access layer switch when dual uplinks are connected to distribution layer BackboneFast – enables fast convergence in network backbone (core) after STP change

18. STP Configuration Commands TaskCLI-BasedIOS-Based Enable STPSet spantree enable [all] ¾ Spantree vlan- list View STPShow spantree vlan 1 Show spantree ¾ Show spantree vlan 1 Show spantree ¾ Set Bridge Prioriety Set spantree priority 34544 vlan 1 Spanning-tree vlan 2 priority 33333

19. Configuration Commands Cont’d Set Root BridgeSet spantree root [secondary] vlan 1[dia 4] [hello 6] Set port costsSet spantree portcost ¾ 25 Spanning-tree [vlan 2] cost 45 Set port prioritySet spantree portpri ¾ 565 Set spantree portvlanpri ¾ priority [vlans] Spanning-tree [vlan 3] port- priority 45

20. Configuration Commands Cont’d Set STP timersSet spantree hello 5 vlan 5 Set spantree root [secondary] [vlan 4] [dia 5] hello 5 Spanning-tree vlan 5 hello- time 4 Set portfastSet spantree portfast ¾ enable Spanning-tree portfast Set UplinkFastSet spantree uplinkfast enable rate 20 all-protocols on Spanning-tree uplinkfast [maxupdate-rate 45] Set backbone fastSet spantree backbonefast enable